Co-occurring Plant Species Research Articles

Decomposition and nitrogen transformation rates in a temperate grassland vary among co-occurring plant species

Decomposition of organic matter varies depending upon interactions between the composition of the organic matter and the source of the microbial community, with differences in these interactions among vegetation types leading to the Home Field Advantage (HFA) hypothesis whereby decomposition of litters is faster in soils previously conditioned by them. It is possible that HFA operates on smaller scales within plant communities with ecosystem processes responding to subtle changes of plant community dominance. Using field measurements and laboratory incubations, we found a strong plant species effect on nitrogen availability and transformations and the relative importance of autotrophic and heterotrophic processes to nitrification. We found that the origin of the soil microbial community had little influence on litter decomposition when litter quality was high but was important with low-quality litter, most of which was root material. Our results demonstrate that plant species identity has a substantial impact on both litter decomposition and N cycling even within a single vegetation type and on an extremely local scale via both litter chemistry and specificity of the associated soil microbial community. Therefore, changes in botanical composition could alter decomposition and nutrient release altering ecosystem productivity and carbon sequestration potential.

Dynamic niche sharing in dry acidic grasslands -a 15N-labeling experiment

In this study we investigated the temporal variability of N-source utilization of pioneer plant species in different early successional stages of dry acidic grasslands. Current theory states that plant species occupy distinct ecological niches and that there are species-specific, temporal N-uptake patterns. We hypothesized that small-scale dynamics in the natural habitat may affect niche differentiation among plant species. We investigated N-uptake patterns of two co-occurring plant species from different functional groups (Corynephorus canescens, Rumex acetosella) under natural conditions using 15N-labeled nitrate and ammonium in three different early successional stages during early and late summer. We found (1) marked seasonal dynamics with respect to N-uptake and N-source partitioning, and (2) different uptake rates across successional stages but a similar N-form utilization of both species. Nitrate was the main N-source in the early and later successional stages, but a shift towards enhanced ammonium uptake occurred at the cryptogam stage in June. Both species increased N-uptake in the later successional stage in June, which was associated with increasing plant biomass in C. canescens, whereas R. acetosella showed no significant differences in plant biomass and root/shoot-ratio between successional stages. Ammonium uptake decreased in both species across all stages with increasing drought. Nevertheless, the peak time of N-uptake differed between the successional stages: in the early successional site, with the lowest soil N, plants were able to extend N-uptake into the drier season when uptake rates in the other successional stages had already declined markedly. Hence, we found a pronounced adjustment in the realized niches of co-occurring plant species with respect to N-uptake. Our results indicate that ecological niches can be highly dynamic and that niche sharing between plant species may occur instead of niche partitioning.

Intra- and interspecific plant-soil interactions, soil legacies and priority effects during old-field succession

1. Legacy effects of plant influences on abiotic and biotic soil properties can result in priority effects that influence the structure and composition of plant communities. To better understand the role of these plant–soil interactions, here we expand the concept of plant–soil feedbacks from a within-species approach (intraspecific plant–soil feedback) to a between-species approach (interspecific plant–soil interactions). 2. In a greenhouse experiment, we tested how the early successional Jacobaea vulgaris affects its own performance and the performance of 30 co-occurring plant species via changes in abiotic and biotic soil conditions. In addition, we examined the reciprocal effect of the co-occurring species on J. vulgaris. 3. Our study had three important results. First, J. vulgaris exhibits strong negative plant–soil feedback. Secondly, there were large differences among the co-occurring species in interspecific plant–soil effects on J. vulgaris growth. Approximately, half the species reduced J. vulgaris performance, whereas the other half had no effect. Thirdly, soil conditioned by J. vulgaris had a positive or neutral effect on the growth of the co-occurring species. 4. To test the soil effects of entire plant communities, in 10 old-fields that differed in time since abandonment we recorded the identity of all plants surrounding J. vulgaris individuals. We calculated the weighted soil effect of this community on J. vulgaris and the reciprocal effect of J. vulgaris on the community. There was a positive linear relationship between time since abandonment and the weighted feedback effect of J. vulgaris on the plant community. 5. We suggest three mechanisms how the legacy of plant–soil interactions may enhance the rate of succession through priority effects: early successional plant species exert negative plant–soil feedback; co-occurring plant species cause negative interspecific plant–soil effects to the early successional species; and the early successional species have overall positive interspecific plant–soil effects on the co-occurring plant species. 6. Synthesis. The performance of an early successional species can be reduced directly by the legacy effects of intraspecific plant–soil feedback, as well as indirectly by the legacy effects of both intra- and interspecific plant–soil interactions. These intra- and interspecific plant–soil interactions can prioritize transitions of plant species in plant communities.

