Growth of Transplanted and Native Shoots in Perennials with Contrasting Genet Architecture

Flora (1991) 185: 335-344 Gustav Fischer Verlag Jena

Growth of Transplanted and Native Shoots in Perennials with Contrasting Genet Architecture BERNHARD SCHMID and F AKHRI A. BAZZAZ Department of Organismic and Evolutionary Biology, Harvard University Cambridge, U.S.A.

Summary Seedlings and young shoots from rhizome cuttings of Aster lanceolatus, a species with spreading genet architecture, and of Solidago canadensis, a species with compact genet architecture, were transplanted to the field and observed together with shoots produced from rhizomes in situ and still connected to parent clones ("native" shoots). No differences in growth were found between transplants derived from seeds or from rhizomes in either species, indicating that they were of similar genetic and physiological quality. Accordingly, home- and away-transplants had similar performance among both sexual and clonal progeny. The native shoots grew much faster than transplants. The rapid growth of native shoots was very likely made possible by parental support via clonal integration from perennating below-ground organs (rhizomes, roots). This effect was larger in S. canadensis than in A. lanceolatus. Furthermore, most native shoots of S. canadensis completed their life cycle with the production of seeds and rhizomes whereas those of A. lanceolatus usually did not. We suggest that, due to the compact genet architecture, clonal integration is stronger and therefore parental support more effective in S. canadensis than in A. lanceolatus.

Introduction In established stands of clonal perennial plants seedlings are often rare or absent, and shoot population dynamics are dominated by clonal growth of existing genets. This may be so because the "starting capital" of a shoot produced from a seed is usually much smaller than for example that of a shoot produced from an underground rhizome. Seedlings that are much smaller than emerging clonal shoots at the beginning of the growing season have an extreme probability of early death. If they were the same size at the beginning of the growing season it may be expected that both should perform equally well. However, if in closed vegetation seedlings of an outcrossing species would then still perform worse than clonal shoots it could be suggested that seedlings were on average genetically less well adapted to the environment of parents than were clonal shoots of proven genotypes (WILLIAMS 1975). These hypotheses can be tested by transplant experiments. However, in clonal perennial plants a further ecological feature may influence success of shoot establishment. Shoots produced from rhizomes in situ may remain connected to parent clones and may therefore in addition to the resources in their own rhizome use resources stored in older organs (see PITELKA & ASHMUN 1985). If this is the case it is expected that experimentally transplanted seedlings and clonal shoots should perform worse than these integrated "native" shoots. The degree to which clonal shoots may benefit from this parental support depends on the duration and intensity of clonal integration. For example, if underground rhizomes of clonal perennials persist for several years and remain

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physiologically connected to each other and new rhizomes, the shoots produced from these rhizomes may share a large pool of resources acquired by earlier generations of aerial shoots, which would not be available if connections were no longer functional. It has been suggested that such variation in the degree of clonal integration is correlated with variation in genet architecture among species: shoot development seems to be more closely controlled by clonal integration in compact genets than in spreading genets (HARTNETT & BAZZAZ 1985a, HARPER 1985). . Ecologically this would make sense because in compact genets clonal shoots of a single genet stand close together and are expected to "cooperate" whereas in spreading genets clonal shoots of a single genet are interspersed with other plants and are expected to respond to their immediate local environments. These hypotheses can be tested by comparing the performance of connected native shoots with that of transplants in species with compact genets and in species with spreading genets. We use the two clonal perennials Aster lanceolatus WILLD. and Solidago canadensis L. to address the hypotheses presented above. These two species, which commonly are found in old-fields in eastern Massachusetts and which are also representative for invaders of disturbed habitats in Europe, have similar size and ecology but contrasting genet architecture (SCHMID & BAZZAZ 1987). Aerial shoots appear each year in spring from underground rhizomes and die at the end of the growing season. The new shoots within a clone are connected to the rhizome and root system of their parents. Aster lanceolatus has long, rooted rhizomes (median length> 10 cm, maximum length> 100 cm) and in dense stands different genets intermingle. Thus, pollen transfer among different genotypes is made very likely by pollinators. This species represents the spreading genet architecture. Solidago canadensis has short, initially unrooted rhizomes (median length < 2 cm, maximum length < 10 cm) and represents the compact genet architecture. Crossing experiments have shown that this species is an obligately outcrossing species (VOSER 1983, SCHMID in prep.). Both species require summer temperature regimes for germination (personal observations). Because seeds are dispersed in autumn they usually germinate in the summer of the following year. During the remaining time of the growth period the seedlings may produce small plants and after the second winter resume growth at the same time in spring as the established plants do. We report results from two parallel experiments carried out in established field populations with no observed regeneration from seeds. In each, we compare the growth and development of native shoots with that of same-sized seedling and rhizome transplants from "home" and "away" sites.

