Impact of different cutting regimes on the performance of Brachypodium pinnatum in Dutch chalk grassland

Biological Conservation 56 (1991) 1-21

Impact of Different Cutting Regimes on the Performance of Brachypodiumpinnatumin Dutch Chalk Grassland R. Bobbink & J. H. Willems Department of Plant Ecology and Evolutionary Biology, University of Utrecht, PO Box 80084, 3508 TB Utrecht, The Netherlands (Received 8 January 1990; revised version received 15 May 1990; accepted 23 May 1990)

A BS TRA C T During the past decade the usual management in Dutch chalk grassland, hay making in autumn, has not prevented a sharp increase of Brachypodium pinnatum and a drastic decrease in species diversity. To optimize management, the impact of different experimental cutting regimes in controlling Brachypodium was investigated in three- and five-year experiments in two chalk grassland sites. Dominance of Brachypodium was reduced within three years by cutting the vegetation in mid-summer. Relative phytomass of this species decreased )Crom 80% to less than 35% during five years of summer cutting. Forb phytomass doubled in these stands, although total above-ground phytomass markedly decreased. Cutting in summer prevented Brachypodium from effectively overtopping other species. Due to this change in vegetation structure, light penetrated more deeply in the canopy and species number and Shannon index of diversity increased considerably. Man)' characteristic chalk grassland species, especially short-lived forbs or plants with low stature, benefited from this cutting regime. Analyses of N and P in the above- or below-ground plant material ofBrachypodium did not demonstrate nutrient deficiencies due to summer cutting. No relation between N removal by cutting and the reduction of dominance of Brachypodium was jound. It is suggested that the decrease in non-structural carbohydrates in the rhizomes of Brachypodium is an important factor in the observed growth reduction. It is concluded that cutting the vegetation in mid-summer with removal oJ the plant parts is adequate to control Brachypodium. It restores a characteristic forb-rich chalk grassland vegetation within five years. 1

Biol. Conserv.0006-3207/91/$03'50 © 1991 ElsevierScience Publishers Ltd, England. Printed in Great Britain

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R. Bobbink, J. H. Willems

INTRODUCTION Chalk grassland is a semi-natural, species-rich vegetation which was once widespread in Western Europe. These grasslands decreased considerably in area during the second half of this century, when they lost their economic function as grazing or mowing areas in the greater part of their range (e.g. Ellenberg, 1978; Wolkinger & Plank, 1981; Willems, 1982; Ratcliffe, 1984). Since World War II a number of remnants have been set aside as nature reserves in several Western European countries. To prevent their natural succession into woodland most of the Dutch chalk grasslands have been cut in late autumn with removal of the hay. During the past decade this management has been insufficient to prevent an increase in phytomass of the grass species Brachypodium pinnatum and a drastic change in the structure of the vegetation. The increase of Brachypodium is negatively correlated with species number and diversity (Bobbink & Willems, 1987). Experimental application of nutrients has shown a strong growth stimulation of only this species after N enrichment, resulting in a sharp decrease in diversity (Bobbink et al., 1988, 1989). Because one of the major aims of acquisition and management of chalk grassland reserves is to preserve their former or existing floristic diversity (e.g. Margules & Usher, 1981; Westhoff, 1985), optimization of the management is necessary and may provide a possible way of controlling the success of Brachypodium and restoring a forb-rich community. It has been hypothesized that cutting the vegetation earlier in the season may affect the dominance of Brachypodium by a combination of a higher level of nutrient stress and a higher impact of population reduction (Huston, 1979). To test this two experiments with different cutting regimes have been set up to determine the effects in controlling Brachypodium. During three years an experiment with different cutting time or frequency was carried out to quantify the impact on phytomass production of Brachypodium and the removal of nutrients, especially of N, from the system. A second five-year experiment was done to determine in detail the changes in diversity under two cutting regimes, because the effective duration of the cutting treatments was only two years in the first experiment.

SITES A N D M E T H O D S Study area

The research was carried out in two chalk grassland nature reserves situated in the extreme south of the province of Limburg, The Netherlands (50 ° 51' N,

Effect of cutting on Brachypodium pinnatum

3

5 ° 54' E)--the Wrakelberg Nature Reserve (site I), situated on a south-facing slope (inclination 15°) and Laamhei (site II), a northwest-facing slope (inclination 20 °) in the Gerendal Nature Reserve. The subsoil of both sites is Upper Cretaceous chalk covered by a rendzina soil (pH 7-8) and the vegetation is at present classified as a Mesobrometum erecti (Willems, 1982). From the 1950s (site II) or the early 1960s (site I) the vegetation has been mown annually in autumn (October/November) with removal of the plant parts. For further details see Bobbink and Willems (1987, 1988). Methods

Experiment 1 In May 1984, four randomized blocks with four permanent plots (70 × 70cm) were laid out in both sites. Each plot was randomly selected for one of the following cutting treatments: (1) autumn cutting (early November), control treatment (AC); (2) summer cutting (early August) (SC); (3) early summer cutting (late June) (EC); and (4) cutting twice (late June and early November) (2 x C). From 1984 to 1986 the vegetation was cut annually with an electric handmowing device (Gardena 6) at 3 cm above the soil surface. The vegetation in the central part of the plot (50 x 50 cm) was collected and the samples were sorted into all phanerogamic species. Standing dead and living plant material were taken together. For each plot species density per 0.25 m 2 was determined and Shannon index of diversity (H') was calculated based on phytomass data. In late June 1986, just before the first cutting, vertical profiles for penetration of photosynthetically active radiation (PAR, 400-700 nm) in the canopy were measured on bright sunny days at 5-10 cm intervals with a line sensor having a sensitive surface of 50 × 0.4 cm (TFDL, Wageningen). In December 1986 root samples were taken randomly to a depth of 10 cm in each experimental plot with a root auger (diameter 8"0 cm). The soil cores were transported to the laboratory and stored at - 20°C. Later on, the roots were washed from the soil above a sieve (mesh width 0.5 mm). The belowground plant parts were sorted into rhizomes and attached roots of Brachypodiurn; non-Brachypodium root material; and remaining fine rootlets of unknown origin. Dry weight of the above- and below-ground plant material was measured after drying at 70°C for 48 h. The above-ground plant parts of all species of each plot were pooled in the following groups for nutrient analyses: Brachypodiurn; other graminoids (grasses, Carex spp., Luzula spp.); and forbs. After grinding the above- and below-ground plant material was digested with diluted salicylic/sulphuric acid. The nitrogen (N) and