Impact on soil quality of a 10-year-old short-rotation coppice poplar stand compared with intensive agricultural and uncultivated systems in a Mediterranean area

Bioenergy crops play an ecologically and economically fundamental role as an alternative to agri-food productions and as renewable energy sources. Little attention has been focused on soil quality following conversion of agricultural lands to biomass crops. Here, we assessed the impact of a 10-year-old short-rotation coppice (SRC) poplar stand on the main soil chemical parameters, microbial biomass carbon, soil respiration, and arbuscular mycorrhizal fungi (AMF), compared with intensive agricultural and uncultivated systems. Three different harvest frequencies of poplar SRC (annual T1, biannual T2 and triennial T3 cutting cycles) were evaluated. Multivariate analysis showed that poplar SRC improved soil quality compared with intensive agricultural and uncultivated systems. T1 and T2 positively affected AMF inoculum potential and root colonisation of a co-occurring plant species, while T3 improved the majority of soil chemical and biochemical parameters. Moreover, three different AMF morphospecies belonging to the genera Glomus and Scutellospora were found in poplar SRC, while morphospecies belonging exclusively to genera Glomus were recorded in intensive agricultural and uncultivated systems. Such aspects have agro-ecological implications, since the positive changes of soil nutrient availability and carbon content together with a high abundance and diversity of soil biota show clear soil sustainability of poplar SRC.

Evidence for ecological matching of whole AM fungal communities to the local plant–soil environment

The range of ecological roles exhibited by arbuscular mycorrhizal (AM) fungi depends on functional differences among naturally occurring local assemblages of AM species. While functional differences have been demonstrated among AM fungal species and among geographic isolates of the same species, almost nothing is known about functional differences among whole communities of naturally occurring AM fungi. In the greenhouse, we reciprocally transplanted whole AM fungal communities between plant-soil systems representing a serpentine grassland and a tallgrass prairie, using as hosts two grasses common to both systems. For Sorghastrum nutans, native fungi consistently enhanced plant growth more than fungi switched from the alternate system. For Schizachyrium scoparium, foreign and native fungi promoted plant growth similarly in both the serpentine and prairie systems. Thus, the use of foreign inoculum in restoration could change the relative performance, and potentially the competitive abilities, of co-occurring plant species. Moving AM fungal inocula into foreign environments also caused changes in the taxonomic composition of the resultant spore communities, demonstrating their response to environmental influences. These results provide strong evidence for functional differences among naturally occurring AM communities and suggest that a particular AM fungal community may be better matched ecologically to its local habitat than communities taken from other locations.

Carry-over effects of bumblebee associative learning in changing plant communities leads to increased costs of foraging

Flower visitors learn to avoid food-deceptive plants and to prefer rewarding ones by associating floral cues to rewards. As co-occurring plant species have different phenologies, cue-reward associations vary over time. It is not known how these variations affect flower visitors’ foraging costs and learning. We trained bumblebees of two colonies to forage in a community of deceptive and rewarding artificial inflorescences whose flower colours were either similar or dissimilar. We then modified the community composition by turning the rewarding inflorescences into unrewarding and adding rewarding inflorescences of a novel flower colour. In the short term, bees trained to similar rather than dissimilar inflorescences experienced higher costs of foraging (decreased foraging speed and accuracy) in the novel community. The colonies differed in their speed-accuracy trade-off. In the longer term, bees adapted their foraging behaviour to the novel community composition by increasingly visiting the novel rewarding inflorescences.

