bluuebunny/crossref_metadata_2025_split huggingface dataset:
The exploitation of heterogeneity by a clonal plant in habitats with contrasting productivity levels
Authors: Kleijn David , Van Groenendael Jan M. 
Published: March 2003

Summary 1
We investigated habitat exploitation by the rhizomatous grass Elymus repens in a heterogeneous environment in both productive and unproductive habitats. We tested whether there was selective entry of rhizomes into favourable microsites or random exploration of the habitat, and whether the benefits of selective exploitation were higher in productive than unproductive habitats. 2
Trays were divided into four quadrants around a common central area. Homogeneous environments were created by planting vegetation throughout the trays; heterogeneous environments were created by planting only centre and two opposite quadrants. Contrasting productivity levels were established by fertilizing half of the trays of each environment type. A single rhizome fragment of E. repens was planted in the centre of the trays and allowed to exploit the trays for two growing seasons. 3

Elymus repens effectively exploited habitat heterogeneity at both productivity levels. However, only unproductive trays showed preferential growth of rhizomes into bare quadrants. 4  In the productive trays individual rhizomes that grew into bare quadrants showed a significantly greater production of shoots and total biomass than those that grew into vegetated quadrants. Smaller, non‐significant effects were observed at lower productivity. 5  Increased growth of primary rhizomes, once they had entered a bare quadrant, with or without selective growth into these patches, led to similar increases in heterogeneous compared to homogeneous trays at the two productivity levels (3.0 vs. 2.7). Heterogeneity was not therefore more effectively exploited at high productivity. 6  We suggest that sectorial transport of nutrients through the rhizomes, resulting in directional outgrowth of bud meristems into the bare quadrants, may have been responsible for the selective placement of rhizomes in the unproductive trays. Our results suggest that selective entry of rhizomes into favourable microsites may complement morphological plasticity in enabling rhizomatous plants to forage in heterogeneous environments.
Environmental heterogeneity and species diversity of forest sedges
Authors: Bell Graham , Lechowicz Martin J. , Waterway Marcia J. 
Published: March 2003

Summary 1 A field experiment was designed to investigate the relationship between environmental heterogeneity and species diversity in a group of sedges (Cyperaceae: Carex) growing in old‐growth forest. 2 A measure of environmental quality, as perceived by the sedges, was obtained from the survival of clonal ramets of 11 species of Carex planted at 10‐m intervals along each of three 1‐km transect lines. 3 The resident assemblage of sedges was censused along the same three transect lines and along a further 24 km of survey lines in the same forest. 4 The general state of a site was represented by the overall survival of the experimental implants at that site. The general environmental variance between sites provided a measure of environmental heterogeneity. This could be partitioned into a specific variance (mean environmental variance of species) and an environmental covariance. The rate of increase of the general and specific variances with distance between sites reflected environmental structure. 5 The three transects differed in scale. The species diversity of the resident Carex assemblage was correlated with general environmental quality both among and within transects. 6 The three transects differed in structure. The number of resident species, relative to the number expected from the number of individuals sampled, was greatest on the most coarse‐grained transect (steepest increase in general environmental variance with distance). 7 Within each transect, species diversity increased with general environmental variance because the specific correlation of performance (correlation among species of survival in pair‐wise combinations of sites) decreased as the general environmental variance increased. 8 The effect of specific environmental variance was weaker. Overall survival of a species on the transects was not correlated with its abundance in the forest. Neither the transects nor a targeted implant experiment provided evidence for a close relationship between the distribution of species and the state of the environment. 9 As a general explanation of our results, we propose a ‘marginal‐specialist’ model in which the species that dominate the most productive sites also have the broadest ranges, whereas other species are superior in a more restricted range of less productive sites.
The replacement series
Authors: Jolliffe Peter A. 
Published: March 2003