4

R. Bobbink, J. H. Willems

phosphorus (P) concentrations were determined colorimetrically using a continuous-flow auto-analyser (Skalar analytical, Breda). Non-structural carbohydrate contents of the rhizomes of Brachypodium were measured colorimetrically (Hewitt, 1958). Data were statistically analysed using BMDP4V for two-way analysis of variance with repeated measurement and a posteriori testing using the T'and GTz-method. Where necessary, data were log or arc-sine transformed (Sokal & Rohlf, 1981).

Experiment 2 In July 1982 four permanent plots, each with eight subplots of 25 x 25 cm, were laid out at site I in a homogeneous part of the vegetation dominated by Brachypodium (cover c. 70%). Half of the plots were cut 3 cm above the soil surface in mid-summer (early August; SC) from 1982 to 1986, whereas the time of cutting in the other plots remained the same as usual (early November; AC). Unfortunately, one of the control plots was accidently destroyed in 1983. The cut plant material was removed from the field and the vegetation samples were sorted to all phanerogamic species, taking standing dead and living plant material together. Dry weight and diversity index were determined as described in experiment 1. Before cutting in July 1986 PAR through the canopy was measured in duplicate in each plot. Differences compared with the data from 1982 were tested using pair-wise non-parametric statistics (Sokal & Rohlf, 1981). Nomenclature of species follows Heukels and Van Der Meijden (1983).

RESULTS

Experiment 1

Above-ground phytomass Total above-ground phytomass decreased markedly after introduction of summer cutting (SC): from c. 380 g m - 2 in 1984 to c. 240 g m - 2 in 1986 at site I (p<0.01) and from 400 to 3 0 0 g m -z at site II (p <0-01), respectively. Cutting twice a year (2 x C) caused only a slight decrease in total aboveground dry weight in June 1986, compared with 1984 (ns). Early summer cutting (EC) and continuation of the usual management, i.e. autumn cutting (AC), did not change total above-ground phytomass (Table 1). The dry weight of some species groups was strongly affected by the cutting treatment. At site I the phytomass of Brachypodium decreased in the 2 x C treatment from 1 7 0 g m -2 in 1984 to 9 0 g m -2 in 1986 (p < 0"01), and in SC even from 275 to 115 g m -2 (p < 0.01). Only a slight decrease in weight of

Effect of cutting on Brachypodium pinnatum

5

TABLE I Total Above-ground Dry Weight of the Vegetation and Dry Weight of other Graminoids and Forbs (g m - 2 + 1 SE in parentheses) from 1984 to 1986 under Different Cutting Treatments in Two Chalk Grassland Sites in South Limburg Site I, Wrakelberg

Cutting Treatment a

1984

Graminoids (dry 2 x C-June EC SC AC Forbs (dry wt) 2 × C-June EC SC AC

Site II, Gerendal

1985

1986

1984

1985

1986

65 ( 1 5 ) 96 (26) 116 (27) 94 (26)

50(18) 83 (29) 94 (35) 76 (28)

wt) 36 31 39 46

(11) (10) (9) (22)

31 (8) 42 (6) 39 (8) 40 (16)

34(11) 32 (2) 47 (12) 36 (12)

120(21) 120 (30) 150 (11) 150 (35)

24 30 63 30

(6) (4) (11) (5)

43 (6) 51 (8) 85 (13) 42(6)

43 (13) 51 (5) 76 (12j 34(1)

51 (13) 59 (17) 107 (40) 50(15)

Total above-ground dry wt of the vegetation 2 × C-June 231 (11) EC 232 (14) SC 378 (19) AC 356 (29)

216(19) 263 (13) 312 (12) 376 (20)

171 205 238 365

(23) (19) (6) (3)

238(11) 244 (24) 401 (33) 320 (35)

62 65 74 51

(15) (17) (25) (11)

210(15) 276 (10) 346 (7) 283 (16)

66 (17) 66 (15) 71 (22) 62(15)

203(16) 258 (9) 299 (18) 307 (10)

a EC, early summer cut; SC, summer cut; AC, autumn cut; 2 × C, cut twice-a-year (data for June given).

Brachypodiumwas observed at EC in 1986 (p < 0.05). The phytomass of this species remained invariably high at AC during the course of the experiment (c. 2 9 0 g m - 2 ; 80% of total dry weight)(Fig. 1). At site II, on the contrary, where the proportion of Brachypodiumin the vegetation in 1984 was lower than at site I, the shifts in dry weight of this species were less distinct or even the opposite of those at site I. At EC and AC the phytomass of Brachypodium increased, respectively, from 65 to 110 g m - 2 and from 120 to 170 g m - 2. Thus, with the usual management, the relative phytomass of this species became almost 60%, compared with 40% in 1984. Even in 2 × C a tendency to increase was observed for this grass. Only at SC did the dry weight of Brachypodium not increase during the experimental period (p < 0.01) (Fig. 1). The dry weight of the other graminoids was not affected by the cutting treatment at site I. However, at site II, a decrease in phytomass was found from 1984, though no significant differences between treatments were observed. The dry weight of the forbs was not significantly influenced by the cutting treatment in this three-year experiment (Table 1).