PLANT–SOIL FEEDBACK: TESTING THE GENERALITY WITH THE SAME GRASSES IN SERPENTINE AND PRAIRIE SOILS

Plants can alter soil properties in ways that feed back to affect plant performance. The extent that plant-soil feedback affects co-occurring plant species differentially will determine its impact on plant community structure. Whether feedback operates consistently across similar plant communities is little studied. Here, the same grasses from two eastern U.S. serpentine grasslands and two midwestern tallgrass prairie remnants were examined for plant-soil feedback in parallel greenhouse experiments. Native soils were homogenized and cultured (trained) for a year with each of the four grasses. Feedback was evaluated by examining biomass variation in a second generation of (tester) plants grown in the trained soils. Biomass was lower in soils trained by conspecifics compared to soils trained by heterospecifics in seven of 15 possible cases; biomass was greater in conspecific soils in one other. Sorghastrum nutans exhibited lower biomass in conspecific soils for all four grasslands, so feedback may be characteristic of this species. Three cases from the Hayden prairie site were explained by trainer species having similar effects across all tester species so the relative performance of the different species was little affected; plants were generally larger in soils trained by Andropogon gerardii and smaller in soils trained by S. nutans. Differences among sites in the incidence of feedback were independent of serpentine or prairie soils. To explore the causes of the feedback, several soil factors were measured as a function of trainer species: nutrients and pH, arbuscular mycorrhizal (AM) spore communities, root colonization by AM fungi and putative pathogens, and functional diversity in bacterial communities as indicated by carbon substrate utilization. Only variation in nutrients was consistent with any patterns of feedback, and this could explain the greater biomass in soils trained by A. gerardii at Hayden. Feedback at Nottingham (one of the serpentine sites) differed, most notably for A. gerardii, from that of similar past studies that used different experimental protocols. To understand the consequences of feedback for plant community structure, it is important to consider how multiple species respond to the same plant-induced soil variation as well as differences in the feedback detected between greenhouse and field settings.

Mycorrhizal source and neighbour identity differently influence Anthemis cotula L. invasion in the Kashmir Himalaya, India

Whilst the importance of arbuscular mycorrhizal fungi (AMF) in invasive spread of some alien plant species has been recently established, it is unclear how such plants respond to AMF from different geographical sources. Besides, influence of different co-occurring plant species on the AMF-mediated invasiveness of alien species, though rarely investigated, is pivotal to the explicit understanding of the feedback between arbuscular mycorrhizal fungi and alien plant invasions. We therefore examined the influence of local (Kashmir Himalyan isolates) and foreign (isolates from Rajasthan, India) AMF on invasiveness of alien Anthemis cotula vis-à-vis the effect of three common co-occurring neighbours, Conyza canadensis, Galinsoga parviflora and Sisymbrium loeselii. Field studies revealed high incidence of Arum-type mycorrhizal colonization in natural populations of A. cotula and the pot trials confirmed the reliance of its invasiveness on AMF with the more favourable effect of local than foreign AMF. The mycorrhizal colonization intensity in field populations of A. cotula was strongly influenced by neighbour identity with major reduction recorded in the presence of S. loeselii (a cruciferous non-host) in comparison to other con-familial neighbours. Pot experiments confirmed the differential effect of co-occurring species on A. cotula's invasive traits. These results signify that the facilitative role of AMF communities in invasion by some alien plants is contingent upon neighbour identity and the present approach provides not only a framework for further examination of the invasive plant–soil biota feedback, but also many important management implications.

