Early Successional Plant Research Articles

Early post-fire plant succession in Peninsula Sandstone Fynbos: The first three years after disturbance

The early post-fire plant succession in fynbos vegetation in the Mediterranean-type climate area of South Africa was studied. Relatively little has been published on this early stage of plant succession in fynbos. Annual sampling over the first three post-fire years confirmed a steady, but relatively slow increase in plant canopy cover of shrubs and graminoids (mainly Restionaceae), whereas cover of geophytes and other herbs peaked in the first year and declined significantly, thereafter. Cover of annual plants increased each year, which may relate to the persistence of a relatively open vegetation cover by the third year. The responses of reseeder and resprouter species of the Restionaceae to the post-fire environment appeared to be habitat dependent. Cover of the reseeders increased rapidly in seep areas, but their recovery was distinctly delayed in dryland areas outside the seeps. Re-establishment of the many reseeder Erica species appeared to be delayed until the second post-fire year. Seed banks of these species were possibly negatively impacted by the fire, and required dispersal of seed from unburnt areas for recruitment. In contrast to some current generalisations, species richness appeared to increase after the fire; less certainly from the first to the second year, but more certainly from the second to the third year. Therefore, this study does not support a short-term monotonic decline in species richness after fire in fynbos.

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.

The soil seed bank and its relationship to the established vegetation in urban wastelands

Industrial and traffic areas are particularly characteristic of the urban environment. Due to frequent soil transport and vegetation disturbance, soil seed banks and seed production play an essential role for vegetation establishment in these sites. Since researchers have scarcely focussed on these traits, it was analysed in three railway and loading areas in Munich. Seed numbers in soil ranged from 1300 m −2 on ruderalized calcareous grassland at Virginia Depot to 2800 m −2 on the rail track beds in Steinhausen and 7600 m −2 on ballast and calcareous gravel at Gleislager Neuaubing. The corresponding annual seed production was 17 000, 50 600 and 49 400 seeds m −2, respectively. The seed bank of all three sites was dominated by biennial and perennial ruderals, and species naturally growing on rocky or sandy habitats. Grassland species and woody plants frequently grew in the established vegetation but scarcely occurred in the soil samples. The abundance of invasive alien species in the seed bank significantly exceeded their share in the vegetation. The seed banks also harboured endangered plants. Differences between the seed bank and the established vegetation reflect ongoing succession. While the seed bank was mainly composed of persistent seeds of early-successional plants, the established vegetation was dominated by long-lived species with transient seeds that increasingly suppress regeneration of the short-lived species. These results imply that recurring soil disturbance could re-activate the seed bank of both alien and endangered plants. Consequences for vegetation management and nature conservation are discussed.

Inoculation with a Native Soil Community Advances Succession in a Grassland Restoration

Restoration on post‐agricultural land may be hindered by the degradation of the soil community, which has been shown to contribute to structuring plant communities and driving succession. Our experiment tested the effect of inoculation with remnant grassland whole soil with or without nurse plants on the survival and growth of uninoculated early and late successional plant species. In 2007 and 2008, we planted uninoculated early, mid, and late successional plant species 0.25–2 m away from a central point of inoculated nurse plants. We found a negative response to inoculation on early successional plants and a positive response to inoculation on mid to late successional plants. This work suggests that the restoration of the soil community is critical to establishing a late successional plant community and that the benefit of inoculated plants can spread to neighbors.