Summary 1 The replacement series has been used widely to assess interference, niche differentiation, resource utilization, and productivity in simple mixtures of species. Correctly used, the approach can lead to some valid interpretations. In order to avoid criticisms, however, researchers should appreciate the assumptions and limitations of this methodology. 2 A replacement series contains confounded species density treatments. Replacement series experiments therefore provide collective results and cannot distinguish separate contributions to interference by the constituents of a mixture. 3 In a replacement diagram, trends in observed yields per species can be due to a multitude of possible levels of intra‐ and interspecific interference. Similarly, trends in expected yields per species do not represent specific levels of intra‐ and interspecific interference. A replacement series is therefore unsuitable for the quantitative evaluation of interference or niche differentiation. 4 A standard replacement series, but not a proportional replacement series, may be used to detect an imbalance between intra‐ and interspecific interference for a component of a mixture. If competition for resources is assumed or known to be the sole cause of interference, then such experiments yield a qualitative evaluation of complementary resource utilization by the mixture components. 5 The possibility of total density dependence impedes generalization from a replacement series result. Other biases, due to initial size and time of observation, may occur with replacement series and related experimental structures. 6 Results from a replacement series are of questionable value in predicting the long‐term outcome of an association between species. 7 Replacement series are a valid setting, but not the only possible setting, for some kinds of yield comparisons. These include comparing productivity of monocultures with that of simple species mixtures. 8 In some cases, interpretations obtained from replacement series have not been confirmed using other methods but, on the whole, conclusions from replacement series do not seem to be characteristically different from those obtained using other approaches.
Ecological mechanisms of reproductive isolation between diploid and tetraploid Chamerion angustifolium
Authors: Husband And Brian C. , Schemske Douglas W. 
Published: March 2003

Summary 1  Theoretical models indicate that coexistence of diploid and polyploid individuals in sympatric populations is unlikely when mating is random among cytotypes and hybrids are inviable. However, coexistence may be facilitated by prezygotic isolating mechanisms that reduce intercytotype mating and increase assortative mating. 2
We examined the ecological factors regulating intercytotype mating in Chamerion (formerly Epilobium) angustifolium by measuring floral morphology, flowering synchrony and insect foraging preferences in a mixed population within a diploid–tetraploid contact zone. We also calculated the minimum rate of between‐cytotype mating from estimates of the frequency of triploid offspring. 3  Diploids had significantly shorter and narrower petals as well as shorter styles than tetraploids. Inflorescences were significantly taller in tetraploids than diploids, but the mean number of open flowers per inflorescence did not differ. Diploid and tetraploid flowering periods overlapped by 51%, with diploid stems flowering earlier. 4  In a plot of 20 diploid and 28 tetraploid stems that were flowering simultaneously, only 26% of all bee flights were between flowers on different stems. Of the total flights between inflorescences, only 15% were between different cytotypes. The combined effects of flowering asynchrony and insect foraging reduced the opportunities for intercytotype mating from 49% expected if mating is random to about 2% of the total number of pollinator flights. 5  A computer simulation indicated that the deficiency of pollinator flights between cytotypes was due largely to the spatial structure of cytotypes and limited pollinator flight distances within the observation plot. The frequency of triploid offspring produced during the period when both cytotypes were in flower was 6.6%, similar to the proportion of flights observed between flowers on different cytotypes (4%). 6
The results indicate that flowering phenology and insect behaviour (as influenced by clonal structure) have a significant effect on prezygotic mating isolation and triploid production, and may contribute to the maintenance of mixed cytotype populations in Chamerion angustifolium.
Soil seed bank dynamics in hayfield succession
Authors: Bekker Renée M. , Verweij Geurt L. , Bakker Jan P. , Fresco Latzi F. M. 
Published: March 2003

Summary 1  Changes in the species composition of the soil seed bank were determined in a dry and a more species‐rich wet chronosequence. Each sequence represented a 25‐year hayfield succession following cessation of fertilizer application in the Drentse A Nature Reserve (NL), where the vegetation has been studied since 1972. 2  The number of seeds of many late successional species showed a significant increase during succession with only two characteristic late species present in the seed bank of the early stage of each series. Most of the early successional species showed a decrease in the number of seeds, which paralleled their relative frequency within the vegetation. Overall, seed density and species diversity of the seed bank was higher in the wet than the dry chronosequence. 3  Mean values for seed longevity were calculated for each successional phase in each series using a published seed bank data base. When based simply on the presence of species, the seed longevity index decreased significantly during the wet‐series succession in both surface and deeper seed banks and in the vegetation. When based on the relative frequency of species present, the index decreased significantly in both series but only in the surface seed bank and the vegetation. 4
Comparison with data for an ancient undisturbed wet hay meadow in England suggests that seed longevity of hayfield species is generally low. Similarity between seed bank and established vegetation was low in both the undisturbed British meadow (only 47%) and in the relatively young Dutch study sites (c. 50%). 5  The soil seed bank is unlikely to determine hayfield succession in the Drentse A Nature Reserve, since the composition of the soil seed bank tended to follow that of the above‐ground vegetation. Increases in plant species richness following the cessation of fertilizer application, the main goal of restoration management, therefore depend to a large extent on an influx of the seeds of most species from outside the site.
Persistent soil seed banks in Phacelia secunda (Hydrophyllaceae): experimental detection of variation along an altitudinal gradient in the Andes of central Chile (33 S)
Authors: Cavieres Lohengrin A. , Arroyo Mary T. K. 
Published: March 2003