R. Bobbink, J. H. Willems Brachypodlum Site I

~

300

Site II

2O0

// ,+/2m7m29s/ /

,e4

'es

'se

,84

's5

'se

Fig. 1. Above-groundphytomassof Brachypodium (g dry weightm- 2 + 1SE) from 1984to 1986 underdifferentcutting treatmentsin two chalk grassland sites.

Below-ground phytomass Total below-ground phytomass was markedly higher than above-ground phytomass, but no significant differences between cutting regimes were found at the end of the experiment. Total below-ground phytomass and below-ground dry weight of Brachypodium tended to be somewhat higher at AC than in the other treatments at site I (Table 2). Nutrient concentration in the shoots N and P concentrations in the shoot material of the different species groups in 1984 and 1986 are given in Table 3. Although the concentrations of these elements decreased from June to November in each year, no significant differences between the plant material from the different cutting treatments were observed from 1984 to 1986. Nutrient concentration in the below-ground plant parts To avoid disturbance of the experimental plots, N and P concentrations in the below-ground plant material were only determined at the end of the experiment. At site I, N concentration in the rhizomes (p < 0.10) and in the attached rootlets of Brachypodium (p < 0"05) increased at EC and SC compared with those in AC. The N concentration in the rootlets of Brachypodium from 2 × C at site I decreased compared with AC (/9 < 0.10),

Effect of cutting on Brachypodium pinnatum

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TABLE 2 Below-ground Dry Weight of Brachypodium, other Species and Remaining Fine Rootlets (g dry weight m-2_+ 1SE) under Different Cutting Treatments in December 1986 in Two Chalk Grassland Sites Cutting treatment Brachypodium 2×C EC SC AC

Site I

Site II

201 228 211 330

(33) (44) (26) (63)

247 t50) 150 (36) 178 (52) 145 (28)

275 317 457 335

(128) (111 ) (90) (118)

368 299 268 405

Other species 2×C EC SC AC

(125) (90t (31) (95)

Remaining rootlets 2×C EC SC AC

72 (5) 102 (27) 74 (14) 145 (16)

93 (18) 101 (6) 100 (32) 93 (17)

548 646 743 809

708 550 546 642

Total below-ground phytomass 2×C EC SC AC

(118) (145) (92) (117)

(167) (68) (108) [89)

although the N concentration in the rhizome of this species did not change compared with AC. The N concentration in the root material of the other species was not significantly influenced by the different cutting regimes. At site II no significant differences in N concentration in the belowground phytomass were found (Table 4). In contrast to N no significant differences between treatments were observed in below-ground P concentration at site I. At site II, the P concentration in the below-ground parts of Brachypodium increased considerably at EC, SC and 2 x C (p < 0.05), compared with AC. The P concentration in the below-ground phytomass of the other species increased at EC only (p < 0-05) (Table 4). At both sites the non-structural carbohydrate content in the rhizomes of Brachypodium decreased strongly at 2 × C, compared with AC (p < 0.01). The concentration in the rhizomes at EC and SC were intermediate, but considerably lower than at AC (p < 0-01) (Fig. 2).

TABLE 3

AC

SC

EC

Site II 2 × C (June data)

AC

SC

EC

Site I 2 x C (June data)

1984 1986 1984 1986 1984 1986 1984 1986

1984 1986 1984 1986 1984 1986 1984 1986 14-4 (0.6) 14-0 (0"5) 16"2 (0-9) 13"8 (0-4) 11"5 (0"9) 12.2 (0'8) 10-6 (0.2) 8-4 (0-4)

14.9 (0'5) 14.9 (0-6) 14.3 (0'8) 13.9 (0"5) 10-8 (0-3) 11-5 (0.3) 9.6 (0.8) 7.9 (0-6)

Brachypodium

14.6 (0-1) 13.9 (0"5) 15-7 (0.9) 13-4 (0'5) 10-4 (0"5) 11.7 (0.1) 11.3 (0-1) 9'6 (0.5)

15.8 (0.1) 15-3 (0"5) 15-4 (0"4) 15.5 (0-4) 10.4 (0.6) 11.9 (0.4) 12-2 (0.6) 11-I (0.4)

Graminoids

Nitrogen

17.6 (0'2) 19.9 (0-2) 19.2 (0.1) 18-5 (0-6) 13-5 (0.2) 14"7 (0.7) 9.9 (0.1) 9'5 (0'7)

18-9 (0-1) 18-8 (0-6) 18-0 (0-4) 18.8 (0"7) 11.9 (0.8) 13.6 (0-5) 10.6 (0-6) 9.7 (0-4)

Forbs

0"93 (0'07) 0"87 (0"03) 0.93 (0-06) 0"80 (0"03) 0-69 (0-01) 0.73 (0-06) 0.53 (0-01) 0'42 (0-02)

0'82 (0-05) 0-81 (0"03) 0-70 (0"05) 0-60 (0"12) 0.54 (0.02) 0.57 (0.02) 0.47 (0-04) 0-39 (0-04)

Brachypodium

1.09 (0"24) 1.08 (0"08) 1-00 (0-05) 0.99 (0-06) 0"70 (0-05) 0-78 (0"08) 0"67 (0"06) 0-73 (0-09)

