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The fate of morphologically different populations of Arrhenatherum elatius transplanted into arable and semi-natural habitats
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Agriculture Ecosystems & Environment ELSEVIER
Agriculture, Ecosystems and Environment60 (1996) 107-120
The fate of morphologically different populations of Arrhenatherum elatius transplanted into arable and semi-natural habitats A.C.E. Miller a, *, G.W. Cussans a, R.J. Froud-Williams b a Crop and Disease Management Department, IACR-Rothamsted, Harpenden, AL5 2JQ, UK b Department of Agricultural Botany, University of Reading, Reading, RG6 6A U, UK Accepted 5 August 1996
Abstract Fates of three morphologically distinct populations of Arrhenatherum elatius were monitored following seedling transplantation into a winter wheat crop, or an Italian ryegrass (Lolium multiflorum) and red clover (Trifolium pratense) ley. Two populations were corm-forming, the third was not. One corm-forming population was an arable weed, the other two populations were collected from semi-natural grassland. Survival within all populations of A. elatius was greater in wheat than grass and clover ley. Ramet numbers of all A. elatius populations increased following cultivation of the wheat plots. Productivity, measured in terms of tiller number and plant biomass, partially reflected site of origin. Non-corm-forming plants were most productive in the grass and clover ley habitat, and plants of arable origin were most productive in the wheat habitat. Mean productivity of corm-forming plants of semi-natural origin was least in both habitats. Arable bulbous plants produced more corms than semi-natural plants. Productivity was generally greater in wheat than grass and clover ley, when comparisons could be made. Non-corm-forming plants, by definition, had lower maximum stem diameters than those which formed corms. The maximum diameter of corms from the population of arable origin was significantly greater than that of the corm-forming population of semi-natural origin in ley, and tended to be greater in wheat. Phenotypic categorisation highlighted differences between the non-corm-forming population and the corm-forming populations. Mean phenotype of non-corm-forming plants in grass and clover ley plots changed over time, plant stems becoming marginally thicker. The two corm-forming populations could not be distinguished by phenotype in ley plots, although in wheat there was a tendency for plants of arable origin to have a more bulbous phenotype.
* Correspondingauthor. Tel.: 01582 763133 x 2761; fax: 01582 760981; e-mail: [email protected]. 0167-8809/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 67-8809(96)01088-2
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A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
Corm-forming plants delayed regrowth from corm reserves until late spring, whereas non-corm-forming plants grew throughout the winter. This life-cycle difference may explain their different niches. Keywords: Corms; Onion couch; Bulbous; Regrowth; Cultivation; Ley; Wheat
1. Introduction Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl, 'false oat-grass', is a perennial species common in rough grasslands throughout the UK. It is polymorphic for a range of morphological characters such as node hairiness and colour (Jenkin, 1931; Ducousso et al., 1990), and erectness of stem (Mahmoud et al., 1975), but the most noted variation is in its capacity for corm-formation (e.g. Jenkin, 1931; Pfitzenmeyer, 1962; Hubbard, 1984; Khan, 1991). European populations of A. elatius have been categorised into four 'subspecies' (Romero-Zarco, 1985). Two Mediterranean subspecies, sardoum and baen'cum, exist as diploids, although tetraploid populations of ssp. baeticum are also found. The remaining two subspecies, elatius and bulbosum, are tetraploid. Subspecies sardoum and elatius are noncorm-forming, whereas baeticum and bulbosum do produce corms. Only the tetraploid subspecies elatius and bulbosum are known in the UK (e.g. Grime et al., 1988). Genetic exchange between these two extreme growth forms of A. elatius results in a spectrum of intermediate forms (Jenkin, 1931). In the field, seed from a large bulbous population which is not in close proximity to non-bulbous plants will tend to give rise to highly bulbous plants (Khan, 1987). Thus, the propensity for corm-formation appears to have a genetic basis, although corm size may be modified by soil type (Underwood, 1912) and plant density (Miller, 1994). Niche differentiation between these two extreme growth forms is not clear-cut. Subspecies bulbosum alone is found as a weed in arable systems, where it is commonly known as 'onion couch'. Both cormforming and non-corm-forming types of A. elatius are found in semi-natural grassland, ssp. bulbosum predominantly in the west of England, Wales and Scotland, and ssp. elatius in the east of England (Miller et al., 1992; Cussans et al., 1993). Various
factors may influence subspecific distributions. It has been suggested that ssp. bulbosum may be more suited to wetter climatic regions with mild winters (Pfitzenmeyer, 1959), also that ssp. elatius may be better adapted to less fertile soils (Grime et al., 1988). Khan (1987) suggested that corm formation imposes a fitness penalty within this species. Onion couch is a weed that has been favoured by practices of both minimal tillage, and continuous winter cereal cropping (e.g. Sherrott and Rees, 1992). Whilst it is not usually considered a common weed, surveys of UK regions suggest that 1-5% of cereal fields may be infested (Chancellor and Froud-Williams, 1983; Scragg and Kilgour, 1984). Yield loss from onion couch can be in excess of 60% (Rees and Sherrott, 1991). Chemical control measures for this subspecies in winter cereals often seem to have limited efficiency due to a low ratio of foliage area to corm number (Ayres, 1977; Tanphipbat and Appleby, 1990). An earlier control method suggested by Jenkin (1931) was the practice of laying the land 'down to grass for a number of years'. The rationale for this was that there was little regrowth of ssp. bulbosum plants between autumn and spring, and hence other wintergrowing plants would outcompete them. However, Jenkin proposed that plants intermediate between the extremes (i.e. between corm-forming and non-cormforming) might be most persistent. Onion couch problems may develop following the implementation of the agricultural policy of 'setaside' (Milne, 1987). Set-aside is a practice in which farmers are paid to keep a proportion of their land out of production for one or more years, with only a very restricted use of herbicides permitted. If onion couch, or persistent intermediate plants derived from a cross between non-bulbous false oat-grass and bulbous onion couch, became established in set-aside fields, there could be a potentially serious weed problem on reversion to arable cropping. This paper assesses persistence and morphology of plants from three different populations of Arrhen-
A.C.E. Miller et al./Agriculture, Ecosystems and Environment 60 (1996) 107-120
atherum elatius, following transplantation into either winter wheat, or an Italian ryegrass and red clover ley. The populations chosen consisted of morphologically typical examples of three broad categories of A. elatius population (Miller et al., 1992): arable bulbous ('onion couch'), semi-natural bulbous, and semi-natural non-bulbous.
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selected populations by tapping inflorescences to release ripe seeds into a bowl. Arrhenatherum elatius populations chosen were as follows: I. plants with no corms ('non-bulbous') from a derelict allotment (i.e. semi-natural habitat) in Earley, Berkshire; 2. plants with a few small corms per stem ('bulbous') from a roadside verge (i.e. semi-natural habitat) in Goonhavern, Cornwall; 3. plants with several, often relatively large, corms per stem ('bulbous') from a dense onion couch infestation in a winter wheat field (i.e. arable habitat) in Broad Chalke, Wiltshire. Arrhenatherum elatius seeds were sown into seed trays of compost on 1 and 2 October 1990. Single seedlings were pricked out into polystyrene modules between 11 and 15 October 1990. This was to reduce the effects of intraspecific competition and
2. Materials and methods 2.1. Plant material
Plant material derived from seed was used for this study, to represent the potential range of genotypes in the gene pool, rather than just the 'successful' survivors. Seeds of A. elatius were collected in summer 1990 from a number of randomly chosen plants in
Table 1 Calendar of field operations in a study of the fate of morphologically different populations of Arrhenatherum elatius transplanted into a winter wheat and a grass/clover ley habitat Date
Grass and clover ley
27/9/1990
l / 1 1 / 1 9 9 0 - 7 / 1 l/1990
A. elatius seedlings transplanted into field
12/3/1991 5/4/199 I 18/3/1991-18/4/1991 20/7/1991-13/8/1991 l 1/9/1991-13/9/1991 2/10/1991
Post-transplantation count I st destructive harvest
Bromoxynil/ioxynil/mecoprop (Swipe, Ciba-Geigy) applied to kill annual dicots Nitram fertiliser (34.5% N) applied @ 580 kg haPost-transplantation count 1st destructive harvest Combine harvested and straw removed
Tine cultivation ( × 2); rotorgrubbed to maximum cultivation depth of 10 cm; 'Mercia' sown @ 162 kg ha- 1 Low dose (0.19 kg a.i. ha- t ) diclofop-methyl (Hoegrass, Hoechst) applied for volunteer Italian ryegrass
25/2/1992
10/3/1992-18/3/1992 3 / 4/1992 9/4/1992
2nd destructive harvest
4/6/1992-24/6/1992 30/6/1992
3rd destructive harvest Cut to 10 cm height and produce removed
22/6/1993-15/7/1993
Winter wheat cv. 'Mercia' sown @ 166 kg ha- ~, into areas within an established ley, previously cleared with 360 g a.i. ha ~ glyphosate (Roundup) followed by cultivation A. elatius seedlings transplanted into field
Cut to 10 cm height and produce removed
11/10/1991
3/8/1992-3/9/1992 16/9/1992
Wheat
Nitram fertiliser (34.5% N) applied @ 580 kg ha- t Low dose (0.19 kg a.i. ha- ~) diclofop-methyl (Hoegrass, Hoechst) applied for volunteer Italian ryegrass
2nd destructive harvest Cut to 10 cm height and produce removed 4th destructive harvest
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transplant shock, and to facilitate transplantation into the field. After initial establishment in an unheated glasshouse, seedlings were hardened off outside for a few days. Seedlings were transplanted into the field between 1 and 7 November 1990, when the largest were beginning to tiller.
2.2. Experimental design The experiment was sited at IACR-Rothamsted, in an established ley comprising Italian ryegrass (Lolium multiflorum) and red clover (Trifolium pratense) (ley plots). A calendar of operations on this experiment is given in Table 1. Each of the three populations of A. elatius were transplanted into blocks consisting of five adjacent 2.5 m × 1.25 m subplots, in adjoining wheat and ley plots. There were four replicates of the two habitats. The 2.5 m X 1.25 m subplots were divided into 25 cm × 25 cm squares, using a 1.25 m × 1.25 m quadrat positioned over each half of the subplot in turn. One A. elatius seedling was planted in the centre of each square, next to a white plastic label. Hence each subplot contained 50 A. elatius seedlings.
2.4. Measurements 2.4.1. Survival Original planting sites, marked with plastic labels, were located, and presence/absence of each original A. elatius transplant was recorded. The initial posttransplantation count was a non-destructive assessment of the whole experiment. Subsequent plant counts were made during the destructive harvest of single subplots, with squares being excavated for below-ground material if no growth was visible above-ground. Survival figures are given as a percentage of the original number of transplants, since establishment was nearly 100% in each population at the post-transplantation count. 2.4.2. Tiller number per plant Counts were made of total tiller number and number of mature tillers with developed leaves (or at least 10 cm in length, depending on development of plants at harvest). In the June 1993 harvest, only mature tillers were counted. 2.4.3. Maximum stem diameter The diameter at the widest part of the corm or stem was measured to the nearest millimetre. March
2.3. Assessments Original plants were located by their proximity to the plastic labels, in each of the ley subplot assessments, and in the first destructive assessment of the wheat plots. Four of the five ley subplots within each plot had been harvested by the termination of the experiment. For the first destructive assessment of the wheat plots, the fifth subplot was systematically harvested. This was to increase the width of pathways between plots of different biotypes, to reduce risk of contamination during cultivation. Following cultivation, it was clearly inappropriate to search for original subplots. Instead, strips of twenty 25 cm × 25 cm squares running the length of the wheat plots were harvested. All measurements, with the exception of survival, were taken from the central 24 plants from subplots within each of the four replicates. Substitutions from surrounding transplants were used where central plants were deceased, to retain a total of 24 plants in the assessments. Survival assessments were made on complete subplots of 50 original plants.