Summary

It is unclear whether soil seed banks in alpine environments are a result of selection for extended seed dormancy, of ideal conditions for seed preservation in the soil, or a combination of both.
The nature of the soil seed bank was investigated in the perennial herb Phacelia secunda (Hydrophyllaceae) through reciprocal burial experiments using seeds obtained from populations growing at 1600, 2200, 2900 and 3400 m a.s.l. in the mediterranean Andes of central Chile. At the four elevations, six replicates of 50 seeds each, from each of the four elevational sources, were buried at 5 cm depth in mesh envelopes, placed in wire cages to protect against predation. Seeds were retrieved 1, 2 and 3 years after burial, and numbers of ungerminated seeds, and of those that remained viable, were determined.
At all elevations of seed burial, increasing proportions of seeds remained ungerminated, and proportionately more of the ungerminated seeds remained viable, as elevation of source of seed increased. For seed from low elevation, more seeds remained viable in the soil at the higher burial sites. However, for the seeds from higher elevation sources, seeds remaining viable were similar in number at all sites for seed burial. Half‐lives of seed buried at the source elevation, calculated from the log‐logistic distribution, increased with elevation, and ranged from 188 days at 1600 m to 354 days at 3400 m.
The results suggest that the propensity to form a persistent seed bank in P. secunda increases with elevation. Seeds from the highest elevations had the longest persistence, and this was largely unaffected by soil preservation conditions. Seeds from lower elevations survived for less time, and this was modifiable by preservation conditions. These results suggest that both selective effects and soil conditions may be involved in soil seed bank expression in the alpine environment.
Male and femaleSilene latifoliaplants differ in per‐contact risk of infection by a sexually transmitted disease
Authors: Kaltz Oliver , Shykoff Jacqui A. 
Published: March 2003

Summary Behavioural, physiological or immunological constraints often render one sex more susceptible to parasites, thereby potentially generating sex‐specific trade‐offs between traits associated with infection risk and other life‐history characters. The fungal pathogenMicrobotryum violaceumsystemically infects the dioecious plantSilene latifoliawhen pollinators deposit fungal spores on the flowers of healthy plants. Male plants produce many short‐lived flowers, whereas females produce few flowers that remain connected with the plant after fertilization. We investigated how variation in flower production and flower longevity affects the infection risk for males and females. In glasshouse experiments, we varied the number of flowers inoculated (4 vs. 16 per plant) with spores and the time until these flowers were removed (1 or 2 days for both sexes, 14 days for females only). We also measured the longevity of male flowers receiving simulated visits, with or without spores, to test for an abscission response to visitation and/or contamination. In a field survey, we measured male and female disease prevalence in 17 natural populations. Varying the number of inoculated flowers did not affect infection probability, but females retaining inoculated flowers for 14 days became diseased more often (20.0%) than did plants with flowers removed within 2 days (7.3%). Males that had dropped more inoculated flowers prematurely were more likely to remain uninfected. Spore‐bearing visits shortened male flower longevity (38.4 ± 2.8 h) relative to non‐spore visits (47.9 ± 5.2 h). Female field disease prevalence (19.7 ± 3.5%) was higher than that of males (14.3 ± 2.6%), especially in populations with a high disease incidence. Continuing physical connection during fruit ripening appears to increase invasion time and thus the per‐contact infection risk in females. This is consistent with higher female field prevalences, although other explanations, unrelated to disease transmission, are possible. These results illustrate how interactions between plant reproductive behaviour and pollinator activity may affect disease spread. Female mating behaviour may evolve towards lower attractiveness to pollinators to minimize infectious contacts, while males can afford to be more promiscuous with an attractive, but disposable, floral display.
Community assembly from the local species pool: an experimental study using congeneric species pairs
Authors: Tofts Richard , Silvertown Jonathan 
Published: March 2003