Variation in the guild composition of herbivorous insect assemblages among co-occurring plant species

Variation in plant traits among plant species may promote the development of a characteristic functional assemblage of insect herbivores associated with each plant species. However, only a small number of studies have detailed the representation of several herbivore guilds among co-occurring plant species to determine whether the functional structure of herbivorous insect assemblages varies widely and consistently among plant species. The present study provides one of the few published data sets reporting on the density of several guilds of insect herbivores among numerous plant species. Variation in guild associations with plant phenology and season are also described. Insect herbivores were divided into 10 guilds, and the representation of these guilds was examined for 18 co-occurring plant species. Guild densities and assemblage composition varied significantly among plant species, even when variation over time was taken into account. Variation in guild densities and assemblage composition were not strongly related to the taxonomic relationships of the plants. The highest densities of several guilds occurred in spring and summer, although other guilds were not strongly seasonal. Certain guilds were strongly associated with the presence of new leaves, whereas other guilds appeared to prefer mature leaves. This resulted in assemblage differences between samples containing new and mature leaves and samples containing mature leaves only. Even though the timing and duration of leaf and flower production varied among plant species, this did not explain all variation in guild densities among plant species. It is suggested that additional factors, including plant traits, are contributing to the wide and consistent variation in herbivore assemblage composition among plant species.

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

Plants may experience reduced reproductive success at low densities, due to lower numbers of pollinator visits or reduced visit quality. Co-occurring plant species that share pollinators have the potential to facilitate pollination by either increasing numbers of pollinator visits or increasing the quality of visits, but also have the potential to reduce plant reproductive success through competition for pollination. I used a field experiment with a common distylous perennial (Piriqueta caroliniana) in the presence and absence of a co-flowering species (Coreopsis leavenworthii) in plots with one of four different distances between conspecific plants. I found strong negative effects of increasing interplant distance (related to conspecific density) on several components of P. caroliniana reproductive success: pollinator visits to plants per plot visit, visits received by individual plants, conspecific pollen grains on stigmas, outcross pollen grains on stigmas, and probability of fruit production. Although P. caroliniana and C. leavenworthii share pollinators, the co-flowering species did not affect visitation, pollen receipt or reproductive effort in P. caroliniana. Pollinators moved very infrequently between species in this experiment, so floral constancy might explain the lack of effect of the co-flowering species on P. caroliniana reproductive success at low densities. In co-occurring self-incompatible plants with floral rewards, reproductive success at low density may depend more on conspecific densities than on the presence of other species.

Allelopathic effects of Ceratiola ericoides (Empetraceae) on germination and survival of six Florida scrub species

Allelopathic inhibition of germination by Florida scrub plants has been demonstrated in the greenhouse and lab, but not in the field. We studied the allelopathic effects of Florida rosemary (Ceratiola ericoides) roots, leaves, and litter leachates on field germination and three-month survival of six Florida scrub species, three habitat generalists (Lechea deckertii, Palafoxia feayi, and Polygonella robusta), and three rosemary scrub specialists (Hypericum cumulicola, Lechea cernua, and Polygonella basiramia). We used AIC and model averaging to evaluate support for a series of non-exclusive hypotheses. Species varied in germination (2.7–24.6%) and survival (39.2–71%) percentages, and in their sensitivity to leachates. Germination of scrub species was most negatively affected by leaf > root > litter leachates, although not all species followed the overall trend. Additional germination suppression by leachate combinations (relative to single leachates) was minimal. Sites did not vary in germination, but seedling survival did differ among sites. This study further documents the negative impact of Florida rosemary leachates on the germination of co-occurring plant species. Allelopathy may be partly responsible for bare sand gaps in Florida rosemary scrub, and therefore be one of the forces structuring Florida rosemary scrub ecosystems.

First report of Carex macrocephala in eastern North America with notes on its co-occurrence with Carex kobomugi in New Jersey1

Abstract The discovery of three populations of the large-headed sedge, Carex macrocephala, in New Jersey described in this paper, represents the first report of this species from Eastern North America. Carex macrocephala grows at much lower densities in New Jersey than does the closely related Asiatic sand sedge, Carex kobomugi. We thus expected invasion by C. macrocephala to have less impact of on the abundance and diversity of native dune plants than have been seen in previous studies of the closely related invasive sedge, C. kobomugi. This expectation was largely supported: the presence of C. macrocephala in New Jersey's coastal dunes had no significant impact on the abundance of co-occurring plant species. Indeed, species richness and diversity were actually significantly higher in areas occupied by C. macrocephala than in surrounding areas. New Jersey appears to be one of the few places in the world where C. macrocephala grows in close proximity to C. kobomugi, suggesting the possibility of hybridiza...