1
2
4
5
t 3
6
Fig. 1. Typical stems of Arrhenatherum elatius plants in six phenotypic categories (after Miller, 1994). 1, non-bulbous; 2, non-bulbous/intermediate; 3, intermediate/non-bulbous; 4, intermediate/bulbous; 5, bulbous/intermediate; 6, bulbous.
A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120 1992 values reflect the diameter o f corms produced during the previous season, n e w ones not b e i n g fully developed at this harvest.
2.4.4. N u m b e r o f corms Eight categories of internode shape can be broadly defined in A. elatius, as points in a c o n t i n u u m (Miller, 1994). F o r this study, these were c o n d e n s e d into two groups: 1. club-shaped corms, swollen at the base then tapering toward the higher node, with a c o r m height:corm width ratio o f at least 2; 2. f l a t / r o u n d e d corms, with n o tapering, and a corm height:corm width ratio o f less than 2. March 1992 values reflect the n u m b e r o f corms produced d u r i n g the previous season, n e w ones not being fully d e v e l o p e d at this harvest. 2.4.5. Dry weight Plants were o v e n - d r i e d at 80°C for approximately 16 h, then w e i g h e d in grams to an accuracy of two decimal places. In the June 1993 grass and clover ley plot assessment, samples were separated into swollen basal parts, and stem plus leaves.
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2.4.6. Phenotypic categories Six subjective categories of bulbosity were defined, based on the n u m b e r and shape o f basal i n t e m o d e s on a plant. These were n u m b e r e d 1 ( n o n b u l b o u s ) through to 6 (extremely bulbous) (Fig. 1). All small plants with one or two flattened or r o u n d e d corms per stem were placed in category 6, since it was n o t possible to distinguish these from category 5 plants at that stage. Data were analysed both as six categories ( 1 - 6 ) and as five categories, with the original categories 5 and 6 merged ( 1 - 5 ) . This was to see whether the inclusion of small plants, which were frequently from the b u l b o u s population o f semi-natural origin, biased data interpretation.
2.5. Data analysis A n a l y s e s o f variance were performed on m e a n plot data, using Genstat 5.0. Data were log~ transf o r m e d (i.e. log¢ ( x + 0.5)) to normalise data where appropriate. Standard errors of the difference between two m e a n s (SEDs) are given, along with P values.
Table 2 The effect of habitat type on survival and growth form of Arrhenatherum elatius. Survival and growth form values are calculated from plot means of three A. elatius populations in each of two habitats (Italian ryegrass and clover Icy, and winter wheat). All values are given for the August 1991 harvest only, with the exception of Survival, for which the April 1991 assessment could be included. Significance levels, and SEDs are given for Habitat and the Habitat x Population interaction. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses. Maximum standard errors of differences between means are given for interaction terms Habitat Survival (%) April 1991 Aug. 1991 Total tiller number (per plant) Mature tiller number (per plant) Max. stem diameter (per plant) Corm no. (flat) (per plant) Corm no. (club) (per plant) Dry weight c (per plant) Phenotypic categories (1-6) (i) All populations (ii) Bulbous populations only
Sig. level a
SED b
Sig. level a
SED b
Ley
Wheat
(Habitat)
(Habitat)
(Habitat × Pop.)
(Habitat × Pop.)
99.1 93.5 0.95 (2.1) 0.74 (1.6) 1.69 (4.9) 1.11 (2.5) 0.04 (0.5) 5.67 (0.3)
99.3 99.0 2.80 (15.9) 2.45 (11.1) 1.93 (6.4) 2.87 (17.1) 1.87 (6.0) 9.25 (10.4)
NS * *** *** ** *** *** ***
1.07 0.11 0.05 0.02 0.06 0.07 0.15
NS NS NS NS ** ** NS NS
4.30 5.92
4.13 5.69
NS, P > 0.05; * P _ < 0 . 0 5 ; * * P ' ; 0 . 0 1 ; * * * P < 0 . 0 0 1 . b SED, standard error of the difference between two means. c Log e values and SEDs on data in milligrams.
a
NS NS
** *
0.05 0.11
0.097 0.111
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A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
W h e a t a n d ley p l o t r e s u l t s f r o m A u g u s t 1991 are c o m p a r e d ( T a b l e 2). H o w e v e r , o w i n g to t h e d i f f e r e n c e in h a r v e s t d a t e b e t w e e n w h e a t a n d l e y p l o t s in
the second year of the study, and because of the effect of cultivation on the nature of the later wheat p l o t d a t a , it w a s n o t c o n s i d e r e d v a l i d to u n d e r t a k e
Table 3 Survival and growth form data for morphologically different populations of Arrhenatherum elatius transplanted into grass and clover ley plots. Means for survival and growth form data from three populations of A. elatius transplanted into a grass/clover Icy are given. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses. Maximum standard errors of differences between means are given for interaction terms. Population of Arrhenatherum elatius
Survival (%) Apr. 1991 Aug. 1991 Mar. 1992 June 1992 June 1993 Mean (Pop.) Slope
Non-bulbous
Arable bulbous
Semi-natural bulbous
99.0 92.5 87.5 80.0 79.5 87.7 - 0.75
99.5 96.0 96.5 92.5 89.0 94.7 - 0.38
98.7 92.0 82.0 71.0 62.0 81.1 - 1.45
Total tiller number Aug. 1991 Mar. 1992 June 1992 Mean (Pop.)
(per plant) 1.27 (3.1) 3.87 (47.5) 2.89 (17.6) 2.68 (14.1)
Sig. level a
SED b
Sig. level a
SED b
(Pop.)
(Pop.)
(Pop. X Time)
(Pop. X Time)
**
2.6
*
0.25
*
5.3
0.97 (2.1) 2.69 (14.2) 1.85 (5.9) 1.84 (5.8)
0.61 (1.4) 2.37 (10.2) 1.60 (4.4) 1.53 (4.1)
***
0.14
***
0.17
Mature tiller number (per plant) Aug. 1991 0.82 (1.8) Mar. 1992 3.62 (37.0) June 1992 2.89 (17.4) June 1993 3.55 (34.3) Mean (Pop.) 2.72 (14.7)
0.86 2.40 1.81 2.26 1.83
0.56 (1.3) 2.04 (7.2) 1.60 (4.4) 1.52 (4.1) 1.43 (3.7)
***
0.10
***
0.19
Maximum stem diameter (per plant) Aug. 1991 0.80 (1.7)
2.18 (8.3)
2.10 (7.7)
* * * (AP); * * (BP)
0.02 (AP); * * * (AP); 0.01 (BP) NS (BP)
0.05 (AP)
Mar. 1992 June 1992 June 1993 Mean (Pop.)
2.20 2.39 2.39 2.29
(8.5) (10.4) (10.4) (9.4)
2.08 (7.5) 2.36(10.1) 2.35 (9.9) 2.22 (8.7)
Corm number (fiat) (per plant) Aug. 1991 Mar. 1992 June 1992 June 1993 Mean (Pop.) -
1.43 (3.7) 1.49 (3.9) 3.17 (23.3) 4.26 (70.6) 2.59 (12.8)
0.79 (1.7) 0.64 (1.4) 2.40 (10.5) 2.86 (16.9) 1.67 (4.8)
0.10
NS (p = 0.051)
0.19
Corm number (club) (per piano Aug. 1991 June 1992 June 1993 Mean (Pop.) -
0.12 1.67 2.57 1.45
0.18
NS (p = 0.059)
0.26
1.10 (2.5) 1.46 (3.8) 1.48 (3.9) 1.21 (2.9)
(1.9) (10.5) (5.6) (9.1) (5.7)
(0.6) (4.8) (12.6) (3.8)
- 0.05 (0.5) 1.14 (2.6) 1.51 (4.0) 0.87 (1.9)
A.C.E. Miller et a l . / Agriculture, Ecosystems and Environment 60 (1996) 107-120
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Table 3 (continued) Population of Arrhenatherum elatius
Sig. level a
SED b
Sig. level a
SED b
Non-bulbous
(Pop. × Time)
(Pop. × Time)
Dry weight ¢ (per piano Aug. 199l - 0 . 0 8 (0.4) Mar. 1992 0.84 (1.8) June 1992 3.08 (21.2) June 1993 3.72 (40.7) Mean (Pop.) 1.89 (6.1) Corm weight (% of total) June 1993 -
Arable bulbous
Semi-natural bulbous
(Pop.)
(Pop.)
- 0 . 1 3 (0.4) 0.05 (0.6) 1.76 (5.3) 2.42 (10.8) 1.03 (2,3)
- 0 . 4 0 (0.2) - 0.28 (0.3) 1.20 (2.8) 1.43 (3.7) 0.49 (1.1)
***
0.12
*
3.04
* * * (AP); NS (BP)
0.038 (AP)
78.4
62.7
Phenotypic categories (1-6) Aug. 1991 1.07
5.88
5.96
Mar. 1992 June 1992 June 1993 Mean (Pop.)
5.94 5.91 5.91 5.91
5.83 5.97 5.92 5.92
1.05 1.06 1.32 1.13
***
* * (AP); NS (BP)
0.20
0.062 (AP)
NS, P > 0.05; * P < 0.05; * * P < 0.01; * * * P_< 0.001. b SED, standard error of the difference between two means. c Transformed and back-transformed values given in grams. AP, all populations analysed; BP, bulbous populations only analysed. a
analyses combining wheat and ley plot data after the first harvest, and trends are discussed instead. The ley plot harvest data from each assessment date were combined for the analyses (Table 3). For ley plot survival data, a regression was done for each plot over time, and analysis of variance was carried out on the slopes. Separate analyses are given for wheat plot data on each harvest date (Table 4).
3. Results
The first year's results have been documented previously in Miller et al. (1992), but are re-stated here to give complete experimental results.
(P < 0.05). Whilst plant numbers in ley plots continued to steadily decrease, cultivation of wheat plots broke up plants, and hence increased ramet numbers considerably. 3.1.2. Grass and clover ley plots The number of original plants recovered from ley plots decreased with time ( P < 0 . 0 0 1 ) (Table 3). There was less mortality amongst bulbous plants of arable origin than within populations of semi-natural origin, either bulbous or non-bulbous ( P < 0 . 0 1 ) . Rate of mortality varied amongst populations (P < 0.05), with most rapid decline in the bulbous population of semi-natural origin. Even in this population, though, more than 60% of plants had survived into the third year after transplantation.
3.1. Survival 3.1.1. Grass and clover ley plot vs. wheat plot trends Initial counts (April 1991) showed that establishment was similar in both ley and wheat (Table 2). By August 1991, survival was lower in ley than wheat
3.1.3. Wheat plots There was no interpopulation difference in survival by the spring after transplantation (Table 4). By the summer, greatest mortality was observed in the bulbous population of semi-natural origin. Cultiva-
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tion subsequently increased ramet numbers in all populations to over twice the original planted number in both the bulbous population of arable origin
and the non-bulbous population. However, owing to a high degree of variability, a significant interpopulation difference was not detected.
Table 4 Survival and growth form data for morphologically different populations of Arrhenatherum elatius transplanted into wheat plots. Means for survival and growth form data from three populations of A. elatius transplanted into winter wheat are given. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses Sig. level a
Population of Arrhenatherum elatius
Survival (%) Apr. 1991 Aug. 1991 Aug. 1992 Total Aug. Aug. Aug.