Materials and Methods

Experimental design In late October and early November 1984 we collected rhizomes and seeds from apparently homogeneous populations of Aster lanceolatus sensu lato (including one population of the closely related and initially not distinguishable A. longifolius LAM.) and Solidago canadensis at several localities in eastern Massachusetts, USA (see SCHMID & BAZZAZ 1990 for detailed description). Two localities were designated as "home" sites, the remaining localities collectively as "away" sites. For both species home and away were represented by collections from four patches each (referred to as "collection" effect). Seeds were germinated on filter paper in Petri dishes (14-h day, 24/18 0c), rooted in vermiculite, and transplanted to 6-cm diameter peat pots in mid-January 1985 and placed in a heated glasshouse. Rhizome cuttings were made by breaking off the new rhizomes or rhizome tips from the same shoots that served as seed parents. The rhizomes were treated with rooting hormone and planted in flats

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filled with vermiculite. After growing for at least two weeks in a growth chamber set at 14-h lightjlO-h dark and 24/20 °C temperature the rhizomes were also transplanted to 6-cm diameter peat pots. The growth medium in the peat pots was a mixture of peat, perlite, and sand (2: 1 : 1 by volume). All pots were regularly watered with a diluted nutrient solution (1/4-strength "HOAGLAND"). The plants were transferred to a colder room (11°C night temperature) when they had developed a rosette ofc. 5 cm diameter and were kept on an Il-h day-length regime until planting. This procedure was used to increase the hardiness and to standardize the initial size (as measured by rosette diameter) of transplants. For both species fourty seedlings and fourty rhizome cuttings were selected to be transplanted to the A. lanceolatus and the S. canadensis test site at Broadmoor Wildlife Sanctuary (Middlesex Co., MA), respectively. Because of experimental problems unequal numbers of away-plants and home-plants had to be used: 26 home- vs. 14 away-seedlings and 30 home- vs. 10 away-rhizome cuttings in A.lanceolatus, 30 home- vs. 10 away-seedlings and 18 home- vs. 22 away-rhizome cuttings in S. canadensis. Between 30 April to 9 May 1985, when field populations resumed growth, all rosettes were transplanted to the two experimental sites. A soil cylinder of c. 10 cm diameter was cut out with its vegetation and replaced by the pot with a seedling or rhizome cutting. Loose soil was put back around the pot to fill the gap and watered in with the transplant. The distance between transplants and other shoots of Aster or Solidago was as great or greater than among native shoots. Extreme care was taken to avoid "transplant shock": in addition to the preconditioning at the appropriate temperature and day length, plants were not removed from their peat pots, regularly surveyed, and, during the first ten days, watered as necessary. This procedure was tested in the experimental garden where identically prepared transplants grew faster and required less time to flowering than native shoots in the field. The test site of S. canadensis was a relatively dry, south facing slope dominated by S. canadensis and perennial grasses (mainly Poa pratensis L., Phleum pratense L., Dactylis glomerata L.). The transplant site of A. lanceolatus was a wet depression with dense A. lanceolatus, Rhus radicans L., and Polygonum sagitta tum L. At both sites we selected eight micro-sites (blocks) with A. lanceolatus or S. canadensis. Into each we transplanted five seedlings and five rhizome cuttings within an area of about I m 2 and in addition selected five emerging native shoots. Home and away transplants were assigned randomly to each block. Native and transplanted shoots of S. canadensis were located along the border of S. canadensis clumps while those of A. lanceolatus were located within A. lanceolatus stands. This ensured that for all of them light conditions were as uniform as possible within each block. Seedlings, rhizome cuttings, and native shoots were marked with coloured wire around the base of the stem and their positions mapped. For each marked plant we determined shoot height (cm), stem diameter (to nearest 0.1 mm), number of green main-stem leaves, number of new main-stem leaves since previous observation, maximum leaf length (mm), leaf width (mm, longest leaf), number of elongated main-stem branches, number of side shoots, phenological status (rosette, bolted, appearance of inflorescences, flowering, flowers wilted, fruiting), condition of shoot apex (intact, damaged, lost), and survival on six occasions throughout the growing season (height and phenological status on eight occasions). On 15 October 1985 we harvested all native shoots and the transplants from half of the blocks for S. canadensis. In the other blocks, the transplants were left in the field for more than one year so that their second-generation shoots could be compared with twenty newly-marked native shoots during the growing season in 1986. These 1986 plants were only measured twice and subsequently harvested. Since the site with A. lanceolatus was damaged by falling trees during a hurricane (27 September 1985), all marked plants of A. lanceolatus were harvested on 19 October 1985. Only a few of them were killed or lost due to the hurricane and were considered as missing for the harvest data. The harvested plants were washed and separated into above- and below-ground parts. The following characters were assessed for each: length and width of inflorescence (cm), number of rhizomes, maximum rhizome length (mm), maximum rhizome diameter (to nearest 0.1 mm, longest rhizome), above-ground biomass (mg dry wt), below-ground biomass (mg dry wt). To obtain additional data sets for comparison, fully grown shoots that reached canopy height were randomly harvested at the same time and sites as marked shoots. Subsequently, they are referred to as "canopy" shoots. For the more variable A.lanceolatus we measured thirty-five canopy shoots from seven micro-sites and for the less variable S. canadensis twenty canopy shoots from four micro-sites. We recorded stem diameter, height, length and width of inflorescence, number of branches, and above-ground biomass.