Summary 1	Vegetation–environment relationships are well understood at the geographical scale, but it is still not clear what role chance and historical contigency play in the assembly of communities from the local species pool. We addressed this question experimentally by comparing the performance of congeneric species introduced into a grassland community. 2 We used the National Vegetation Classification and distribution maps to define the local pool of grassland species suited to our experimental site. From this we selected five pairs of congeneric species. One member of each pair was native to the experimental site (‘present’ species) and one member was represented in the local species pool but absent from the community (‘absent’ species). 3 If community assembly processes are deterministic, then absent species should underperform present congeners. Any other result would imply that chance, dispersal limitation or historical contingency plays a role in community assembly. Our experimental arrangement allowed us to assess the consistency of comparisons across different grazing treatment combinations. 4 Two field experiments were undertaken. The first investigated the effects of dispersal limitation by introducing seed of congeners and monitoring performance. The second investigated the survival of congeners introduced as small plants at the early stages of growth in ‘plugs’. A laboratory germination test and a combined analysis of both field experiments were also undertaken. 5 Some species showed markedly different responses from their congeners in the two field experiments. Overall, the present species did not consistently perform better than their absent congeners. This indicates that the study community bears the strong imprint of dispersal limitation or other historical factors, a finding consistent with results previously obtained for the same community using a trait‐based model. 6 We conclude that the accuracy of predictions about community composition will be limited by the success with which factors such as propagule dispersal and differential effects of seed predation and herbivory can be modelled. We predict the circumstances under which modelling of community assembly is most likely to be successful.
Contrasting effects of grazing and hay cutting on the spatial and genetic population structure of Veratrum album, an unpalatable, long‐lived, clonal plant species
Authors: Kleijn David , Steinger Thomas 
Published: March 2003

Summary 1  Vegetation change induced by large herbivores is driven by the effects of grazers on populations of individual plant species. Short‐term experimental or demographic studies may be insufficient when investigating the population responses of long‐lived clonal plant species. 2
We therefore examined the effects of grazing on such a plant (Veratrum album) by comparing the spatial and genetic structure of populations in grasslands subject to long‐term grazing or mowing for hay. 3

V. album is a locally dominant species that is avoided by large herbivores due to its toxicity. RAPD‐phenotypes of a subsample of c. 50 shoots, and co‐ordinates and dry weight of all shoots, were determined in a 5 × 10 m plot in each of four meadow and four pasture populations. 4
The breeding system of the genus Veratrum was previously unknown but our experimental finding that cross‐pollinated but not self‐pollinated or unpollinated flowers produced as many seeds as freely pollinated flowers suggested that V. album is a predominantly cross‐pollinating species. 5  Both the spatial and genetic population structure differed markedly between the two grassland types. Clonal expansion of established plants in pastures led to populations consisting of larger shoots that were significantly more aggregated at a small spatial scale. Populations also had a higher proportion of flowering shoots, less seedling recruitment and a lower genotypic diversity in pastures than hay meadows. 6
The differences in population structure appear to be due to hay meadow populations reproducing primarily by seeds, whereas clonal reproduction accounts for half of the population growth in pastures. We suggest that, as livestock selectively avoids V. album shoots, grazing indirectly promotes plant growth, which results in an enhanced vegetative reproduction as well as a higher seed production. Experimental studies are, however, needed to determine why and how grazing adversely affects seedling recruitment. 7  Detailed information on population level responses of unpalatable dominant plant species, such as provided by the present study, may help us understand and predict vegetation change in response to changing levels of herbivory.
Modelling individual growth and competition in plant populations: growth curves of Chenopodium album at two densities
Authors: Damgaard Christian , Weiner Jacob , Nagashima Hisae 
Published: March 2003

Summary

We modelled the growth in estimated biomass of individuals in experimental populations of Chenopodium album grown at two densities and measured sequentially nine times over 128 days. Competition is modelled by coupling individual growth equations and, within the population, the growth rate of a plant at any point in time is a function of its size to the power a, a measure of the degree of size‐asymmetry of competition.
The growth of individuals in these crowded populations was significantly better fit by a Richards growth model than by models with one fewer parameter (e.g. logistic or Gompertz growth models). The additional parameter determines the location of the inflection point and provides great flexibility in fitting growth curves. Density had a significant effect on this parameter.
At the higher density, the maximum size that plants achieved was decreased and the exponential phase of growth was reduced. The estimate of the size‐asymmetry parameter a was always greater than 1 and it increased significantly at the higher density. Growth was reduced and the number of very small individuals increased at the higher density, although a few plants still achieved a large size.
Our approach combines several recent innovations in the modelling of stand development, including (a) modelling of individual growth with biologically meaningful growth models and (b) modelling the relationship between size and growth of individuals within the population. Sequential, non‐destructive data on the growth of individuals over time, in combination with modern statistical computing techniques, can lead to major advances in the modelling of plant population development, providing powerful tools for exploring variation in individual growth and for testing a wide range of hypotheses.