Non-bulbous
Arable bulbous
Semi-natural bulbous
99.7 100.0 212.5
99.7 99.5 268.7
98.5 97.5 136.3
tiller number (per plant) 1991 3.03 (20.2) 1992 (r) 3.10 (21.7) 1992 (s) 3.80 (44.4)
SED b
NS ** NS ( P = 0.055)
0.4 42.6
3.03 (20.2) 3.33 (27.4) 4.30 (72.8)
2.34 (9.9) 2.87 (17.1) 3.08 (21.3)
*** * **
0.11 0.11 0.24
2.79 (15.9) 3.04 (20.4) 4.00 (54.3)
2.09 (7.6) 2.42 (10.8) 2.64 (13.6)
*** ** **
0.08 0.09 0.27
Maximum stem diameter (per plant) Aug. 1991 1.21 (2.9) Aug. 1992 (r) 1.16 (2.7)
2.37 (10.2) 2.38 (10.3)
2.22 (8.7) 2.25 (9.0)
* * * (AP); NS ( P = 0.065)(BP) * * * (AP); NS ( P = 0.094) (BP)
0.05 (AP); 0.05 (8P) 0.04 (AP); 0.05 (BP)
Corm Aug. Aug. Aug.
number (flat) 1991 1992 (r) 1992 (s)
3.59 (35.6) 4.09 (59.3) 5.06 (157.7)
2.15 (8.1) 2.82 (16.3) 3.04 (20.5)
Corm Aug. Aug. Aug.
number (club) 1991 1992 (r) 1992 (s)
2.11 (7.1) 2.51 (11.7) 3.46 (31.2)
Dry weight c (per plant) Aug. 1991 9.34(11.4) Aug. 1992 (r) 1.49 (3.9) Aug. 1992 (s) 2.15 (8.0) Phenotypic categories (1-6) Aug. 1991 1.01 Aug. 1992 (r) 1.20 Aug. 1992 (s) 1.15
Mature tiller number Aug. 1991 Aug. 1992 (r) Aug. 1992 (s)
(per plant) 2.47 (11.3) 2.73 (14.8) 3.43 (30.3)
(per plant) (per plant) -
* *
0.13 0.28 0.42
1.63 (4.6) 1.81 (5.6) 2.03 (7.1)
* * *
0.09 0.16 0.34
9.50(13.4) 2.29 (9.4) 3.24 (24.9)
8.89(7.3) 1.54 (4.2) 1.77 (5.4)
* ** *
0.17 0.15 0.33
5.87 5.87 5.87
5.51 5.36 5.50
* * * (AP); * (BP) * * * (AP); NS ( P = 0.10) (BP) * * * (AP); NS ( P = 0.10) (BP)
0.10 (AP); 0.09 (BP) 0.18 (AP); 0.22 (BP) 0.13 (AP); 0.16 (BP)
a NS, P > 0.05; * P_< 0.05; * * P_< 0.01; * * * P_< 0.001. b SED, standard error of the difference between two means. c Log e values and SEDs on data in milligrams. AP, all populations analysed; BP, bulbous populations only analysed. r, data per ramet; s, data per 25 cm × 25 cm square.
*
*
A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
3.2. Tiller number 3.2.1. Grass and clover Icy plot vs. wheat plot trends Tiller number of A. elatius was considerably less ( P < 0.001) in Icy plots than wheat plots in August 1991 (Table 2). This trend was also true for August 1992, when comparing equal areas (i.e. August 1992 'square' not 'ramet' data). A higher percentage of tillers tended to be mature in ley than in wheat in both August 1991 and June/August 1992. 3.2.2. Grass and clover Icy plots Total tiller number varied with time ( P < 0.001) (Table 3). However, this was not purely a simple increase in numbers, with a reduction in total tiller numbers of about two-fifths between March 1992 and the following June. The percentage of tillers that were mature was significantly higher ( P < 0.001) in June 1992 than in March. This was largely attributed to growth stage, as the latter harvest corresponds with time of flowering. In both the bulbous population of arable origin and the non-bulbous population, mature tiller number increased greatly between June 1992 and June 1993. The populations displayed significantly different ( P < 0.001) numbers of tillers, both total and mature only, the non-bulbous population having three times as many as the arable bulbous population by the second summer after transplantation. The percentage of mature tillers was not significantly different between populations, and relative rankings were not consistent for this parameter. Significant ( P < 0.001 ) population X time interactions were noted both for total and mature tiller numbers. In both cases, the more rapid increase in the non-bulbous population between August 1991 and March 1992 will have contributed to this difference. A fall in numbers of mature tillers between June 1992 and June 1993 in the bulbous population of semi-natural origin will also have contributed to a significant interaction. 3.2.3. Wheat plots There was a significantly different number of tillers (both total and mature only) between populations on both harvest dates (Table 4). Analysis based on data per square (25 cm X 25 cm) compared with that on data per ramet made little difference to
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interpretation. The bulbous population of arable origin produced most tillers, both total and mature only, and the highest percentage mature tillers. The bulbous population of semi-natural origin produced least tillers, and had the lowest percentage of mature tillers.
3.3. Maximum stem diameter 3.3.1. Grass and clover ley plot vs. wheat plot trends Maximum stem diameters were greater in wheat than ley in August 1991 ( P < 0.01) (Table 2). There was a population x habitat interaction ( P < 0.01), the difference in values between wheat and ley values for the non-bulbous population being proportionately greater than for the two bulbous populations. By the second summer after transplantation, mean values in ley tended to be greater than those in wheat. Cultivation of wheat plots did not appear to affect maximum stem diameter of A. elatius, values being similar in both years of the experiment. However, values in ley plots did increase after the first season, implying that competition had prevented young plants from producing such substantial tillers. Once nonbulbous plants had become established in ley, they actually attained greater maximum stem diameters than in wheat, whereas bulbous plants produced broadly similar values in both habitats. 3.3.2. Grass and clover ley plots Maximum stem diameter was lower ( P < 0.001) in the first summer after transplantation than in the second and third summers, in each of the three populations (Table 3). Note that the data from March 1992 refer to corms remaining from the previous summer--new ones had not fully developed by this harvest date. There was clearly a significant difference between populations ( P < 0.001), with the non-bulbous population, rather by definition, having much lower values than the two bulbous populations. However, the maximum diameter of corms from the population of arable origin was also significantly higher than that of the semi-natural bulbous population. Change in maximum stem diameter with time was at a different relative rate in the non-bulbous population than in either of the two bulbous populations ( P < 0.001).
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3.3.3. Wheat plots Again, non-bulbous plants clearly had lower maximum stem diameters than bulbous ones ( P < 0.001) (Table 4). Whilst there was a trend for arable bulbous plants to have a greater diameter than semi-natural bulbous plants, this difference was not significant due to size of standard error. 3.4. Corm number per plant 3.4.1. Grass and clover ley plot vs. wheat plot trends Corm number per plant was greater in wheat than ley, both in August 1991 ( P < 0.001) and August 1992 (Table 2). A population x habitat interaction was observed for flat corms in August 1991 ( P < 0.01), with the ratio of the number in wheat to the number in ley being twice as great for the arable bulbous than the semi-natural bulbous population. Percentage flat corms tended to be lower in wheat than ley. 3.4.2. Grass and clover ley plots Number of corms (fiat and total) increased significantly with time ( P < 0.001) (Table 3). The bulbous population of arable origin always had more corms per plant than the one of semi-natural origin ( P < 0.01). The population X time interaction approached the 5% level of significance ( P = 0.051) for number of flat corms per plant i.e. there was a trend for more rapid accumulation of corms in the arable bulbous population. The percentage of flat corms was not significantly different over time, but was significantly greater in arable bulbous plants than semi-natural bulbous plants ( P < 0.05). 3.4.3. Wheat plots There were significantly greater numbers of corms on arable bulbous plants than semi-natural bulbous ones (flat and total; August 1991 P <0.01 and August 1992 P < 0.05) (Table 4). Percentage of flat corms was significantly greater in bulbous plants of arable origin than semi-natural origin (August 1991 P < 0.01 and August 1992 P < 0.05, analyses not presented). 3.5. Dry weight 3.5.1. Grass and clover ley plot vs. wheat plot trends Dry weight per plant was much greater in wheat than ley in August 1991 ( P < 0.001) (Table 2). No
population X habitat interactions were found for this harvest. However, by August 1992, there appeared to be an interaction with the environment, resulting in a difference in the relative productivity of the three populations (ley: non-bulbous > arable bulbous > semi-natural bulbous; wheat (s): arable bulbous > non-bulbous > semi-natural bulbous; wheat (r): arable bulbous > semi-natural bulbous > nonbulbous).
3.5.2. Grass and clover ley plots Dry weight per plant increased over time ( P < 0.001) (Table 3). There was a significant interpopulational difference ( P < 0 . 0 0 1 ) , with non-bulbous plants consistently producing more dry matter than bulbous plants. Arable bulbous plants produced more than semi-natural bulbous plants. A significant ( P < 0.001) population X time interaction was recorded, with initial increase in mass of non-bulbous plants being relatively greater than that of bulbous plants. Dry weight accumulation in bulbous plants of seminatural origin was relatively slow. The two bulbous populations had allocated different proportions of their biomass to corms at the June 1993 assessment ( P < 0.05), with nearly 80% mass in corms in the arable population, and under 65% in the semi-natural population. 3.5.3. Wheat plots Dry weight was significantly different between populations ( P < 0.05) both in August 1991, and when August 1992 data per 'square' were analysed (Table 4). The bulbous population of arable origin produced most dry matter, and the one of semi-natural origin produced least. When ramet data were compared for August 1992, there was still a significant difference ( P < 0.01), but relative rankings had changed, with the nonbulbous plants producing least dry matter per ramet. 3.6. Phenotypic categories 3.6.1. Grass and clover ley plot vs. wheat plot trends No significant differences were found between wheat and Icy plots in August 1991 (Table 2). However, a population x habitat interaction was found, both for all populations ( P < 0.01), and for the two bulbous populations ( P < 0.05). This was because
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plants from the semi-natural bulbous population were slightly less bulbous in wheat than in ley. This trend was also seen in the later harvest.
3.6.2. Grass and clover ley plots There was a significant difference between phenotypic categories over time when all three populations were assessed, but not when comparing the two bulbous populations alone (Table 3). A similar observation was made about the population × time interaction. These results would appear to derive from the change from a static mean phenotype within the non-bulbous population over the first 2.5 years of the experiment, which then increased slightly in bulbosity the following year. The mean phenotype of the non-bulbous population was clearly at the opposite end of the spectrum from those of the two bulbous populations, as would be expected from the original definition of the populations. Combining categories 5 and 6 affected significance levels, but not the interpretation of results (analysis not shown). 3.6.3. Wheat plots There was a clear difference between mean phenotype of the non-bulbous and bulbous populations (P < 0.001) (Table 4). In the first season, the arable bulbous plants had a slightly more bulbous mean phenotype than the semi-natural bulbous plants (P < 0.05). This remained a trend in the second year, although it was not statistically significant (P--0.100). Combining categories 5 and 6 made the difference between arable and semi-natural bulbous populations non-significant in the first summer after transplantation (analysis not shown).