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Statistical analysis Preliminary statistical analyses were carried out with computer programs of the BMDP-package (DIXON 1985). All model fitting and significance testing was then done using the GUM statistical language (generalised linear interactive modelling system, release 3.77; PAYNE 1985) assuming normal errors for continuous variables, transformed if necessary, and POISSON erros for frequency counts. Results were summarized for the two species separately in analysis of variance and analysis of deviance tables (continuous variables and frequency counts, respectively) according to the scheme given in Table 1. A formal description of the relationship between linear (analysis of variance) and log-linear (analysis of deviance) models can be found in MCCULLAGH & NEWER (1983). The factor "block" and the treatment factor "native vs. transplanted" defined the main model. The effects of "seedling vs. rhizome", "home vs. away", and of "collection" were estimated in a compound submodel for transplanted shoots (see COCHRAN & COX 1957). Since collection was nested within home vs. away, the effects of home vs. away and of home vs. away crossed with seedling vs. rhizome were tested against the corresponding effects involving collection, as indicated in Table I, and not, as with the other terms, against the residual. To account for possible differences in initial size within the groups of native or transplanted shoots, the first set of measurements made after the start of the experiments were used as covariates in the analysis of variance (see Table I). Two standard errors of the differences between means (S.E.D.) and the means themselves, adjusted for covariate and collection effects, are represented in figures. Table I. Skeleton analysis of variance [deviance] for characters measured on 120 shoots of Aster lanceolatus (same for Solidago canadensis). Source of variation

Sum of squares [Deviance change] 1)

d.f. [dJ. change]

Mean squares [Mean change]

Variance-ratio [approx. FF)

block transpl. vs. native initial size 2) home vs. away collection seedlings vs. cuttings home/away x seedl./cutt collection x seedl./cutt. residual

Sb Sn Se Sh Sp S, Shs Sps S, [res. dev.]

7 1 2 1 6 1 1 3

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Sb/S sn/s se/s Sh/Sp sp/s ss/s

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sp,/s

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Results From May until August, seedlings, rhizome cuttings, and native shoots of both species had very low (less than 5%) mortality rates. Two months later, in mid-October, 48% of the native shoots and 71 % of the transplants of A. lanceolatus had died without completing their lif cycle (difference between the two numbers significant at P < 0.05; plants destroyed by the hurricane not included in calculations). In S. canadensis only 10% of all shoots (native or transplanted) were dead by this time. In either species rhizome cuttings had the same mortality rates as seedlings and home transplants the same mortality rates as away transplants.