Different native arbuscular mycorrhizal fungi influence the coexistence of two plant species in a highly alkaline anthropogenic sediment

Different species of arbuscular mycorrhizal fungi (AMF) can produce different amounts of extraradical mycelium (ERM) with differing architectures. They also have different efficiencies in gathering phosphate from the soil. These differences in phosphate uptake and ERM length or architecture may contribute to differential growth responses of plants and this may be an important contributor to plant species coexistence. The effects of the development of the ERM of AMF on the coexistence of two co-occurring plant species were investigated in root-free hyphal chambers in a rhizobox experimental unit. The dominant shrub (Salix atrocinerea Brot.) and herbaceous (Conyza bilbaoana J. Remy) plant species found in a highly alkaline anthropogenic sediment were studied in symbiosis with four native AMF species (Glomus intraradices BEG163, Glomus mosseae BEG198, Glomus geosporum BEG199 and Glomus claroideum BEG210) that were the most abundant members of the AMF community found in the sediment. Different AMF species did not influence total plant productivity (sum of the biomass of C. bilbaoana and S. atrocinerea), but had a great impact on the individual biomass of each plant species. The AMF species with greater extracted ERM lengths (G. mosseae BEG198, G. claroideum BEG210 and the four mixed AMF) preferentially benefited the plant species with a high mycorrhizal dependency (C. bilbaoana), while the AMF species with the smallest ERM length (G. geosporum BEG199) benefited the plant species with a low mycorrhizal dependency (S. atrocinerea). Seed production of C. bilbaoana was only observed in plants inoculated with G. mosseae BEG198, G. claroideum BEG210 or the mixture of the four AMF. Our results show that AMF play an important role in the reproduction of C. bilbaoana coexisting with S. atrocinerea in the alkaline sediment and have the potential to stimulate or completely inhibit seed production. The community composition of native AMF and the length of the mycelium they produce spreading from roots into the surrounding soil can be determinant of the coexistence of naturally co-occurring plant species.

Vegetation–environment relationships and ecological species groups of an Arizona Pinus ponderosa landscape, USA

Pinus ponderosa forests occupy numerous topographic and soil complexes across vast areas of the southwestern United States, yet few data exist on species distributions and vegetation–environment relationships for these environmentally diverse landscapes. We measured topography, soils, and vegetation on 66, 0.05-ha plots within a 110,000-ha P. ponderosa landscape in northern Arizona, USA, to discern vegetation–environment relationships on this landscape. We analyzed associations of environmental variables with plant communities and with single-species distributions, and we classified ecological species groups (co-occurring plant species exhibiting similar environmental affinities). Gradients in community composition paralleled gradients in soil texture, available water, organic C, total N, and geographic precipitation patterns. Soil parent material, affected by the presence or absence of volcanic activity, is a primary factor constraining vegetation patterns on this landscape. Using discriminant analysis, we built a model that correctly classified the most important of four grasses (Bouteloua gracilis, Muhlenbergia montana, Sporobolus interruptus, or Festuca arizonica) on 70–80% of plots based on five environmental variables related to soil moisture and resource levels. We also classified 52 of the 271 detected plant species into 18 ecological species groups. Species groups ranged from Phacelia and Bahia groups occupying xeric, volcanic cinder soils low in organic C and total N, to Festuca and Lathyrus groups characterizing moist, loam and silt loam soils. We applied the species groups by estimating P. ponderosa diameter increment in a regression tree using abundances of species groups. The most rapid P. ponderosa diameter growth of 5 mm/year occurred on plots with high importance of the Festuca and Lathyrus groups. Our results on this semi-arid landscape support several general ecological species group principles chiefly developed in temperate regions, and suggest that vegetation–environment research has great potential for enhancing our understanding of P. ponderosa forests occupying vast areas of the southwestern United States.