0-99 (0.25) 1"06 (0-14) 0'95 (0-16) 0"99 (0-20) 0.63 (0.13) 0-79 (0.09) 0-82 (0-06) 0-82 (0-07)

Graminoids

Phosphorus

1.24 (0"09) 1.18 (0"06) 1-21 (0-03) 1-22 (0'06) 0-87 (0.04) 0'87 (0-05) 0'51 (0-06) 0.51 (0'08)

1-08 (0"06) 1.08 (0-07) 0"99 (0.10) 1-06 (0'06) 0.77 (0-10) 0.85 (0.04) 0.66 (0.01) 0.54 (0.02)

Forbs

N and P Concentrations (mg g dry weight- 1 + 1 SE) in the Above-ground Phytomass of Brachypodium, other Graminoids and Forbs under Different Cutting Treatments in Two Chalk Grassland Sites

O~

Effect of cutting on Brachypodium pinnatum

9

TABLE 4

N and P Concentrations (mg g dry weight- 1 + 1 SE) in the Below-ground Phytomass of Brachypodium, other Species and Remaining Fine Rootlets after Different Cutting Treatments in December 1986 in Two Chalk Grassland Sites Brachypodium

Other species

Remaining rootlets

Rhizome

Attached root

Nitrogen Site I 2×C EC SC AC

7'3 (2'0) 8.7 (0.2) 9'8 (0"6) 4'3 (0"6)

2'7 (0.1) 7.7 (0-4) 9.2 (0.3) 4.8 (0"2)

3'3 (1"4) 8-9 (1.7) 7.9 (0-9) 6.2 (1-3)

2"6 (0"21 8.6 (0.4) 9.0 (0-3) 5.4 10-4)

Site II 2xC EC SC AC

7.3 (0"8) 6-2 (0"5) 6.7 (0.7) 6.7 (0"61

5-3 (0-4) 4.8 (0"4) 4.9 (0"4) 5.1 (0"6)

6.7 (0"5) 5-9 (0'7) 6'6 (0'7) 6.2 (0'6)

7.5 (0-5) 7.0 (0.3) 7-1 (0.5) 6-3 (0.4)

Phosphorus Site l 2xC EC SC AC

0"24 (0"09) 0"24 (0-01) 0-34 (0-07) 0'20 (0"01)

0.14 (0"01) 0'22 (0"01) 0'27 (0"02) 0-24 (0"03)

0'46 (0.13) 0'60 (0.24) 0.83 (0-31) 0"80 (0.26)

0.14 (0.01) 0.26 (0-01) 0.29 [0'03) 0.31 (0'01)

Site II 2xC EC SC AC

0.45 (0.11) 0"33 (0'05) 0'36 (0-08) 0.13 (0.01)

0.19 (0"08) 0.24 (0.04) 0'23 (0.02) 0-06 (0'03)

0"26 (0.05) 0"73 (0.04) 0-43 (0.03) 0"25 (0.04)

0.19 (0-04) 0.42 (0.02) 0.37 (0'03) 0.14 (0-02)

N removal with different cutting regimes T h e a m o u n t o f N r e m o v e d with 2 x C in the first year was 5"02 g N m - 2 at site I (p < 0-01) a n d 4.79 g N m - z at site II (p < 0.05), c o m p a r e d with 3.44 and 3 . 1 0 g N m -2 at AC, respectively. C u t t i n g earlier in the season (EC, SC) r e m o v e d 1 0 - 2 5 % m o r e N t h a n AC, but the difference was n o t significant. T h e a m o u n t o f N r e m o v e d in 1984 with the material o f Braehypodium was h a r d l y affected by the o n c e - a - y e a r cutting t r e a t m e n t s ; o n l y 2 x C r e m o v e d m o r e N f r o m the system t h a n A C (p < 0"05) (Table 5). In the third y e a r o f the e x p e r i m e n t , the r e m o v a l o f N at SC a n d 2 x C d e c r e a s e d at site I, c o m p a r e d with the a m o u n t s r e m o v e d in 1984 (p < 0-01). T h e r e m o v a l o f N with the s h o o t m a t e r i a l o f Braehypodium decreased especially in these t r e a t m e n t s , because s h o o t d r y weight was r e d u c e d since

10

R. Bobbink, J. H. Willems "-" i =,

0.3

.? 0

o

0.2 "O =.,

m o -i

~ 2

iliiiiiil

0.1

I E 0 Z

0

:i::ii ~i~i~i~I 2xC E¢ SC AC Site I

2xC EC S(; AC

Site II

Fig. 2. Non-structural carbohydrate concentration in the rhizomes of Brachypodium (g g dry weight ~ _+ 1 SE) in December 1986 after three years of different cutting treatments in two chalk grassland sites.

1984 (p<0.05). At AC the amounts of N removed remained the same between 1984 and 1986 (Table 5). At site II, N removal by the different cutting treatments hardly changed during the experiment. Only for 'graminoids' was a tendency to decrease observed in all regimes. The total removal of N during the three-year experimental period decreased in general as follows 2 x C > SC > EC > AC. At site I, however, even the maximum difference between treatments (9.04 g N m - 2 compared with 11.46 g N m-2) was not significant. In contrast, the amount of N removed by 2 x C, SC or EC (respectively, 12.5, 12-4 and 11-5 g N m - 2) at site II was markedly higher than by AC (8.7 g N m -e) (p < 0.01).