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Fig. 1. Plant height over time for seedling (diamonds) and rhizome transplants (triangles), and for native shoots (squares). (a) Aster lanceolatus, (b) Solidago canadensis (shaded symbols for second year after transplanting). Bars indicate two S.E.D.s.

Despite their similar initial heights, transplants grew slower and remained smaller than native shoots (P < 0.001 at most observation dates for height, stem diameter, number of new leaves, number of live leaves, number of branches, and for above- and below-ground biomass at harvest; Figs 1, 2). In both species, leaf size was less affected by transplanting than was overall plant size (Table 2). Transplanted shoots of A. lanceolatus actually produced wider leaves than native shoots during the second half of the experiment (P < 0.01). The difference in growth between transplanted and native shoots was greater in s. canadensis than in A. lanceolatus (see examples in Table 2). However, where results are compared between experiments, the statements in this paragraph can not formally be supported with P-values for statistical significance. We calculated relative growth rates for the heights shown in Fig. 1 (see BRADBURY 1981). In A. lanceolatus native shoots grew 50% faster than transplanted shoots during the first time interval (P < 0.001). During the second and third time interval, however, transplanted shoots grew twice as fast as native shoots and during the following time intervals rates were not statistically different. In S. canadensis the relative height growth rates of native shoots were five to six times as large as those of transplanted shoots during the first time interval and twice as large during the second time interval (P < 0.001). The early rates of native shoots were never approached by the transplanted shoots also during the later time intervals in 1985. The differences in height and leaf number between second-generation shoots produced by the transplants in 1986 and newly marked native shoots were smaller than the differences observed in 1985,

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Fig. 2. Leaf demography of seedling (dashes lines, diamonds) and rhizome transplants (dotted lines, triangles), and of native shoots (full lines, squares). (a) Aster lanceolatus, (b) Solidago canadensis (shaded symbols for second year after transplanting). Top lines in each graph: cumulative leaf births; symbols: number of live leaves; bottom lines: cumulative leaf deaths. Bars indicate two S.E.D.s for comparisons of the three lines with symbols.

Table 2. Performance of seedling and rhizome transplants and of native shoots of Aster lanceolatus and Solidago canadensis in early August 1985 ("home" and "away" combined). Means and coefficients of variation (%, in parentheses) are given. Character

Seedlings

Rhizomes

Natives

Aster lanceolatus Height (em) Stem diameter (mm) No. of leaves Leaf length (mm) Leaf width (mm)

37.4 (52) 1.83 (31) 5.5 (67) 66.6 (35) 11.6 (24)

31.0 (57) 1.93 (36) 5.3 (51) 71.9 (32) 11.6 (20)

76.5 (36) 3.18 (30) 9.6 (38) 77.8 (22) 10.3 (16)

Solidago canadensis Height (em) Stem diameter (mm) No.ofleaves Leaf length (mm) Leafwidth (mm)

15.1 (79) 1.74 (21) 16.4 (62) 58.8 (35) 9.0 (21)

26.9 (71) 1.84 (17) 22.9 (42) 60.5 (27) 8.8 (19)

107.4 (23) 4.32 (20) 75.6 (30) 87.1 (22) 11.1 (23)

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Table 3. Performance of native shoots marked after emergence and of shoots that reached canopy height at the end of the growing season of Aster lanceolatus and Solidago canadensis in mid October 1985. Means and coefficients of variation (%, in parentheses) are given for metric characters. Character Aster lanceolatus 1) Height (cm) Stem diameter (mm) Survival (%) Flowering (%) Solidago canadensis Height (cm) Stem diameter (mm) Survival (%) Flowering (%)

Native shoots

Canopy shoots

96.6 (31) 3.12 (29) 52 3

122.3 (13) 3.65 (15) (lOW) 71

108.3 (20) 4.40 (20) 92

124.4 (9) 3.99 (15) (lOW) 100

72

1) Shoots killed or lost during hurricane excluded. 2) Only live shoots were collected, i.e. survival 100% by definition.