Vorkommen der Kleinen Teichrose Nuphar pumila und des Hybrids N. x intermedia in der Schweiz

Kozlowski G. and Eggenberg S. 2005. Distribution of the least water-lily (Nuphar pumila) and the hybrid N. x intermedia in Switzerland. Bot. Helv. 115: 125–136. The least water-lily (N. pumila (TIMM.) DC) is very rare and endangered in Switzerland and Central Europe. To provide a basis for its conservation, we present a distribution map with all natural, re-introduced and extinct populations in Switzerland. Only four natural populations of N. pumila still exist in Switzerland: Lac de Lussy and Lac des Joncs (FR), Grappelensee (SG), and Kammoosteich (ZH). These populations and their sites (lakes) are described, and the co-occurring plant species are listed. Results suggest that the populations from Kammoosteich and Lac de Lussy are affected by hybridisation and competition with N. lutea. In these two lakes, the hybrid N. x intermedia was very abundant. The presence of N. lutea and N. intermedia in Kammoosteich and Lac de Lussy should be regarded as one of the most serious threats of pure N. pumila populations. To facilitate further investigation and conservation field work, a detailed morphological description of the two taxons is presented and illustrated by original drawings. To escape extinction in Switzerland, N. pumila needs further appropriate action plans for all natural and newly re-introduced sites as well as an improved knowledge of its genetic structure and habitat preferences.

Contrasted effects of water limitation on leaf functions and growth of two emergent co-occurring plant species, Cladium mariscus and Phragmites australis

Phragmites australis tolerates a wider range of ecological conditions than Cladium mariscus and thus may be better adapted to disturbance resulting from human manipulations. We hypothesised that the difference in ecological conditions was related to differences in photosynthetic ability under water stress. We compared the reactions of the two species to water deficit related to the summer drawdown in the Mediterranean regions that may be enhanced by drainage for human activities. Stress tolerance was evaluated in a pot experiment measuring pigment contents, chlorophyll fluorescence and gas exchanges, growth rates, living and dead biomass, for plants grown in waterlogged conditions or submitted to permanent or temporary drainage. Reed was favoured by the first days of drainage with an increase in CO 2 assimilation by approximately 5 μmol m −2 s −1 and an increase of the elongation rates from 1 to 1.5 cm d −1, indicating waterlogged conditions were not optimal for this species. While pigment content showed no significant effects, growth rate, percentage live above-ground biomass, chlorophyll fluorescence and net rate of CO 2 assimilation declined significantly for both species from days 5 to 7 after drying started. Assimilation rates reached 2.3 and 0.3 μmol m −2 s −1 on day 9 for sawsedge and reed, respectively. Sawsedge had a 50% greater assimilation rate in the waterlogged conditions, but its responses to water loss in terms of declining growth rate and CO 2 assimilation started on day 5 versus 7 for reed. By contrast, the condition of PSII as indicated by chlorophyll fluorescence was affected more in reed. Both growth rates and rates of CO 2 assimilation had completely recovered for both species within 3 days of re-wetting. Our results reflect the evergreen character of sawsedge leaves that are hardly damaged by stress but decrease their activity. Reed leaves die upon excessive stress and plants may shed leaf as an adjustment to stress. Sawsedge photosynthesis will be optimal in waterlogged conditions whereas that of reed will be constant within a broad range from slightly drained to mild stress. Those responses to drainage at the leaf scale may partly explain the difference in habitat where both species are usually found.