4. Discussion
4.1. Persistence and productivity The three populations of A. elatius behaved differently from each other, and in the two habitats. In terms of plant numbers, the arable bulbous population was most persistent in both habitats, followed by the non-bulbous population. This result agrees with
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Cussans et al. (1993), who suggested that the weed strategy of onion couch was based on extreme persistence, rather than invasiveness or competitiveness. In the arable habitat, this persistence strategy was emphasised by the success of ramet establishment following stem separation by cultivation. Although each population showed a gradual decline in numbers in the grass and clover ley habitat, most of the transplants survived into the third year after transplantation. In contrast, a comparison of plant tiller and biomass production gives a different angle to the data interpretation. In the grass and clover ley plots, it is the non-bulbous plants that have produced by far the greatest number of tillers and biomass. Rather surprisingly, the bulbous population of semi-natural origin was less productive in the ley habitat than the population of arable origin. In the wheat plots, the arable bulbous population was most productive, but once again the semi-natural bulbous population was least productive. In the case of the non-bulbous and arable bulbous populations, then, relative performance was exactly as might be expected from a knowledge of their original habitats (e.g. Grime et al., 1988). However, the relative growth and persistence of the semi-natural bulbous plants, which had the highest mortality and lowest productivity in both habitats, did not fit into the expected pattern.
4.2. Phenotypic categories and morphology In most instances, there was a big difference in phenotype between plants originating from a nonbulbous population and those from bulbous populations. The only time that plants in any of the three most bulbous phenotypic categories were found in a non-bulbous plot was in August 1992 following cultivation of wheat plots. Owing to the small number of plants involved, and in light of their complete absence in other harvests, this occurrence was most likely attributable to movement of ramets during cultivation. The amalgamation of categories 5 and 6 in the analyses, to check the potential for the automatic placement of small plants into category 6 to bias differences between the generally larger arable bulbous plants and the smaller semi-natural ones, revealed that this did not have much effect.
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The maximum stem diameters of A. elatius plants grown in wheat were stable over the 2 years assessed. However, those grown in ley were initially lower than those grown in wheat, increasing between the first and second years before stabilising. This would suggest that plants in ley had their growth suppressed by competition until they were well established. It is interesting, though, that the 'stable' values in ley tended to be higher than those in wheat, particularly for the two semi-natural populations. This may be due to disturbance caused by cultivation in the wheat plots, or it may be a reaction to herbicide applied - - although the latter is unlikely to be a major factor, as a graminicide was only used in the second year of the experiment, with two applications of a very low dose (approximately one-sixth of a typical field rate). It may also be that stems or corms of greater width provide A. elatius with a competitive advantage in a ley habitat which is not so necessary for plants growing in a wheat crop.
4.3. Life-cycle differences The sequence of assessments in August 1991, March 1992 and June 1992, in a semi-natural habitat, highlighted an important life-cycle difference between non-bulbous and bulbous plants (Jenkin, 1931). Whereas bulbous plants stored reserves overwinter, which were largely not mobilised before March, non-bulbous plants continued growing throughout this period. This life-cycle difference has important implications for persistence and productivity of the different morphological types. Subspecies bulbosum is more likely to be adversely affected by competition from faster-growing herbs in semi-natural habitats, including the non-corm-forming ssp. elatius where this occurs. In arable habitats, annual cultivation and re-seeding of the land minimises this competitive disadvantage. There is also a positive effect of delay in corm sprouting in modem times, because it reduces the impact of foliar-acting herbicides, which are largely translocated into the corm from which the treated shoot derives (Tanphiphat and Appleby, 1990).
4.4. Response of the bulbous population of semi-natural origin The non-bulbous population and the arable bulbous population did not produce surprising results, in
light of their origins. The bulbous population of semi-natural origin did, by not responding in a manner consistently akin to the other bulbous population, or to the other population of semi-natural origin. The most consistent result arising from the semi-natural bulbous population was that it was least persistent and least productive of the three populations studied. This was not necessarily the result of an atypical population of low vigour having been selected, as other research has also revealed a relatively poor response from semi-natural bulbous plants (J.W. Cussans, personal communication, 1993). This leads to the question of why semi-natural bulbous plants are not outcompeted in their native habitats by the apparently more competitive growth forms of A. elatius. It is possible that climatic variation between the south-west and south-east of the UK may be a factor accounting for this response. Pfitzenmeyer (1959) suggested that semi-natural bulbous plants are better adapted to wetter areas. It is also possible that this population has other attributes contributing to its survival in its native habitat, such as heavy metal tolerance, or soil acidity, that were not investigated in this study. The slightly less bulbous mean phenotype in wheat than in ley for this population, compared with the more static phenotype of the arable bulbous population, suggests that there may be a greater degree of phenotypic plasticity in the semi-natural population. The direction of plasticity is inconsistent with Khan's hypothesis (Khan, 1987) that corm-formation imposes a fitness penalty with respect to competitivity. Were this the case, one would not expect plants to have a more bulbous phenotype in the highly competitive grass and clover ley than in the wheat, as was recorded. The different selection pressures acting on bulbous plants of arable and semi-natural origin must have influenced the degree of genetic separation between such populations. The former biotype will be subject to cultivation and herbicides, and the latter to defoliation (Miller et al., 1992).
4.5. Wider implications This study is obviously limited by the number of populations which could feasibly be studied. There is a possibility that the populations under consideration
A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
are atypical for their morphological category (see Section 4.4), although the allogamous nature of reproduction in the species results in a broad genetic base for individual populations. If the responses of the populations are assumed to be typical, then wider implications can be suggested. Cussans et al. (1993) suggested that arable A. elatius was more persistent than semi-natural forms. Our study does not dispute this hypothesis. However, the performance of the semi-natural bulbous form precludes a simple correlation between persistence and corm-formation. Cultivation increased ramet numbers in the nonbulbous population as well as in the bulbous populations. Why then is this form not apparent as an arable weed? Herbicides may play the major role in the absence of ssp. elatius from arable fields (Miller et al., 1992), or it may be that clumps of ssp. bulbosum may be damaged less by abrasion from cultivation of stony soils than non-bulbous ramets (Ingram, 1975). The potential for cultivation to disperse ramets of A. elatius was indicated dramatically by the increase in ramet numbers in the wheat plots. This is of particular significance in a species with a phalanx growth form (Lovett Doust, 1981), in which natural spread is in the immediate vicinity of the parent. Dispersal via farm machinery can spread weeds from one field or farm to the next (Schippers et al., 1993). In A. elatius there is evidence, using Random Amplified Polymorphic DNA (RAPD) markers on DNA of individual ramets, that cultivation may spread clones over considerable distances in the field (Miller et ai., 1995). The findings of this paper are relevant in the context of set-aside and fallow. This study has shown that onion couch has the potential to persist within a competitive grass and clover ley for several years, albeit with less vigour than its non-bulbous relative. This means that established weed infestations of onion couch, whilst not spreading rapidly in a grass and clover ley, will not be eradicated rapidly either. Hence the weed control potential of such a strategy (Jenkin, 1931) is not worthwhile in the short term. It is likewise interesting that ssp. elatius was very successful in an arable habitat, where herbicide use was kept to a minimum. The increase in ramet numbers of these non-bulbous plants following cultivation implies that farming systems employing a low
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input regime, for example organic farming, may have a potential weed problem in ssp. elatius as well as ssp. bulbosum.
5. Conclusions Morphologically distinct populations of Arrhenatherum elatius do not produce identical growth patterns in different habitats. One likely factor is their propensity for corm-formation, and the effect this has on the seasonality of plant growth. Bulbous A. darius, selected through agricultural practices, was most persistent, and was separated into the greatest number of viable ramets by cultivation. Non-bulbous A. elatius, derived from a lessmanaged habitat, was the most competitive of the three populations studied in a competitive grass and clover ley. Bulbous A. elatius, derived from a different semi-natural habitat, was least vigorous of the three populations, possibly through a differential climatic or edaphic tolerance. Cultivation fragments A. elatius plants, and can result in dispersal of ramets within a field. It did not kill established plants of ssp. elatius, which may therefore have the potential to become competitive weeds in low herbicide systems.
Acknowledgements This study was funded by the Joint Agriculture and the Environment Programme (JAEP). We would like to thank everyone who assisted with this study in any way, particularly those involved in the transplantation of seedlings into the field. Also, thanks to Alan Todd (Statistics Department, Rothamsted) for carrying out the Genstat analyses.
References Ayres, P., 1977.The growth of Arrhenatherum elatius var bulbosum (Willd.) Spenn. in spring barley,as influencedby cultivation. Weed Res., 17: 423-428. Chancellor, R.J. and Froud-Williams,R.J., 1983.Weeds of cereals in central southernEngland. Tenth Report 1982-1983, AFRC Weed Research Organization,Oxford,pp. 27-32. Cussans, J.W., Morton,A.J. and Khan, A.U., 1993. An ecological
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comparison of weed and non-weed forms of Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl. Proc. of the Brighton Crop Protection Conference on Weeds, 1993, pp. 515-522. Ducousso, A., Petit, D., Valero, M. and Vernet, P., 1990. Genetic variation between and within populations of a perennial grass: Arrhenatherum elatius. Heredity, 65: 179-188. Grime, J.P., Hodgson, J.G. and Hunt, R., 1988. Comparative Plant Ecology. A Functional Approach to Common British Species. Unwin Hyman, London, pp. 102-103. Hubbard, C.E., 1984. Grasses, 3rd edn. (revised by J.C.E. Hubbard). Penguin Books, Harmondsworth, UK, pp. 232-233. Ingrain, G.H., 1975. The distribution of perennial weed grasses in the arable regions of the United Kingdom. Proc. of the EWRS Syrup. on the Status and Control of Grassweeds in Europe, Pards, 1975, pp. l-8. Jenkin, T.J., 1931. Swollen stem internodes and other characters in Arrhenatherum Beauv. Bull. Welsh Plant Breeding Station, Series H, 12: 126-147. Khan, A.U., 1987. An agro-ecological study of Arrhenatherum elatius. Ph.D. Thesis, University of London. Khan, A.U., 1991. Agro-ecology of onion couch (Arrhenatherum elatius var. bulbosum). 1. Distribution of onion couch related to the source and soil texture. Ann. Appl. Biol., 118: 645-650. Lovett Doust, L., 1981. Population dynamics and local specialization in a clonal perennial (Ranunculus repens). I. The dynamics of ramets in contrasting habitats. J. Ecol., 69: 743-755. Mahmoud, A., Grime, J.P. and Furness, S.B., 1975. Polymorphism in Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl. New Phytol., 75: 269-276. Miller, A.C.E., 1994. Aspects of the survival and spread of Arrhenatherum elatius ssp. bulbosum. Ph.D. Thesis, University of Reading. Miller, A.C.E., Cussans, J.W., Cussans, G.W. and Morton, A.J., 1992. A preliminary study into the distribution patterns of different growth forms of Arrhenatherum elatius (L.) J. & C. Presl. Proc. of the lX~me Colloque International sur la Biologic des Mauvaises Herbes, Dijon, 1992, pp. 259-268.