Light penetration in the canopy Light penetration (PAR) inside the canopy was considerably influenced by the cutting treatment, especially at site I. Here, the 50%-light transmission point was 4-4-5.5cm at SC and 2 x C, compared with 13"6cm at AC (p < 0.01). At EC it was intermediate, but different from AC (p < 0-05). Near the soil surface light penetration was 27-28% of incoming P A R radiation at SC and 2 x C and only 8-5% at AC (p < 0"01). No differences between EC and AC were found. At site II the once-a-year cutting regimes did not affect light penetration. Only at 2 × C did the 50%-light transmission point decrease (p < 0-01), and more light reached the soil surface here than at AC (p < 0.05) (Table 6).

Effect of cutting on Brachypodium pinnatum

11

TABLE 5

N Removal (g m - 2 + 1 SE) with Different Cutting Treatments in 1984 and 1986 in Two Chalk Grassland Sites, including Total Amount of N Removed during the Experimental Period Brachypodium

Graminoids

Forbs

Total

Site 1 2 × C 1984-June 1984-total 1986-June EC 1984 1986 SC 1984 1986 AC 1984 1986

2-41 (0-27) 3-60 (0-34) 1.39 (0-20) 2.41 (0.12) 1.68 (0.19) 2.89 (0.19) 1.32 (0.13) 2.62 (0.40) 2.32 (0.17)

0.55 (0.16) 0.71 (0.21) 0"51 (0.17) 0-47 (0'14) 0-49 (0"12) 0"42 (0"11) 0"57 (0"14) 0"51 (0"21) 0'41 (0.14)

0-45 (0.10) 0-71 (0"16) 0.81 (0'24) 0.54 (0.05) 0.91 (0.07) 0.73 (0.11) 1.03 (0"15) 0-31 (0"03) 0-33 (0'05)

5'02 (0-22) 2.72 (0-38) 3-42 (0"24) 3-08 (0'24) 4.04 (0"09) 2"92 (0-12) 3"44 (0-24) 3-06 (0-18)

Site II 2 × C 1984-June 1984-total 1986-June EC 1984 1986 SC 1984 1986 AC 1984 1986

0'96 (0'20) 1.66 (0'28) 1.22 (0.24) 1'05 (0'l 1) 1.50 (0-21) 1.66 (0-22) 1.66 (0-32) 1.27 (0-05) 1.22 (0-09)

1.72 (0.24) 2.14 (0"36) 0'69 (0'25) 1.80 (0"36) 1.09 (0-39) 1"54 (0"10) 1.16 (0"53) 1'33 (0'25) 0'69 (0'25)

0.83 (0.16) 1.00 (0.19) 1.25 (0'23) 1.09 (0'27) 1-20 (0-23) 0-98 (0-47) 1-02 (0.28) 0-49 (0"15) 1.25 (0.15)

4'79 (0"30) 3.15 (0"23) 3.94 (0"25) 3"79 (0'04) 4'19 (0.48) 3'83 (0.39) 3'10 (0.37) 3.15 (0"06)

Total amount removed (1984-86) EC Site I 10"02 (0-52) Site II 11-48 (0-35)

SC 11.11 (0'06) 12.35 (0.77)

AC 9'04 (0-53) 8"66 (0-46)

2×C 11'46 (0.59) 12.48 (0.60)

TABLE 6

50% Light Transmission Point (cm above soil surface+ 1SE) and the Proportion of Incoming Radiation (% PAR _+ 1 SE) at 2 cm Above Soil Surface in Two Chalk Grassland Sites under Different Cutting Regimes (end of June 1986) Site I

2×C EC SC AC

Site 11

(cml

(% PAR)

{cm)

(% PAR)

5"5 (1"7) 7-3 (1.3) 4-4 (0-6) 13.6 (0.3)

28-3 (2.9) 11-8 (1-8) 27.3 (4"2) 8-5 (2-0)

7.3 (1.2) 15-8 (1.0) 14-5 (1.7) 15"3 (1'3)

14-8 (2"9) 4-8 (0"5) 5"0 (0"6) 4-0 (1.3)

12

R. Bobbink, J. H. Willems

Diversity The total number of vascular plants increased from 22 0.25 m -2 in 1984 to 27 0-25m -2 in 1986 at EC (p < 0.05) and at SC (p < 0-10) at site I. No significant shifts in species number were observed at 2 × C and AC during the experimental period. At site II no significant change in species number was found in the plots which were cut once a year. Only at 2 × C did species richness become slightly higher in 1986 than in 1984 (p < 0"10) (Table 7). The Shannon index of diversity increased markedly from 1984 to 1986 at SC, EC and 2 x C at site ! (p < 0-01), whereas in AC it remained invariably low. At site II the Shannon index of diversity was already considerably higher at the start of the experiment than at site I. Cutting regimes hardly induced shifts in diversity indices at this site; at AC a decreasing tendency was found, whereas only at SC could an increasing tendency be seen (p < 0.10) (Table 7).

Experiment 2

Above-ground phytomass Total above-ground phytomass decreased from 375 (+ 19) g m - 2 in 1982 to 235 (_+9)gm -z in 1986 at SC (p<0.01). At AC total above-ground phytomass neither increased nor decreased significantly but fluctuated between 350 and 410 g m - 2. The proportion of Brachypodium and forbs in the phytomass was strongly affected by earlier cutting (Fig. 3). At SC the relative phytomass of TABLE 7 Species Number (per 50 × 50cm; ___1 SE) and Shannon Index of Diversity (H' + 1 SE) in 1984 and 1986 under Different Cutting Regimes in Two Chalk Grassland Sites Species number 1984

1986

Site I EC SC AC 2xC

21.8 21.8 238 218

(1.3) (2.0) (1'1) (30)

27-8 26.3 240 235

Site II EC SC AC 2×C

26.0 25.0 22-3 21.5

(1-5) (1-4) (1-7) (1.7)

25"8 (1'8) 27.0 (1.3) 24.8 (1.3) 26.0 (1.4)

(1.6) (1.5) (19) (21)

H' 1984

1.07 1.11 085 105

1986

(0.09) (0.13) (014) (022)

1.58 (0.09) 1.82 (0.12) 0-83 (013) 1-65 (031)

2.04 (0.09) 1.76 (0.09) 1.73 (0.03) 2.04 (0-09)

1'89 (0.07) 1.85 (0.10) 1-48 (0.07) 1.89 (0.07)

Effect of cutting on Brachypodium pinnatum

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R. Bobbink, J. H. Willems

0

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Light transmission

Fig. 4.