but still highly significant (P < 0.001; Figs 1,2). Native shoots of A. lanceolatus were more variable than native shoots of S. canadensis with regard to characters that measured overall plant size, i.e. height, stem diameter, number of leaves (Tables 2, 3). Although seedlings had bigger but fewer leaves than rhizome cuttings at the beginning of the experiment (P < 0.001 for A. lanceolatus and for S. canadensis), they subsequently did not differ from each other in any character measured in either of the two species. The only two exceptions were that rhizome transplants of A. lanceolatus had wider leaves than seedling transplants late in the season (P < 0.05 and P < 0.001 for last two observation dates) and that shoot apices were more often damaged in seedling transplants than in rhizome transplants of S. canadensis at the time of harvest (P : : : ; 0.01). In A. lanceolatus, collection effects were only slightly more often significant than expected by chance (i.e. P < 0.05 in five out of fifty-eight analyses). In S. canadensis, however, collection effects were significant in twenty out of fifty-eight analyses (significance levels ranging from from 0.05 to 0.001). Home- and away-transplants of both taxa showed similar performance both if grown from seedlings or from rhizomes (home vs. away effects and home vs. away x seedling vs. cutting interactions not significant). If anything and at least for the rhizome cuttings, away transplants tended to grow bigger than home transplants in both species. While only about half of all native shoots of A. lanceolatus marked at the beginning of the growing season survived up to the hurricane and subsequent harvest, almost all native shoots of S. canadensis survived. Differences between the two species were even larger for proportion of shoots flowering (Table 3). In A. lanceolatus, surviving native shoots on the average were much smaller than canopy shoots sampled at harvest (P < 0.001 for above-ground biomass, number of branches, phenological status, P < 0.01 for height, and P < 0.05 for stem diameter). However, variation in size among these surviving native shoots was large (Table 3). That is, only a small proportion of all shoots of A. lanceolatus present as rosettes in spring successfully completed their life cycle. and reached the size of canopy shoots. In contrast, the majority of shoots marked at the beginning of the growing season in S. canadensis did complete their life cycle with the production of seeds, and at harvest could have been included in the category of canopy shoots (Table 3).

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Discussion Shoots of A. lanceolatus and S. canadensis survived and grew but remained small if transplanted to the field at the rosette stage. In contrast, most rosettes of similar size to transplants marked in the field grew rapidly ("native" shoots). This indicates that initial shoot size is not the only factor necessary for successful shoot development in established populations. Native shoots were connected to the below-ground rhizome and root systems of the clones to which they belonged and transplants were not. Thus, clonal integration allowed native shoots to use resources stored in the perennating organs of old parent shoots and this could explain the growth differences between transplanted and native shoots. It might be argued that there were no controls that had been disconnected from their parents but were left in the field (see HARTNETT & BAZZAZ 1983). We tested this procedure and found that, while it could have been done in A. lanceolatus, disconnected native shoots would not have survived in S. canadensis. This was not the case in HARTNETT & BAZZAZ' study because their material belonged to another taxon of S. canadensis, now recognized as S. altissima L., which has longer, earlier-rooting rhizomes than the typical taxon (SCHMID et al. 1988). A further concern might be that the ground around the native shoots was not disturbed in the same way as was necessary for transplanting. However, this was done with greatest care, and it is unlikely that the experimental procedure itself explained the very large differences - which persisted for more than one growing season in S. canadensis between transplanted and native shoots. Uncovering entire rhizome systems for mapping and putting the loose soil back has no significant effects on subsequent shoot growth and development in S. altissima (MEYER & SCHMID in prep.). In conclusion, the results of the present study indicate that in the investigated established populations naturally occurring seedlings would not be able to grow to the same final size as adjacent native shoots even if they were of the same initial size - because they lack the parental support via clonal integration. This conclusion is supported by the results from studies made by GOLDBERG & WERNER (1983) and HARTNETT & BAZZAZ (1983, 1985a). To test if other differences than those related to mode of regeneration existed between sexual offspring and clonal offspring, we transplanted both seedlings and rhizome cuttings obtained from the same plants and grown to the same initial size to the field. The results showed no differences in shoot growth and survival between the two types in both species. Apparently, the physiological and genetic quality of sexual progeny was comparable to that of clonal progeny and therefore would not explain the failure of naturally emerging seedlings to establish. The fact that home- and away-plants statistically had the same performance both if they were seedling transplants or if they were rhizome transplants further supports the conclusion that clonal progeny was not better adapted to the environment of parents than was sexual progeny. In accordance with the results from other studies the performance of sexual progeny was not more variable than that of clonal progeny (see SCHMID & BAZZAZ 1990 and references therein). The observed differences between transplanted and native shoots marked at the rosette stage were much greater in S. canadensis than in A. lanceolatus. This was especially pronounced if relative height growth rates were compared: transplanted shoots had very low rates throughout the experiment in S. canadensis whereas in A. lanceolatus after an initial lag they reached the same values as native shoots. Furthermore, differences between these native shoots and native shoots that reached canopy height at the end of the growing season were smaller in S. canadensis than in A.lanceolatus (see Figs 1, 2; Tables 2, 3). These contrasts between the two taxa are correlated with differences in genet architecture in the predicted way. First we discuss the pattern typical for the compact architecture found in S. canadensis and than the pattern typical for the spreading architecture found in A. lanceolatus.