Miller, A.C.E., Brookes, C.P., Loxdale, H.D. and Cussans, G.W., 1995. Use of the PCR-RAPD technique in a study on reproductive strategy of bulbous oatgrass (Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl subsp, bulbosum (Willd.) Schiib. & Mart.). WSSA Abstr., 35: 55. Milne, R., 1987. Putting the land out to grass. New Sci., 116: 10-11. Pfitzenmeyer, C.D.C., 1959, The autecology of Arrhenatherum elatius (L.) J. & C. Presl and its intergeneric relationships. M.Sc. Thesis, University of Wales. Pfitzenmeyer, C.D.C., 1962. Biological flora of the British Isles, No. 81. Arrhenatherum elatius (L.) J. & C. Presl. J. Ecol., 50: 235-245. Rees, L. and Sherrott, A.P., 1991. Repeated herbicide treatments for the long term control of Arrhenatherum elatius in winter cereals. Proc. of the Brighton Crop Protection Conference on Weeds, Brighton, 1991, BCPC, Farnham, pp. 937-944. Romero-Zarco, C., 1985. Revision del genero Arrhenatherum Beauv. (Gramineae) en la Peninsula lberica. Acta Bot. Malacitana, 10: 123-154. Schippers, P., ter Borg, S.J., van Groenendael, J.M. and Habekott~, B., 1993. What makes Cyperus esculentus (yellow nntsedge) an invasive species? A spatial model approach. Proc. of the Brighton Crop Protection Conference on Weeds, Brighton, BCPC, Farnham, 1993, pp. 495-504. Scragg, E.B. and Kilgour, D.W., 1984. Perennial grass weeds in barley and wheat in N.E. Scotland. Proc. Crop Protection in Northern Britain, Dundee, 1984, pp. 38-43. Sherrott, A. and Rees, L., 1992. Onion couch control. Farmers Weekly, 31 January 1992, Spring Herbicides Supplement, pp. 6-7. Tanphiphat, K. and Appleby, A.P., 1990. Absorption, translocation, and phytotoxicity of glyphosate in bulbous oatgrass ( Arrhenatherum elatius var. bulbosum). Weed Sci., 38: 480483. Underwood, L.M., 1912. A note on onion couch. J. Agric. Sci., 4: 270-272.
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Agriculture Ecosystems & Environment ELSEVIER
Agriculture, Ecosystems and Environment60 (1996) 107-120
The fate of morphologically different populations of Arrhenatherum elatius transplanted into arable and semi-natural habitats A.C.E. Miller a, *, G.W. Cussans a, R.J. Froud-Williams b a Crop and Disease Management Department, IACR-Rothamsted, Harpenden, AL5 2JQ, UK b Department of Agricultural Botany, University of Reading, Reading, RG6 6A U, UK Accepted 5 August 1996
Abstract Fates of three morphologically distinct populations of Arrhenatherum elatius were monitored following seedling transplantation into a winter wheat crop, or an Italian ryegrass (Lolium multiflorum) and red clover (Trifolium pratense) ley. Two populations were corm-forming, the third was not. One corm-forming population was an arable weed, the other two populations were collected from semi-natural grassland. Survival within all populations of A. elatius was greater in wheat than grass and clover ley. Ramet numbers of all A. elatius populations increased following cultivation of the wheat plots. Productivity, measured in terms of tiller number and plant biomass, partially reflected site of origin. Non-corm-forming plants were most productive in the grass and clover ley habitat, and plants of arable origin were most productive in the wheat habitat. Mean productivity of corm-forming plants of semi-natural origin was least in both habitats. Arable bulbous plants produced more corms than semi-natural plants. Productivity was generally greater in wheat than grass and clover ley, when comparisons could be made. Non-corm-forming plants, by definition, had lower maximum stem diameters than those which formed corms. The maximum diameter of corms from the population of arable origin was significantly greater than that of the corm-forming population of semi-natural origin in ley, and tended to be greater in wheat. Phenotypic categorisation highlighted differences between the non-corm-forming population and the corm-forming populations. Mean phenotype of non-corm-forming plants in grass and clover ley plots changed over time, plant stems becoming marginally thicker. The two corm-forming populations could not be distinguished by phenotype in ley plots, although in wheat there was a tendency for plants of arable origin to have a more bulbous phenotype.
* Correspondingauthor. Tel.: 01582 763133 x 2761; fax: 01582 760981; e-mail: [email protected]. 0167-8809/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 67-8809(96)01088-2
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Corm-forming plants delayed regrowth from corm reserves until late spring, whereas non-corm-forming plants grew throughout the winter. This life-cycle difference may explain their different niches. Keywords: Corms; Onion couch; Bulbous; Regrowth; Cultivation; Ley; Wheat
1. Introduction Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl, 'false oat-grass', is a perennial species common in rough grasslands throughout the UK. It is polymorphic for a range of morphological characters such as node hairiness and colour (Jenkin, 1931; Ducousso et al., 1990), and erectness of stem (Mahmoud et al., 1975), but the most noted variation is in its capacity for corm-formation (e.g. Jenkin, 1931; Pfitzenmeyer, 1962; Hubbard, 1984; Khan, 1991). European populations of A. elatius have been categorised into four 'subspecies' (Romero-Zarco, 1985). Two Mediterranean subspecies, sardoum and baen'cum, exist as diploids, although tetraploid populations of ssp. baeticum are also found. The remaining two subspecies, elatius and bulbosum, are tetraploid. Subspecies sardoum and elatius are noncorm-forming, whereas baeticum and bulbosum do produce corms. Only the tetraploid subspecies elatius and bulbosum are known in the UK (e.g. Grime et al., 1988). Genetic exchange between these two extreme growth forms of A. elatius results in a spectrum of intermediate forms (Jenkin, 1931). In the field, seed from a large bulbous population which is not in close proximity to non-bulbous plants will tend to give rise to highly bulbous plants (Khan, 1987). Thus, the propensity for corm-formation appears to have a genetic basis, although corm size may be modified by soil type (Underwood, 1912) and plant density (Miller, 1994). Niche differentiation between these two extreme growth forms is not clear-cut. Subspecies bulbosum alone is found as a weed in arable systems, where it is commonly known as 'onion couch'. Both cormforming and non-corm-forming types of A. elatius are found in semi-natural grassland, ssp. bulbosum predominantly in the west of England, Wales and Scotland, and ssp. elatius in the east of England (Miller et al., 1992; Cussans et al., 1993). Various
factors may influence subspecific distributions. It has been suggested that ssp. bulbosum may be more suited to wetter climatic regions with mild winters (Pfitzenmeyer, 1959), also that ssp. elatius may be better adapted to less fertile soils (Grime et al., 1988). Khan (1987) suggested that corm formation imposes a fitness penalty within this species. Onion couch is a weed that has been favoured by practices of both minimal tillage, and continuous winter cereal cropping (e.g. Sherrott and Rees, 1992). Whilst it is not usually considered a common weed, surveys of UK regions suggest that 1-5% of cereal fields may be infested (Chancellor and Froud-Williams, 1983; Scragg and Kilgour, 1984). Yield loss from onion couch can be in excess of 60% (Rees and Sherrott, 1991). Chemical control measures for this subspecies in winter cereals often seem to have limited efficiency due to a low ratio of foliage area to corm number (Ayres, 1977; Tanphipbat and Appleby, 1990). An earlier control method suggested by Jenkin (1931) was the practice of laying the land 'down to grass for a number of years'. The rationale for this was that there was little regrowth of ssp. bulbosum plants between autumn and spring, and hence other wintergrowing plants would outcompete them. However, Jenkin proposed that plants intermediate between the extremes (i.e. between corm-forming and non-cormforming) might be most persistent. Onion couch problems may develop following the implementation of the agricultural policy of 'setaside' (Milne, 1987). Set-aside is a practice in which farmers are paid to keep a proportion of their land out of production for one or more years, with only a very restricted use of herbicides permitted. If onion couch, or persistent intermediate plants derived from a cross between non-bulbous false oat-grass and bulbous onion couch, became established in set-aside fields, there could be a potentially serious weed problem on reversion to arable cropping. This paper assesses persistence and morphology of plants from three different populations of Arrhen-
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atherum elatius, following transplantation into either winter wheat, or an Italian ryegrass and red clover ley. The populations chosen consisted of morphologically typical examples of three broad categories of A. elatius population (Miller et al., 1992): arable bulbous ('onion couch'), semi-natural bulbous, and semi-natural non-bulbous.
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selected populations by tapping inflorescences to release ripe seeds into a bowl. Arrhenatherum elatius populations chosen were as follows: I. plants with no corms ('non-bulbous') from a derelict allotment (i.e. semi-natural habitat) in Earley, Berkshire; 2. plants with a few small corms per stem ('bulbous') from a roadside verge (i.e. semi-natural habitat) in Goonhavern, Cornwall; 3. plants with several, often relatively large, corms per stem ('bulbous') from a dense onion couch infestation in a winter wheat field (i.e. arable habitat) in Broad Chalke, Wiltshire. Arrhenatherum elatius seeds were sown into seed trays of compost on 1 and 2 October 1990. Single seedlings were pricked out into polystyrene modules between 11 and 15 October 1990. This was to reduce the effects of intraspecific competition and
2. Materials and methods 2.1. Plant material
Plant material derived from seed was used for this study, to represent the potential range of genotypes in the gene pool, rather than just the 'successful' survivors. Seeds of A. elatius were collected in summer 1990 from a number of randomly chosen plants in
Table 1 Calendar of field operations in a study of the fate of morphologically different populations of Arrhenatherum elatius transplanted into a winter wheat and a grass/clover ley habitat Date
Grass and clover ley
27/9/1990
l / 1 1 / 1 9 9 0 - 7 / 1 l/1990
A. elatius seedlings transplanted into field
12/3/1991 5/4/199 I 18/3/1991-18/4/1991 20/7/1991-13/8/1991 l 1/9/1991-13/9/1991 2/10/1991
Post-transplantation count I st destructive harvest
Bromoxynil/ioxynil/mecoprop (Swipe, Ciba-Geigy) applied to kill annual dicots Nitram fertiliser (34.5% N) applied @ 580 kg haPost-transplantation count 1st destructive harvest Combine harvested and straw removed
Tine cultivation ( × 2); rotorgrubbed to maximum cultivation depth of 10 cm; 'Mercia' sown @ 162 kg ha- 1 Low dose (0.19 kg a.i. ha- t ) diclofop-methyl (Hoegrass, Hoechst) applied for volunteer Italian ryegrass
25/2/1992
10/3/1992-18/3/1992 3 / 4/1992 9/4/1992
2nd destructive harvest
4/6/1992-24/6/1992 30/6/1992
3rd destructive harvest Cut to 10 cm height and produce removed
22/6/1993-15/7/1993
Winter wheat cv. 'Mercia' sown @ 166 kg ha- ~, into areas within an established ley, previously cleared with 360 g a.i. ha ~ glyphosate (Roundup) followed by cultivation A. elatius seedlings transplanted into field
Cut to 10 cm height and produce removed
11/10/1991
3/8/1992-3/9/1992 16/9/1992
Wheat
Nitram fertiliser (34.5% N) applied @ 580 kg ha- t Low dose (0.19 kg a.i. ha- ~) diclofop-methyl (Hoegrass, Hoechst) applied for volunteer Italian ryegrass
2nd destructive harvest Cut to 10 cm height and produce removed 4th destructive harvest
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transplant shock, and to facilitate transplantation into the field. After initial establishment in an unheated glasshouse, seedlings were hardened off outside for a few days. Seedlings were transplanted into the field between 1 and 7 November 1990, when the largest were beginning to tiller.
2.2. Experimental design The experiment was sited at IACR-Rothamsted, in an established ley comprising Italian ryegrass (Lolium multiflorum) and red clover (Trifolium pratense) (ley plots). A calendar of operations on this experiment is given in Table 1. Each of the three populations of A. elatius were transplanted into blocks consisting of five adjacent 2.5 m × 1.25 m subplots, in adjoining wheat and ley plots. There were four replicates of the two habitats. The 2.5 m X 1.25 m subplots were divided into 25 cm × 25 cm squares, using a 1.25 m × 1.25 m quadrat positioned over each half of the subplot in turn. One A. elatius seedling was planted in the centre of each square, next to a white plastic label. Hence each subplot contained 50 A. elatius seedlings.