Light transmission in the canopies (% PAR + 1 SE) in July 1986 under summer (open symbols) and autumn cutting (closed symbols) (experiment 2).

Brachypodium decreased, especially in the fourth and fifth years of the experiment, from 77% in 1982 to 34% in 1986 (p<0-01). In AC the proportion of this grass species did not change significantly through time and remained between 75 and 80%. The relative phytomass of the other graminoids was hardly affected by the cutting treatment, whereas the proportion of the forbs increased considerably at SC (p < 0.01). This means that the dry weight of forb species doubled after 1982 (p < 0"05), although total above-ground phytomass production decreased in this sward. Light penetration in the canopy Light penetration in the canopy, measured in July 1986, was considerably higher at SC than at AC. The point of 50% light transmission at SC decreased 14-15 cm compared with AC and significantly more light reached the soil surface (Fig. 4). Diversity and species composition Vascular species number and Shannon index of diversity were positively influenced by SC: the number of species increased from 15-6 per 25 x 25 cm to 21.2 (p < 0"01), while the Shannon index of diversity more than doubled in this five-year period (p < 0-01). At AC the number of species and the diversity index showed a slight tendency to decrease (Fig. 5). AC resulted in hardly any changes in species composition (Table 8). Only three tall or straggling plant species (Origanum vulgare, Knautia arvensis and

Effect of cutting on Brachypodium pinnatum

15

~a 25 Number of species per plot (25x25cm)

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Year

Fig. 5. Vascular species number (per 25 x 25 cm + 1 SE) and Shannon index of diversity (H' + 1 SE) from 1982 to 1986 under summer (open symbols) and autumn cutting (closed symbols) (experiment 2).

Vicia cracca) increased slightly in frequency, whereas a decrease was observed for six species (mostly short-lived or of low stature). Among the frequent species, Carexflacca increased somewhat in dry weight, whereas Leontodon hispidus, Briza media and Centaurea scabiosa decreased in weight. Five years after the introduction of cutting in August species composition had drastically changed. More than ten species increased considerably in frequency, especially almost all short-lived forbs (e.g. Anthyllis vulneraria, Linum catharticum, Gentianella germanica, Daucus carota and Rhinanthus minor) and plants with a low growth form (e.g. Polygala vulgaris, Leucanthemum vulgare and Plantago lanceolata). None of the species consistently decreased in frequency under this cutting regime.

7 8 4 3 4 3

1 0

Decreasing Scabiosa columbaria Achillea millefolium Plantago lanceolata Dactylis glomerata Poa compressa Leucanthemum vulgare

Increasing Knautia arvensis Vicia craeca

1982

2 0

5 6 5 0 1 1

1984

Autumn cutting

4 3

1 3 2 0 1 0

1986

Increasing Hieracium sabaudum Polygala vulgaris Daucus earota Linum catharticum Leucanthemum vulgare Picris hieracioides Gentianella germanica Rhinanthus minor Medicago lupulina Anthyllis vulneraria Taraxacum vulgare (s.I.)

Decreasing None

2 6 7 10 8 1 4 0 1 0 0

1982

13 9 14 14 9 0 5 2 6 0 2

1984

Summer cutting

15 14 15 16 14 9 9 7 6 5 5

1986

TABLE 8 Changes in Species Composition from 1982 to 1986 under Summer (n = 16) or Autumn (n = 8) Cutting Species which increase or decrease in frequency (as the number of the subplots in which they were recorded) are given (A). A species is considered to increase of decrease when the change was more than 25% of the total number of subplots of the treatment. For the frequent species (in average in more than 50% of the subplots) the changes in dry weight (g dry weight m 2 + 1 SE) are also shown (B). (A) Frequency

Carex flacca

Increasing

Centaurea scabiosa Leontodon hispidus Briza media

Decreasing

(B) Phytomass

333 (126)

23-8 (6"8) 7"6 (3-8) 5-4 (0-8)

1982

64'0 (96)

5"8 (1'4) 0"7 (1'1) 1"1 (0-2)

1986

Autumn cutting

Briza media Scabiosa columbaria Leontodon hispidus Sanguisorba minor Plantago lanceolata

Increasing

Brachypodium pinnatum Origanum vulgate Dactylis glomerata Knautia arvensis

Decreasing (19"1) (5"0) (2"7) (2"7)

4"7 (1.7) 0"9 (0-4) 6-7 (1"5) 2' 1 (0'4) 2"9 (1'8)

291"3 17"4 9-7 7"0

1982

25.9 (2'9) 25"8 (8"5) 25"3 (3"8) 8"6 (1"9) 5"7 (1"9)

78"8 (12-8) 7"0 (2" I) 2"1 (1"1) 2"1 (!-1)