The biomass and nutrient levels of Calamagrostis canadensis and Carex stricta under different hydrologic and fungicide regimes

We examined whether fungicide and the subsequent reduction of soilborne pathogenic fungi would differentially enhance the productivity and foliar nutrient content of two coexisting species, Calamagrostis canadensis (Michx.) Beauv. and Carex stricta Lam. This was tested under hydrologic regimes that simulated those in prairie wetlands and included a 32-d cycle (flooded 16 d, dry 16 d), 6-d cycle (flooded 3 d, dry 3 d), flooded, well-watered, and dry hydroperiods. Calamagrostis canadensis biomass increased 26%–45% in the wet hydroperiods (6-d cycle, well watered, and flooded) when fungicide was applied but remained fairly constant over all hydroperiods in nonfungicide treatments. Calamagrostis canadensis grown in the wet hydroperiods without fungicide produced the same biomass and growth rates as plants treated with fungicide in the dry hydrologic regime, suggesting that pathogenic fungi in wet hydrologic regimes have the same effect as major environmental stresses such as drought. In contrast, the biomass and growth rate of Carex stricta generally did not vary significantly with fungicide treatment. The only exception was in the rapidly alternating hydroperiod (6-d cycle), where Carex stricta treated with fungicide produced 48% more biomass and grew 46% faster than plants not treated with fungicide. Mean concentrations of foliar phosphorus generally were not significantly different between the fungicide and nonfungicide treatments for either plant species, while foliar nitrogen concentrations were higher in both species when treated with fungicide in the 32-d cycle, dry, and well-watered hydroperiods. The effect of fungicide on the biomass and foliar nutrients of these two co-occurring plant species depended on the species and the hydrologic regime, and our results suggest that seasonal and interannual changes in hydrologic regimes may confer a temporary advantage to one species or the other that, over the long term, allow them to coexist.Key words: benomyl fungicide, foliar nitrogen, foliar phosphorus, hydroperiod, marsh reed grass, wetland.

Do co-occurring plant species adapt to one another? The response of Bromus erectus to the presence of different Thymus vulgaris chemotypes.

Local modification of the soil environment by individual plants may affect the performance and composition of associated plant species. The aromatic plant Thymus vulgaris has the potential to modify the soil through leaching of water-soluble compounds from leaves and litter decomposition. In southern France, six different thyme chemotypes can be distinguished based on the dominant monoterpene in the essential oil, which is either phenolic or non-phenolic in structure. We examine how soils from within and away from thyme patches in sites dominated by either phenolic or non-phenolic chemotypes affect germination, growth and reproduction of the associated grass species Bromus erectus. To do so, we collected seeds of B. erectus from three phenolic and three non-phenolic sites. Seeds and seedlings were grown on soils from these sites in a reciprocal transplant type experiment in the glasshouse. Brome of non-phenolic origin performed significantly better on its home soil than on soil from a different non-phenolic or a phenolic site. This response to local chemotypes was only observed on soil collected directly underneath thyme plants and not on soil in the same site (<5 m away) but where no thyme plants were present. This is preliminary evidence that brome plants show an adaptive response to soil modifications mediated by the local thyme chemotypes. Reproductive effort was consistently higher in brome of phenolic origin than in brome of non-phenolic origin (on both thyme- and grass-soil), indicating that life-history variation may be related to environmental factors which also contribute to the spatial differentiation of thyme chemotypes. Moreover, we found that brome growing on thyme-soil in general was heavier than when growing on grass-soil, regardless of the origin of the brome plants. This is concordant with thyme-soil containing higher amounts of organic matter and nitrogen than grass-soil. Our results indicate that patterns of genetic differentiation and local adaptation may modify competitive interactions and possible facilitation effects in natural communities.

Multivariate regression trees for analysis of abundance data.

Multivariate regression tree methodology is developed and illustrated in a study predicting the abundance of several cooccurring plant species in Missouri Ozark forests. The technique is a variation of the approach of Segal (1992) for longitudinal data. It has the potential to be applied to many different types of problems in which analysts want to predict the simultaneous cooccurrence of several dependent variables. Multivariate regression trees can also be used as an alternative to cluster analysis in situations where clusters are defined by a set of independent variables and the researcher wants clusters as homogeneous as possible with respect to a group of dependent variables.

The Demographics of Leaves

ECOLOGY/EVOLUTION Leaf life-span (LLS) is a key functional trait of plants, yet co-occurring plant species often display a wide range of LLS. Documenting these patterns and relating them to variation in other traits and external ecological factors help to elucidate the range of ecological strategies