2.4. Measurements 2.4.1. Survival Original planting sites, marked with plastic labels, were located, and presence/absence of each original A. elatius transplant was recorded. The initial posttransplantation count was a non-destructive assessment of the whole experiment. Subsequent plant counts were made during the destructive harvest of single subplots, with squares being excavated for below-ground material if no growth was visible above-ground. Survival figures are given as a percentage of the original number of transplants, since establishment was nearly 100% in each population at the post-transplantation count. 2.4.2. Tiller number per plant Counts were made of total tiller number and number of mature tillers with developed leaves (or at least 10 cm in length, depending on development of plants at harvest). In the June 1993 harvest, only mature tillers were counted. 2.4.3. Maximum stem diameter The diameter at the widest part of the corm or stem was measured to the nearest millimetre. March
2.3. Assessments Original plants were located by their proximity to the plastic labels, in each of the ley subplot assessments, and in the first destructive assessment of the wheat plots. Four of the five ley subplots within each plot had been harvested by the termination of the experiment. For the first destructive assessment of the wheat plots, the fifth subplot was systematically harvested. This was to increase the width of pathways between plots of different biotypes, to reduce risk of contamination during cultivation. Following cultivation, it was clearly inappropriate to search for original subplots. Instead, strips of twenty 25 cm × 25 cm squares running the length of the wheat plots were harvested. All measurements, with the exception of survival, were taken from the central 24 plants from subplots within each of the four replicates. Substitutions from surrounding transplants were used where central plants were deceased, to retain a total of 24 plants in the assessments. Survival assessments were made on complete subplots of 50 original plants.
1
2
4
5
t 3
6
Fig. 1. Typical stems of Arrhenatherum elatius plants in six phenotypic categories (after Miller, 1994). 1, non-bulbous; 2, non-bulbous/intermediate; 3, intermediate/non-bulbous; 4, intermediate/bulbous; 5, bulbous/intermediate; 6, bulbous.
A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120 1992 values reflect the diameter o f corms produced during the previous season, n e w ones not b e i n g fully developed at this harvest.
2.4.4. N u m b e r o f corms Eight categories of internode shape can be broadly defined in A. elatius, as points in a c o n t i n u u m (Miller, 1994). F o r this study, these were c o n d e n s e d into two groups: 1. club-shaped corms, swollen at the base then tapering toward the higher node, with a c o r m height:corm width ratio o f at least 2; 2. f l a t / r o u n d e d corms, with n o tapering, and a corm height:corm width ratio o f less than 2. March 1992 values reflect the n u m b e r o f corms produced d u r i n g the previous season, n e w ones not being fully d e v e l o p e d at this harvest. 2.4.5. Dry weight Plants were o v e n - d r i e d at 80°C for approximately 16 h, then w e i g h e d in grams to an accuracy of two decimal places. In the June 1993 grass and clover ley plot assessment, samples were separated into swollen basal parts, and stem plus leaves.
111
2.4.6. Phenotypic categories Six subjective categories of bulbosity were defined, based on the n u m b e r and shape o f basal i n t e m o d e s on a plant. These were n u m b e r e d 1 ( n o n b u l b o u s ) through to 6 (extremely bulbous) (Fig. 1). All small plants with one or two flattened or r o u n d e d corms per stem were placed in category 6, since it was n o t possible to distinguish these from category 5 plants at that stage. Data were analysed both as six categories ( 1 - 6 ) and as five categories, with the original categories 5 and 6 merged ( 1 - 5 ) . This was to see whether the inclusion of small plants, which were frequently from the b u l b o u s population o f semi-natural origin, biased data interpretation.
2.5. Data analysis A n a l y s e s o f variance were performed on m e a n plot data, using Genstat 5.0. Data were log~ transf o r m e d (i.e. log¢ ( x + 0.5)) to normalise data where appropriate. Standard errors of the difference between two m e a n s (SEDs) are given, along with P values.
Table 2 The effect of habitat type on survival and growth form of Arrhenatherum elatius. Survival and growth form values are calculated from plot means of three A. elatius populations in each of two habitats (Italian ryegrass and clover Icy, and winter wheat). All values are given for the August 1991 harvest only, with the exception of Survival, for which the April 1991 assessment could be included. Significance levels, and SEDs are given for Habitat and the Habitat x Population interaction. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses. Maximum standard errors of differences between means are given for interaction terms Habitat Survival (%) April 1991 Aug. 1991 Total tiller number (per plant) Mature tiller number (per plant) Max. stem diameter (per plant) Corm no. (flat) (per plant) Corm no. (club) (per plant) Dry weight c (per plant) Phenotypic categories (1-6) (i) All populations (ii) Bulbous populations only
Sig. level a
SED b
Sig. level a
SED b
Ley
Wheat
(Habitat)
(Habitat)
(Habitat × Pop.)
(Habitat × Pop.)
99.1 93.5 0.95 (2.1) 0.74 (1.6) 1.69 (4.9) 1.11 (2.5) 0.04 (0.5) 5.67 (0.3)
99.3 99.0 2.80 (15.9) 2.45 (11.1) 1.93 (6.4) 2.87 (17.1) 1.87 (6.0) 9.25 (10.4)
NS * *** *** ** *** *** ***
1.07 0.11 0.05 0.02 0.06 0.07 0.15
NS NS NS NS ** ** NS NS
4.30 5.92
4.13 5.69
NS, P > 0.05; * P _ < 0 . 0 5 ; * * P ' ; 0 . 0 1 ; * * * P < 0 . 0 0 1 . b SED, standard error of the difference between two means. c Log e values and SEDs on data in milligrams.
a
NS NS
** *
0.05 0.11
0.097 0.111
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A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
W h e a t a n d ley p l o t r e s u l t s f r o m A u g u s t 1991 are c o m p a r e d ( T a b l e 2). H o w e v e r , o w i n g to t h e d i f f e r e n c e in h a r v e s t d a t e b e t w e e n w h e a t a n d l e y p l o t s in
the second year of the study, and because of the effect of cultivation on the nature of the later wheat p l o t d a t a , it w a s n o t c o n s i d e r e d v a l i d to u n d e r t a k e
Table 3 Survival and growth form data for morphologically different populations of Arrhenatherum elatius transplanted into grass and clover ley plots. Means for survival and growth form data from three populations of A. elatius transplanted into a grass/clover Icy are given. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses. Maximum standard errors of differences between means are given for interaction terms. Population of Arrhenatherum elatius
Survival (%) Apr. 1991 Aug. 1991 Mar. 1992 June 1992 June 1993 Mean (Pop.) Slope
Non-bulbous
Arable bulbous
Semi-natural bulbous
99.0 92.5 87.5 80.0 79.5 87.7 - 0.75
99.5 96.0 96.5 92.5 89.0 94.7 - 0.38
98.7 92.0 82.0 71.0 62.0 81.1 - 1.45
Total tiller number Aug. 1991 Mar. 1992 June 1992 Mean (Pop.)
(per plant) 1.27 (3.1) 3.87 (47.5) 2.89 (17.6) 2.68 (14.1)
Sig. level a
SED b
Sig. level a
SED b
(Pop.)
(Pop.)
(Pop. X Time)
(Pop. X Time)
**
2.6
*
0.25
*
5.3
0.97 (2.1) 2.69 (14.2) 1.85 (5.9) 1.84 (5.8)
0.61 (1.4) 2.37 (10.2) 1.60 (4.4) 1.53 (4.1)
***
0.14
***
0.17
Mature tiller number (per plant) Aug. 1991 0.82 (1.8) Mar. 1992 3.62 (37.0) June 1992 2.89 (17.4) June 1993 3.55 (34.3) Mean (Pop.) 2.72 (14.7)
0.86 2.40 1.81 2.26 1.83
0.56 (1.3) 2.04 (7.2) 1.60 (4.4) 1.52 (4.1) 1.43 (3.7)
***
0.10
***
0.19
Maximum stem diameter (per plant) Aug. 1991 0.80 (1.7)
2.18 (8.3)
2.10 (7.7)
* * * (AP); * * (BP)
0.02 (AP); * * * (AP); 0.01 (BP) NS (BP)
0.05 (AP)
Mar. 1992 June 1992 June 1993 Mean (Pop.)
2.20 2.39 2.39 2.29
(8.5) (10.4) (10.4) (9.4)
2.08 (7.5) 2.36(10.1) 2.35 (9.9) 2.22 (8.7)
Corm number (fiat) (per plant) Aug. 1991 Mar. 1992 June 1992 June 1993 Mean (Pop.) -
1.43 (3.7) 1.49 (3.9) 3.17 (23.3) 4.26 (70.6) 2.59 (12.8)
0.79 (1.7) 0.64 (1.4) 2.40 (10.5) 2.86 (16.9) 1.67 (4.8)
0.10
NS (p = 0.051)
0.19
Corm number (club) (per piano Aug. 1991 June 1992 June 1993 Mean (Pop.) -
0.12 1.67 2.57 1.45
0.18
NS (p = 0.059)
0.26
1.10 (2.5) 1.46 (3.8) 1.48 (3.9) 1.21 (2.9)
(1.9) (10.5) (5.6) (9.1) (5.7)
(0.6) (4.8) (12.6) (3.8)
- 0.05 (0.5) 1.14 (2.6) 1.51 (4.0) 0.87 (1.9)
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113
Table 3 (continued) Population of Arrhenatherum elatius
Sig. level a
SED b
Sig. level a
SED b
Non-bulbous
(Pop. × Time)
(Pop. × Time)
Dry weight ¢ (per piano Aug. 199l - 0 . 0 8 (0.4) Mar. 1992 0.84 (1.8) June 1992 3.08 (21.2) June 1993 3.72 (40.7) Mean (Pop.) 1.89 (6.1) Corm weight (% of total) June 1993 -
Arable bulbous
Semi-natural bulbous
(Pop.)
(Pop.)
- 0 . 1 3 (0.4) 0.05 (0.6) 1.76 (5.3) 2.42 (10.8) 1.03 (2,3)
- 0 . 4 0 (0.2) - 0.28 (0.3) 1.20 (2.8) 1.43 (3.7) 0.49 (1.1)
***
0.12
*
3.04
* * * (AP); NS (BP)
0.038 (AP)
78.4
62.7
Phenotypic categories (1-6) Aug. 1991 1.07
5.88
5.96
Mar. 1992 June 1992 June 1993 Mean (Pop.)
5.94 5.91 5.91 5.91
5.83 5.97 5.92 5.92
1.05 1.06 1.32 1.13
***
* * (AP); NS (BP)
0.20
0.062 (AP)
NS, P > 0.05; * P < 0.05; * * P < 0.01; * * * P_< 0.001. b SED, standard error of the difference between two means. c Transformed and back-transformed values given in grams. AP, all populations analysed; BP, bulbous populations only analysed. a
analyses combining wheat and ley plot data after the first harvest, and trends are discussed instead. The ley plot harvest data from each assessment date were combined for the analyses (Table 3). For ley plot survival data, a regression was done for each plot over time, and analysis of variance was carried out on the slopes. Separate analyses are given for wheat plot data on each harvest date (Table 4).
3. Results
The first year's results have been documented previously in Miller et al. (1992), but are re-stated here to give complete experimental results.
(P < 0.05). Whilst plant numbers in ley plots continued to steadily decrease, cultivation of wheat plots broke up plants, and hence increased ramet numbers considerably. 3.1.2. Grass and clover ley plots The number of original plants recovered from ley plots decreased with time ( P < 0 . 0 0 1 ) (Table 3). There was less mortality amongst bulbous plants of arable origin than within populations of semi-natural origin, either bulbous or non-bulbous ( P < 0 . 0 1 ) . Rate of mortality varied amongst populations (P < 0.05), with most rapid decline in the bulbous population of semi-natural origin. Even in this population, though, more than 60% of plants had survived into the third year after transplantation.