1986

Summer cutting

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18

R. Bobbink, J. H. Willems

A marked increase in phytomass from 1982 to 1986 was also observed for a number of frequent species (e.g. Briza, Leontodon, Sanguisorba minor and Scabiosa columbaria), whereas a decrease in dry weight under summer cutting, besides that of Brachypodium, was found only for three tall species, namely Origanum, Knautia and Dactylis glomerata. DISCUSSION Chalk grassland needs specific management to prevent natural succession towards woodland (e.g. Wells, 1971, 1974; Ellenberg, 1978). To achieve this, most Dutch chalk grassland reserves have been mown with removal of the hay in late autumn since the 1950s or early 1960s. In the last decade this management proved to be insufficient to control the increasing success of Brachypodium, although succession to woodland has been prevented (Bobbink & Willems, 1987). Dominance of Brachypodium was reduced by cutting the vegetation in mid-summer or twice a year (Fig. 1). After five years the relative phytomass of this species decreased to less than 35% under summer cutting (Fig. 3). Furthermore, in the reserve where Brachypodium is still expanding (site II: from c. 40% in 1984 to c. 60% in 1986 under autumn cutting) this increase is stopped in the vegetation cut in summer. Many studies have focussed on the effects of cutting regime in Western European chalk grasslands. However, almost all have been performed to restore abandoned chalk grassland with a strong accumulation of persistent litter (e.g. Green, 1980; Krfisi, 1981; Burger, 1984; Dierschke, 1985; Schreiber & Schiefer, 1985; Hakes, 1988). In abandoned chalk grassland in Switzerland cutting in June reduced the abundance of Brachypodium whereas Bromus erectus and forbs increased (Kriisi, 1981). In Germany cutting in summer also decreased the proportion of Brachypodium, compared with abandonment (Burger, 1984; Hakes, 1988). In English chalk grassland different cutting treatments also reduced the abundant growth of Brachypodium after a period of abandonment (Green, 1980). These findings are in agreement with our results under summer cutting. One of the major aims of management of chalk grassland is to maintain floristic diversity (e.g. Margules & Usher, 1981). Previous studies have shown a negative correlation between relative phytomass of Brachypodium and diversity in the stand (Bobbink & Willems, 1987). Removal experiments demonstrated the functional aspects of this relationship (Bobbink et al., 1987). In the present study species number and diversity increased in the summercut stands (Table 7; Fig. 5). Within three years this increase is not so marked, but light penetrates deeper in the canopy, indicating a shift in the vertical structure of the vegetation which favours the invasion or growth of a number

Effect of cutting on Brachypodium pinnatum

19

of characteristic chalk grassland species (Table 6). Many of these species are short-lived or of low stature, e.g. rosette plants (Willems, 1985; Grime et al., 1988). The experimental summer cutting, reducing Brachypodium which with its erect growth form would grow higher than the other species, led to an increase of many low-growing species in weight or frequency after five years' cutting (Fig. 3; Table 8). Apart from the higher light intensities near the soil surface (Fig. 4), the red:far-red ratio is higher in such open stands (Schenkeveld & Verkaar, 1984). This might stimulate germination of shortlived species (Silvertown, 1980; Schenkeveld & Verkaar, 1984). After germination the increased light intensity at the level of their leaves will favour the persistence of these species (Verkaar & Schenkeveld, 1984). Almost all of the short-lived species of Dutch chalk grasslands increased strongly under summer cutting (Table 8). A shift in cutting time from November to August is thus a suitable adjustment of the present management to restore a forb-rich community from a sward dominated by

Brachypodium. Nutrient impoverishment of ecosystems is considered an important factor in restoring diversity in nature reserves, especially under conditions of increased productivity and nutrient accumulation in the system (e.g. Green, 1986; Bakker, 1989). It has been demonstrated that dominance of Brachypodium in Dutch chalk grassland is caused by N enrichment (Bobbink et al., 1988, 1989; Bobbink, details to be published). The amount of N removed from the ecosystem by different cutting regimes was, however, not positively correlated with the reduction of Brachypodiurn by those regimes (Table 5; Fig. 1). In the summer-cut stands with the most severe decrease of this species, N removal was hardly higher than in vegetation cut in autumn. Decreased concentrations of N and P in the above- or below-ground plant parts of Brachypodium were not observed under summer cutting (Tables 3 and 4), although phytomass production strongly decreased. Therefore, it is likely that although N enrichment induces dominance of this species, other plant features are responsible for the effects of summer cutting on Brachypodium. In rhizomatous species the non-structural carbohydrate content of the rhizomes is important for shoot formation (e.g. Risser, 1985). It is suggested that the observed decrease in non-structural carbohydrates in the rhizomes of Brachypodiurn in the twice-a-year and summer-cut stands (Fig. 2) hampered shoot formation in the following growing seasons. The aim of the present study was to quantify the impact of different cutting regimes in controlling the success of Brachypodium in managed chalk grassland nature reserves in The Netherlands. Cutting in summer proves to be adequate to reduce the abundant growth of this species. Due to the resulting change in vegetation structure species diversity increases markedly in these stands. Many characteristic chalk grassland species with a

20

R. Bobbink, J. H. Willems

low stature benefit from this cutting regime. Although N enrichment is a key factor for the success of Brachypodium, nutrient impoverishment as achieved by cutting and hay removal does not seem to be the most important factor in controlling this species. ACKNOWLEDGEMENTS The authors wish to express their gratitude to Leonard Bik, Marc Bergkotte, Hannie de Caluwe, D a n n y Esselink, Ren6 Kwant, Jan Schreurs, Ineke Stam and Rudi Zielman for their useful assistance in the field and laboratory and to Marjolein Smithuis for drawing the figures. They are also much indebted to Prof. Marinus Werger for his comments on the manuscript and to the State Forest Service in Limburg for permission to work in their nature reserves. The study was partly supported by the Anjer Fund, Maastricht and the Prins Bernhard Fund, Amsterdam.