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In S. canadensis, continued support via strong clonal integration from large belowground structures of parent shoots was presumably responsible for the high success rate of native shoots. Because they do not produce their own roots until about mid-time trough the growing season, they are actually completely dependent on old rhizome connections and roots for resource uptake during their early development (BRADBURY & HOFSTRA 1977). In accordance with the presumed great importance of clonal integration shoot development in S. canadensis appears to be more centrally controlled rather than controlled by the immediate local environment (cf. HARPER 1985). The ability of compact genets to buffer differential environmental influences has been observed in other populations of Solidago (HARTNETT & BAZZAZ 1985 b). The relatively better growth of transplanted shoots of A. lanceolatus compared with S. canadensis, and the low success rate of native shoots of A. lanceolatus are consistent with data obtained for this and other experimental systems comparing spreading with compact genet architectures (see PITELKA & ASHMUN 1985, SCHMID & HARPER 1985, SCHMID & BAZZAZ 1987). All these data suggest that shoots of spreading genets are clonally less integrated and more influenced by their immediate environment than are shoots of compact genets. Features of the biology of A. lanceolatus that are consistent with this view are rooted rhizomes which lack dormancy - rosettes appear in autumn and overwinter and rhizome connections that decay within less than two years (personal observation). Compared with species with compact genets, local response might be favoured by selection in species with spreading genets, both, if long rhizomes are more expensive to maintain than short rhizomes, and if parents are less able to "predict" or influence their distant environment (> 10 cm) than their proximate environment « 10 cm). Our study involved only two species to address a general hypothesis about the correlation between shoot development and genet architecture. Other studies with other taxa show the same correlation: in Carex, much higher mortality and size variation have been observed in shoot cohorts of species with spreading genets than of species with compact genets (BERNARD 1977, NOBLE et al. 1979, SCHMID 1984). Considering the potential importance of genet architecture for the population biology of clonal organisms (SACKVILLE HAMILTON et al. 1987), it is to be hoped that future work on this theme will widen the range of species investigated.

Acknowledgements We thank the Massachusetts Audubon Society and the Sanctuary Director, E. LANDRE, for permission to use the field site at Broadmoor and for logistic support and N. FOWLER, D. MATTHIES, J. STOCKLIN, and J. WHITE for comments on the manuscript. B. SCHMID was supported by a fellowship from the Swiss National Science Foundation. The research was supported in part by NSF grant BSR-84-4395 to F. A. BAZZAZ and by grants 3.4441-0.86 and 31-9080.87 from the Swiss National Science Foundation to B. SCHMID.

References BERNARD, J. M. (1977): The life history and population dynamics of shoots of Carex rostrata. 1. Ecol. 64: 1045- 1048. BRADBURY, 1. K. (1981): Dynamics, structure and performance of shoot populations of the rhizomatous herb Solidago canadensis L. in abandoned pastures. Oecologia (Berlin) 48: 271- 276. BRADBURY, 1. K., & HOFSTRA, G. (1977): Assimilate distribution patterns and carbohydrate concentration changes in organs of Solidago canadensis during an annual developmental cycle. Canadian J. Bot. 55: 1121-1127. COCHRAN, W. G., & Cox, G. M. (1957): Experimental Design. Second edition. New York. DIXON, W. J. (1985): BMDP Statistical Software. Berkeley.

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