3.1. Survival 3.1.1. Grass and clover ley plot vs. wheat plot trends Initial counts (April 1991) showed that establishment was similar in both ley and wheat (Table 2). By August 1991, survival was lower in ley than wheat
3.1.3. Wheat plots There was no interpopulation difference in survival by the spring after transplantation (Table 4). By the summer, greatest mortality was observed in the bulbous population of semi-natural origin. Cultiva-
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114
tion subsequently increased ramet numbers in all populations to over twice the original planted number in both the bulbous population of arable origin
and the non-bulbous population. However, owing to a high degree of variability, a significant interpopulation difference was not detected.
Table 4 Survival and growth form data for morphologically different populations of Arrhenatherum elatius transplanted into wheat plots. Means for survival and growth form data from three populations of A. elatius transplanted into winter wheat are given. Where a log e transformation was used for an analysis, transformed mean values and standard errors of differences between means are shown, with back-transformed means given in parentheses Sig. level a
Population of Arrhenatherum elatius
Survival (%) Apr. 1991 Aug. 1991 Aug. 1992 Total Aug. Aug. Aug.
Non-bulbous
Arable bulbous
Semi-natural bulbous
99.7 100.0 212.5
99.7 99.5 268.7
98.5 97.5 136.3
tiller number (per plant) 1991 3.03 (20.2) 1992 (r) 3.10 (21.7) 1992 (s) 3.80 (44.4)
SED b
NS ** NS ( P = 0.055)
0.4 42.6
3.03 (20.2) 3.33 (27.4) 4.30 (72.8)
2.34 (9.9) 2.87 (17.1) 3.08 (21.3)
*** * **
0.11 0.11 0.24
2.79 (15.9) 3.04 (20.4) 4.00 (54.3)
2.09 (7.6) 2.42 (10.8) 2.64 (13.6)
*** ** **
0.08 0.09 0.27
Maximum stem diameter (per plant) Aug. 1991 1.21 (2.9) Aug. 1992 (r) 1.16 (2.7)
2.37 (10.2) 2.38 (10.3)
2.22 (8.7) 2.25 (9.0)
* * * (AP); NS ( P = 0.065)(BP) * * * (AP); NS ( P = 0.094) (BP)
0.05 (AP); 0.05 (8P) 0.04 (AP); 0.05 (BP)
Corm Aug. Aug. Aug.
number (flat) 1991 1992 (r) 1992 (s)
3.59 (35.6) 4.09 (59.3) 5.06 (157.7)
2.15 (8.1) 2.82 (16.3) 3.04 (20.5)
Corm Aug. Aug. Aug.
number (club) 1991 1992 (r) 1992 (s)
2.11 (7.1) 2.51 (11.7) 3.46 (31.2)
Dry weight c (per plant) Aug. 1991 9.34(11.4) Aug. 1992 (r) 1.49 (3.9) Aug. 1992 (s) 2.15 (8.0) Phenotypic categories (1-6) Aug. 1991 1.01 Aug. 1992 (r) 1.20 Aug. 1992 (s) 1.15
Mature tiller number Aug. 1991 Aug. 1992 (r) Aug. 1992 (s)
(per plant) 2.47 (11.3) 2.73 (14.8) 3.43 (30.3)
(per plant) (per plant) -
* *
0.13 0.28 0.42
1.63 (4.6) 1.81 (5.6) 2.03 (7.1)
* * *
0.09 0.16 0.34
9.50(13.4) 2.29 (9.4) 3.24 (24.9)
8.89(7.3) 1.54 (4.2) 1.77 (5.4)
* ** *
0.17 0.15 0.33
5.87 5.87 5.87
5.51 5.36 5.50
* * * (AP); * (BP) * * * (AP); NS ( P = 0.10) (BP) * * * (AP); NS ( P = 0.10) (BP)
0.10 (AP); 0.09 (BP) 0.18 (AP); 0.22 (BP) 0.13 (AP); 0.16 (BP)
a NS, P > 0.05; * P_< 0.05; * * P_< 0.01; * * * P_< 0.001. b SED, standard error of the difference between two means. c Log e values and SEDs on data in milligrams. AP, all populations analysed; BP, bulbous populations only analysed. r, data per ramet; s, data per 25 cm × 25 cm square.
*
*
A.C.E. Miller et al./ Agriculture, Ecosystems and Environment 60 (1996) 107-120
3.2. Tiller number 3.2.1. Grass and clover Icy plot vs. wheat plot trends Tiller number of A. elatius was considerably less ( P < 0.001) in Icy plots than wheat plots in August 1991 (Table 2). This trend was also true for August 1992, when comparing equal areas (i.e. August 1992 'square' not 'ramet' data). A higher percentage of tillers tended to be mature in ley than in wheat in both August 1991 and June/August 1992. 3.2.2. Grass and clover Icy plots Total tiller number varied with time ( P < 0.001) (Table 3). However, this was not purely a simple increase in numbers, with a reduction in total tiller numbers of about two-fifths between March 1992 and the following June. The percentage of tillers that were mature was significantly higher ( P < 0.001) in June 1992 than in March. This was largely attributed to growth stage, as the latter harvest corresponds with time of flowering. In both the bulbous population of arable origin and the non-bulbous population, mature tiller number increased greatly between June 1992 and June 1993. The populations displayed significantly different ( P < 0.001) numbers of tillers, both total and mature only, the non-bulbous population having three times as many as the arable bulbous population by the second summer after transplantation. The percentage of mature tillers was not significantly different between populations, and relative rankings were not consistent for this parameter. Significant ( P < 0.001 ) population X time interactions were noted both for total and mature tiller numbers. In both cases, the more rapid increase in the non-bulbous population between August 1991 and March 1992 will have contributed to this difference. A fall in numbers of mature tillers between June 1992 and June 1993 in the bulbous population of semi-natural origin will also have contributed to a significant interaction. 3.2.3. Wheat plots There was a significantly different number of tillers (both total and mature only) between populations on both harvest dates (Table 4). Analysis based on data per square (25 cm X 25 cm) compared with that on data per ramet made little difference to
115
interpretation. The bulbous population of arable origin produced most tillers, both total and mature only, and the highest percentage mature tillers. The bulbous population of semi-natural origin produced least tillers, and had the lowest percentage of mature tillers.
3.3. Maximum stem diameter 3.3.1. Grass and clover ley plot vs. wheat plot trends Maximum stem diameters were greater in wheat than ley in August 1991 ( P < 0.01) (Table 2). There was a population x habitat interaction ( P < 0.01), the difference in values between wheat and ley values for the non-bulbous population being proportionately greater than for the two bulbous populations. By the second summer after transplantation, mean values in ley tended to be greater than those in wheat. Cultivation of wheat plots did not appear to affect maximum stem diameter of A. elatius, values being similar in both years of the experiment. However, values in ley plots did increase after the first season, implying that competition had prevented young plants from producing such substantial tillers. Once nonbulbous plants had become established in ley, they actually attained greater maximum stem diameters than in wheat, whereas bulbous plants produced broadly similar values in both habitats. 3.3.2. Grass and clover ley plots Maximum stem diameter was lower ( P < 0.001) in the first summer after transplantation than in the second and third summers, in each of the three populations (Table 3). Note that the data from March 1992 refer to corms remaining from the previous summer--new ones had not fully developed by this harvest date. There was clearly a significant difference between populations ( P < 0.001), with the non-bulbous population, rather by definition, having much lower values than the two bulbous populations. However, the maximum diameter of corms from the population of arable origin was also significantly higher than that of the semi-natural bulbous population. Change in maximum stem diameter with time was at a different relative rate in the non-bulbous population than in either of the two bulbous populations ( P < 0.001).
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3.3.3. Wheat plots Again, non-bulbous plants clearly had lower maximum stem diameters than bulbous ones ( P < 0.001) (Table 4). Whilst there was a trend for arable bulbous plants to have a greater diameter than semi-natural bulbous plants, this difference was not significant due to size of standard error. 3.4. Corm number per plant 3.4.1. Grass and clover ley plot vs. wheat plot trends Corm number per plant was greater in wheat than ley, both in August 1991 ( P < 0.001) and August 1992 (Table 2). A population x habitat interaction was observed for flat corms in August 1991 ( P < 0.01), with the ratio of the number in wheat to the number in ley being twice as great for the arable bulbous than the semi-natural bulbous population. Percentage flat corms tended to be lower in wheat than ley. 3.4.2. Grass and clover ley plots Number of corms (fiat and total) increased significantly with time ( P < 0.001) (Table 3). The bulbous population of arable origin always had more corms per plant than the one of semi-natural origin ( P < 0.01). The population X time interaction approached the 5% level of significance ( P = 0.051) for number of flat corms per plant i.e. there was a trend for more rapid accumulation of corms in the arable bulbous population. The percentage of flat corms was not significantly different over time, but was significantly greater in arable bulbous plants than semi-natural bulbous plants ( P < 0.05). 3.4.3. Wheat plots There were significantly greater numbers of corms on arable bulbous plants than semi-natural bulbous ones (flat and total; August 1991 P <0.01 and August 1992 P < 0.05) (Table 4). Percentage of flat corms was significantly greater in bulbous plants of arable origin than semi-natural origin (August 1991 P < 0.01 and August 1992 P < 0.05, analyses not presented). 3.5. Dry weight 3.5.1. Grass and clover ley plot vs. wheat plot trends Dry weight per plant was much greater in wheat than ley in August 1991 ( P < 0.001) (Table 2). No
population X habitat interactions were found for this harvest. However, by August 1992, there appeared to be an interaction with the environment, resulting in a difference in the relative productivity of the three populations (ley: non-bulbous > arable bulbous > semi-natural bulbous; wheat (s): arable bulbous > non-bulbous > semi-natural bulbous; wheat (r): arable bulbous > semi-natural bulbous > nonbulbous).
3.5.2. Grass and clover ley plots Dry weight per plant increased over time ( P < 0.001) (Table 3). There was a significant interpopulational difference ( P < 0 . 0 0 1 ) , with non-bulbous plants consistently producing more dry matter than bulbous plants. Arable bulbous plants produced more than semi-natural bulbous plants. A significant ( P < 0.001) population X time interaction was recorded, with initial increase in mass of non-bulbous plants being relatively greater than that of bulbous plants. Dry weight accumulation in bulbous plants of seminatural origin was relatively slow. The two bulbous populations had allocated different proportions of their biomass to corms at the June 1993 assessment ( P < 0.05), with nearly 80% mass in corms in the arable population, and under 65% in the semi-natural population. 3.5.3. Wheat plots Dry weight was significantly different between populations ( P < 0.05) both in August 1991, and when August 1992 data per 'square' were analysed (Table 4). The bulbous population of arable origin produced most dry matter, and the one of semi-natural origin produced least. When ramet data were compared for August 1992, there was still a significant difference ( P < 0.01), but relative rankings had changed, with the nonbulbous plants producing least dry matter per ramet. 3.6. Phenotypic categories 3.6.1. Grass and clover ley plot vs. wheat plot trends No significant differences were found between wheat and Icy plots in August 1991 (Table 2). However, a population x habitat interaction was found, both for all populations ( P < 0.01), and for the two bulbous populations ( P < 0.05). This was because
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plants from the semi-natural bulbous population were slightly less bulbous in wheat than in ley. This trend was also seen in the later harvest.