REFERENCES Bakker, J. P. (1989). Nature management by grazing and cutting. PhD thesis, University of Groningen. Bobbink, R. & Willems, J. H. (1987). Increasing dominance of Brachypodium pinnatum (L.) Beauv. in chalk grasslands: a threat to a species-rich ecosystem. Biol. Conserv., 40, 301-14. Bobbink, R. & Willems, J. H. (1988). Effects of management and nutrient availability on vegetation structure of chalk grassland. In Diversity and Pattern in Plant Communities, ed. H. J. During, M. J. A. Werger & J. H. Willems. SPB Academic Publishing, The Hague, pp. 183-93. Bobbink, R., During, H. J., Schreurs, J., Willems, J. H. & Zielman, R. (1987). Effects of selective clipping and mowing time on species diversity in chalk grassland. Folia Geobot. et Phytotax., 22, 363-76. Bobbink, R., Bik, L. & Willems, J. H. (1988). Effects of nitrogen fertilization on vegetation structure and dominance of Brachypodium pinnatum (L.) Beauv. In chalk grasslands. Acta Bot. Neerl., 37, 231~,2. Bobbink, R., Den Dubbelden, K. & Willems, J. H. (1989). Seasonal dynamics of phytomass and nutrients in chalk grassland. Oikos, 55, 216-24. Burger, R. (1984). Successional limestone grassland communities of the Kaiserstuhl, with regard to their conservation management. Coll. Phytosoe., 11, 405-20. Dierschke, H. (1985). Experimentelle Untersuchungen zur Bestandesdynamik von Kalkmagerrasen (Mesobromion) in Siidniedersachsen, I. Vegetationsentwicklung auf Dauerfl/ichen 1972 -1984. Miinst. Geogr. Arb., 20, 9-24. Ellenberg, H. (1978). Vegetation Mitteleuropas mit den Alpen. Ulmer, Stuttgart. Green, B. H. (1980). Management of extensive amenity grasslands by mowing. In Amenity Grassland: an Ecological Perspective, ed. I. H. Rorison & R. Hunt. Wiley, Chichester, pp. 155-61.

Effect of cutting on Brachypodium pinnatum

21

Green, B. H. (1986). Controlling ecosystems for amenity. In Ecology and Design in Landscape, ed. A. D. Bradshaw, D. A. Goode & E. Thorp. Blackwell, Oxford, pp. 195-210. Grime, J. P., Hodgson, J. G. & Hunt, R. (1988). Comparative Plant Ecology: a Functional Approach to Common British Species. Allen & Unwin, London. Hakes, W. (1988). Vergleich der Planzenbestandesstruktur geni.itzter und brachliegender Kalk-Halbtrockenrasen in Nordhessen. Phytocoenologia, 16, 289-314. Heukels, H. & Van Der Meijden, R. (1983). Flora van Nederland, 20th edn. WoltersNoordhoff, Groningen. Hewitt, B. R. (1958). Spectrophotometric determination of total carbohydrate. Nature, Lond., 182, 246~,7. Huston, M. (1979). A general hypothesis of species diversity. Amer. Nat., 113, 81-101. Kriisi, B. (1981). Phenological methods in permanent plot research. VerOff~Geobot. Inst. ETH, Stiff. Riibel, Ziirich, 75, 1-116. Margules, C. & Usher, M. B (1981). Criteria used in assessing wildlife conservation potential: a review. Biol. Conserv., 21, 79-109. Ratcliffe, D. A. (1984). Post-medieval and recent changes in British vegetation: the culmination of human influence. New Phytol., 98, 73 100. Risser, P. G. (1985). Grasslands. In Physiological Ecology oj'North American Plant Communities, ed. B. F. Chabot & H. A. Mooney. Chapman & Hall, New York, pp. 232-56. Schenkeveld, A. J. & Verkaar, H. J. (1984). On the ecology of short-lived forbs in chalk grasslands. PhD thesis, University of Utrecht. Schreiber, K. F. & Schiefer, J. (1985). Vegetations- und Stoffdynamik in Griinlandbrachen--10 Jahre Bracheversuche in Baden-Wiirttemberg. Miinst. Geogr. Arb., 20, 155-86. Silvertown, J. (1980). Leaf-canopy-induced seed dormancy in a grassland flora. New Phytol., 85, 109-18. Sokal, R. R. & Rohlf, F. J. (1981). Biometrv. Freeman, San Francisco. Verkaar, H. J. & Schenkeveld, A. J. (1984). On the ecology of short-lived forbs in chalk grasslands: seedling development under low photon flux density conditions. Flora, Jena, 175, 135-41. Wells, T. C. E. (1971). A comparison of the effects of sheep grazing and mechanical cutting on the structure and botanical composition of chalk grassland. In The Scientific Management of Animal and Plant Communities.for Conservation, ed. E. Duffey & A. S. Watt. Blackwell Scientific Publications, Oxford, pp. 325-30. Wells, T. C. E. (1974). Some concepts of grassland management. In Grassland Ecology and Wildlife Management, by E. Duffey, M. G. Morris, J. Sheail, L. K. Ward, D. A. Wells & T. C. E. Wells. Chapman & Hall, London, pp. 163 74. Westhoff, V. (1985). Nature management in coastal areas of Western Europe. Vegetatio, 62, 523-32. Willems, J. H. (1982). Phytosociological and geographical survey of Mesobromion communities in Western Europe. Vegetatio, 48, 227~,0. Willems, J. H. (1985). Growth form spectra and species diversity in permanent grassland plots with different management. Miinst. Geogr. Arb., 20, 35~43. Wolkinger, F. & Plank, S. (1981). Dry Grasslands of Europe. Council of Europe, Strasbourg.