3.6.2. Grass and clover ley plots There was a significant difference between phenotypic categories over time when all three populations were assessed, but not when comparing the two bulbous populations alone (Table 3). A similar observation was made about the population × time interaction. These results would appear to derive from the change from a static mean phenotype within the non-bulbous population over the first 2.5 years of the experiment, which then increased slightly in bulbosity the following year. The mean phenotype of the non-bulbous population was clearly at the opposite end of the spectrum from those of the two bulbous populations, as would be expected from the original definition of the populations. Combining categories 5 and 6 affected significance levels, but not the interpretation of results (analysis not shown). 3.6.3. Wheat plots There was a clear difference between mean phenotype of the non-bulbous and bulbous populations (P < 0.001) (Table 4). In the first season, the arable bulbous plants had a slightly more bulbous mean phenotype than the semi-natural bulbous plants (P < 0.05). This remained a trend in the second year, although it was not statistically significant (P--0.100). Combining categories 5 and 6 made the difference between arable and semi-natural bulbous populations non-significant in the first summer after transplantation (analysis not shown).
4. Discussion
4.1. Persistence and productivity The three populations of A. elatius behaved differently from each other, and in the two habitats. In terms of plant numbers, the arable bulbous population was most persistent in both habitats, followed by the non-bulbous population. This result agrees with
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Cussans et al. (1993), who suggested that the weed strategy of onion couch was based on extreme persistence, rather than invasiveness or competitiveness. In the arable habitat, this persistence strategy was emphasised by the success of ramet establishment following stem separation by cultivation. Although each population showed a gradual decline in numbers in the grass and clover ley habitat, most of the transplants survived into the third year after transplantation. In contrast, a comparison of plant tiller and biomass production gives a different angle to the data interpretation. In the grass and clover ley plots, it is the non-bulbous plants that have produced by far the greatest number of tillers and biomass. Rather surprisingly, the bulbous population of semi-natural origin was less productive in the ley habitat than the population of arable origin. In the wheat plots, the arable bulbous population was most productive, but once again the semi-natural bulbous population was least productive. In the case of the non-bulbous and arable bulbous populations, then, relative performance was exactly as might be expected from a knowledge of their original habitats (e.g. Grime et al., 1988). However, the relative growth and persistence of the semi-natural bulbous plants, which had the highest mortality and lowest productivity in both habitats, did not fit into the expected pattern.
4.2. Phenotypic categories and morphology In most instances, there was a big difference in phenotype between plants originating from a nonbulbous population and those from bulbous populations. The only time that plants in any of the three most bulbous phenotypic categories were found in a non-bulbous plot was in August 1992 following cultivation of wheat plots. Owing to the small number of plants involved, and in light of their complete absence in other harvests, this occurrence was most likely attributable to movement of ramets during cultivation. The amalgamation of categories 5 and 6 in the analyses, to check the potential for the automatic placement of small plants into category 6 to bias differences between the generally larger arable bulbous plants and the smaller semi-natural ones, revealed that this did not have much effect.
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The maximum stem diameters of A. elatius plants grown in wheat were stable over the 2 years assessed. However, those grown in ley were initially lower than those grown in wheat, increasing between the first and second years before stabilising. This would suggest that plants in ley had their growth suppressed by competition until they were well established. It is interesting, though, that the 'stable' values in ley tended to be higher than those in wheat, particularly for the two semi-natural populations. This may be due to disturbance caused by cultivation in the wheat plots, or it may be a reaction to herbicide applied - - although the latter is unlikely to be a major factor, as a graminicide was only used in the second year of the experiment, with two applications of a very low dose (approximately one-sixth of a typical field rate). It may also be that stems or corms of greater width provide A. elatius with a competitive advantage in a ley habitat which is not so necessary for plants growing in a wheat crop.
4.3. Life-cycle differences The sequence of assessments in August 1991, March 1992 and June 1992, in a semi-natural habitat, highlighted an important life-cycle difference between non-bulbous and bulbous plants (Jenkin, 1931). Whereas bulbous plants stored reserves overwinter, which were largely not mobilised before March, non-bulbous plants continued growing throughout this period. This life-cycle difference has important implications for persistence and productivity of the different morphological types. Subspecies bulbosum is more likely to be adversely affected by competition from faster-growing herbs in semi-natural habitats, including the non-corm-forming ssp. elatius where this occurs. In arable habitats, annual cultivation and re-seeding of the land minimises this competitive disadvantage. There is also a positive effect of delay in corm sprouting in modem times, because it reduces the impact of foliar-acting herbicides, which are largely translocated into the corm from which the treated shoot derives (Tanphiphat and Appleby, 1990).
4.4. Response of the bulbous population of semi-natural origin The non-bulbous population and the arable bulbous population did not produce surprising results, in
light of their origins. The bulbous population of semi-natural origin did, by not responding in a manner consistently akin to the other bulbous population, or to the other population of semi-natural origin. The most consistent result arising from the semi-natural bulbous population was that it was least persistent and least productive of the three populations studied. This was not necessarily the result of an atypical population of low vigour having been selected, as other research has also revealed a relatively poor response from semi-natural bulbous plants (J.W. Cussans, personal communication, 1993). This leads to the question of why semi-natural bulbous plants are not outcompeted in their native habitats by the apparently more competitive growth forms of A. elatius. It is possible that climatic variation between the south-west and south-east of the UK may be a factor accounting for this response. Pfitzenmeyer (1959) suggested that semi-natural bulbous plants are better adapted to wetter areas. It is also possible that this population has other attributes contributing to its survival in its native habitat, such as heavy metal tolerance, or soil acidity, that were not investigated in this study. The slightly less bulbous mean phenotype in wheat than in ley for this population, compared with the more static phenotype of the arable bulbous population, suggests that there may be a greater degree of phenotypic plasticity in the semi-natural population. The direction of plasticity is inconsistent with Khan's hypothesis (Khan, 1987) that corm-formation imposes a fitness penalty with respect to competitivity. Were this the case, one would not expect plants to have a more bulbous phenotype in the highly competitive grass and clover ley than in the wheat, as was recorded. The different selection pressures acting on bulbous plants of arable and semi-natural origin must have influenced the degree of genetic separation between such populations. The former biotype will be subject to cultivation and herbicides, and the latter to defoliation (Miller et al., 1992).
4.5. Wider implications This study is obviously limited by the number of populations which could feasibly be studied. There is a possibility that the populations under consideration
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are atypical for their morphological category (see Section 4.4), although the allogamous nature of reproduction in the species results in a broad genetic base for individual populations. If the responses of the populations are assumed to be typical, then wider implications can be suggested. Cussans et al. (1993) suggested that arable A. elatius was more persistent than semi-natural forms. Our study does not dispute this hypothesis. However, the performance of the semi-natural bulbous form precludes a simple correlation between persistence and corm-formation. Cultivation increased ramet numbers in the nonbulbous population as well as in the bulbous populations. Why then is this form not apparent as an arable weed? Herbicides may play the major role in the absence of ssp. elatius from arable fields (Miller et al., 1992), or it may be that clumps of ssp. bulbosum may be damaged less by abrasion from cultivation of stony soils than non-bulbous ramets (Ingram, 1975). The potential for cultivation to disperse ramets of A. elatius was indicated dramatically by the increase in ramet numbers in the wheat plots. This is of particular significance in a species with a phalanx growth form (Lovett Doust, 1981), in which natural spread is in the immediate vicinity of the parent. Dispersal via farm machinery can spread weeds from one field or farm to the next (Schippers et al., 1993). In A. elatius there is evidence, using Random Amplified Polymorphic DNA (RAPD) markers on DNA of individual ramets, that cultivation may spread clones over considerable distances in the field (Miller et ai., 1995). The findings of this paper are relevant in the context of set-aside and fallow. This study has shown that onion couch has the potential to persist within a competitive grass and clover ley for several years, albeit with less vigour than its non-bulbous relative. This means that established weed infestations of onion couch, whilst not spreading rapidly in a grass and clover ley, will not be eradicated rapidly either. Hence the weed control potential of such a strategy (Jenkin, 1931) is not worthwhile in the short term. It is likewise interesting that ssp. elatius was very successful in an arable habitat, where herbicide use was kept to a minimum. The increase in ramet numbers of these non-bulbous plants following cultivation implies that farming systems employing a low
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input regime, for example organic farming, may have a potential weed problem in ssp. elatius as well as ssp. bulbosum.
5. Conclusions Morphologically distinct populations of Arrhenatherum elatius do not produce identical growth patterns in different habitats. One likely factor is their propensity for corm-formation, and the effect this has on the seasonality of plant growth. Bulbous A. darius, selected through agricultural practices, was most persistent, and was separated into the greatest number of viable ramets by cultivation. Non-bulbous A. elatius, derived from a lessmanaged habitat, was the most competitive of the three populations studied in a competitive grass and clover ley. Bulbous A. elatius, derived from a different semi-natural habitat, was least vigorous of the three populations, possibly through a differential climatic or edaphic tolerance. Cultivation fragments A. elatius plants, and can result in dispersal of ramets within a field. It did not kill established plants of ssp. elatius, which may therefore have the potential to become competitive weeds in low herbicide systems.
Acknowledgements This study was funded by the Joint Agriculture and the Environment Programme (JAEP). We would like to thank everyone who assisted with this study in any way, particularly those involved in the transplantation of seedlings into the field. Also, thanks to Alan Todd (Statistics Department, Rothamsted) for carrying out the Genstat analyses.
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comparison of weed and non-weed forms of Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl. Proc. of the Brighton Crop Protection Conference on Weeds, 1993, pp. 515-522. Ducousso, A., Petit, D., Valero, M. and Vernet, P., 1990. Genetic variation between and within populations of a perennial grass: Arrhenatherum elatius. Heredity, 65: 179-188. Grime, J.P., Hodgson, J.G. and Hunt, R., 1988. Comparative Plant Ecology. A Functional Approach to Common British Species. Unwin Hyman, London, pp. 102-103. Hubbard, C.E., 1984. Grasses, 3rd edn. (revised by J.C.E. Hubbard). Penguin Books, Harmondsworth, UK, pp. 232-233. Ingrain, G.H., 1975. The distribution of perennial weed grasses in the arable regions of the United Kingdom. Proc. of the EWRS Syrup. on the Status and Control of Grassweeds in Europe, Pards, 1975, pp. l-8. Jenkin, T.J., 1931. Swollen stem internodes and other characters in Arrhenatherum Beauv. Bull. Welsh Plant Breeding Station, Series H, 12: 126-147. Khan, A.U., 1987. An agro-ecological study of Arrhenatherum elatius. Ph.D. Thesis, University of London. Khan, A.U., 1991. Agro-ecology of onion couch (Arrhenatherum elatius var. bulbosum). 1. Distribution of onion couch related to the source and soil texture. Ann. Appl. Biol., 118: 645-650. Lovett Doust, L., 1981. Population dynamics and local specialization in a clonal perennial (Ranunculus repens). I. The dynamics of ramets in contrasting habitats. J. Ecol., 69: 743-755. Mahmoud, A., Grime, J.P. and Furness, S.B., 1975. Polymorphism in Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl. New Phytol., 75: 269-276. Miller, A.C.E., 1994. Aspects of the survival and spread of Arrhenatherum elatius ssp. bulbosum. Ph.D. Thesis, University of Reading. Miller, A.C.E., Cussans, J.W., Cussans, G.W. and Morton, A.J., 1992. A preliminary study into the distribution patterns of different growth forms of Arrhenatherum elatius (L.) J. & C. Presl. Proc. of the lX~me Colloque International sur la Biologic des Mauvaises Herbes, Dijon, 1992, pp. 259-268.
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