Predicting root defence against herbivores during succession

Summary 1. Root herbivores and pathogens interfere with basic below‐ground plant function, and can thereby affect plant fitness and spatial and temporal patterns in natural plant communities. However, there has been little development of concepts and theories on below‐ground plant defence, a deficit that is in contrast to the abundance of theorizing for above‐ground plant parts. 2. A review of the past 10 years of research on below‐ground plant–herbivore interactions has revealed that, similar to above‐ground tissues, root defences can be expressed constitutively or induced upon herbivore attack, and can be classified into direct and indirect traits, tolerance, and escape. Indeed, it has been shown that roots tolerate herbivory by outgrowing or re‐growing lost tissues, or resist it by producing secondary metabolites that are toxic to herbivores or attract natural enemies of herbivores. 3. We propose that, similar to above‐ground plant–herbivore theories, the partition of abiotic and biotic factors over ecological succession can serve as the basis for predicting investment in defence strategies below‐ground. 4. Investigation of herbivore pressure and root responses along primary and secondary successional gradients suggests that: (i) roots are often fast growing, thinner and softer in early compared to later succession. (ii) Insect and nematode herbivore pressure increases until mid‐succession and later decreases. (iii) Mycorrhizal abundance increases with succession, and the composition of fungal species changes through succession, often shifting from arbuscular mycorrhizae to ecto‐mycorrhizae. 5. Based on these findings, and on classical (above‐ground) plant defence theory, we suggest the following set of testable hypotheses for below‐ground plant defence: (i) During succession, early plants invest most of their resources in growth and less in defences (associated with a general lack of herbivores and pathogens, and with limited availability of resources in the system), therefore relying more on re‐growth (tolerance) strategies. (ii) During mid‐succession, a buildup of herbivore pressure facilitates replacement by plant species that exhibit greater direct and indirect defence strategies. (iii) Constitutive and inducible levels of defences may trade‐off, and early successional plants should rely more on induction of defences after herbivore attack, whereas late successional plants will increasingly rely on constitutively produced levels of physical and chemical defence. (iv) Successional changes in microbial associations have consequences for root defence by improving plant nutrition and defence expression as well as directly competing for root space; however, toxic or impenetrable root defences may also limit association with root symbionts, and so may constrain the expression of root defence.

Quantifying relationships between traits and explicitly measured gradients of stress and disturbance in early successional plant communities

Questions: How can one explicitly quantify, and separately measure, stress and disturbance gradients? How do these gradients affect functional composition in early successional plant communities and to what extent? Can we accurately predict trait composition from knowledge of these gradients? Location: Southern Quebec, Canada. Methods: Using eight environmental variables measured in 48 early successional plant communities, we estimated stress and disturbance gradients through structural equation modelling. We then measured 10 functional traits on the most abundant species of these 48 communities and calculated their community-level mean and variance weighted by the relative abundance of each species. Finally, we related these community-weighted means and variances to the estimated stress and disturbance gradients using general linear models or generalized additive models. Results: We obtained a well-fitting measurement model of the stress and disturbance gradients existing in our sites. Of the 10 studied traits, only average plant reproductive height was strongly correlated with the stress (r2=0.464) and disturbance (r2=0.543) gradients. Leaf traits were not significantly related to either the stress or disturbance gradients. Conclusions: The well-fitting measurement model of the stress and disturbance gradients, combined with the generally weak trait–environment linkages, suggests that community assembly in these early successional plant communities is driven primarily by stochastic processes linked to the history of arrival of propagules and not to trait-based environmental filtering.

Habitat Value of Intensively Established Pine Plantations for Northern Bobwhite

ABSTRACT Declines in northern bobwhite (Colinus virginianus; bobwhite) populations in the southeastern United States may be partially attributable to loss of early successional plant cover associated with greater use of herbicides in forest management. We tested effects of 5 levels of operational plantation establishment intensity on vegetation communities and structure important for bobwhite in 1–5‐year‐old loblolly pine (Pinus taeda) plantations (n = 4) in the Outer Coastal Plain Mixed Forest of Mississippi. We compared results with values reported in the literature to calculate usable space for winter food, loafing, nesting, and brood‐rearing at 2 levels of spatial resolution. Treatments (k = 5) reflected a range of management intensities and were combinations of mechanical site preparation, chemical site preparation (CSP), and herbaceous weed control (HWC). Coverage of winter food plants in the least intensive treatment was more than double that in the most intensive; however, differences in usable space of winter food cover were negligible due to improved accessibility in more intensive treatments. Although CSP reduced coverage of nonpine woody plants across all years, loafing cover reached adequate levels by year 3 in all treatments. Usable nesting cover was <4% across all years and treatments and was nearly eliminated by the reduction in herbaceous cover and visual screening cover following broadcast HWC. Optimal brood‐rearing habitat was virtually absent in all treatments and years due to the lack of conjoint occurrence of bare ground and forb canopy. Although bobwhite habitat may have been promoted by formerly widespread plantation establishment methods that involved wide‐scale soil disturbance, those established using newer methods with less soil disturbance are likely inadequate for most bobwhite habitat requirements. Efforts to provide bobwhite habitat in plantation‐dominated landscapes may have to rely on management of thinned mid‐rotation stands and permanent landscape features such as rights‐of‐way. The value of young plantations for bobwhite may be increased by reducing management intensity or increasing spacing between planting rows, thus increasing time before crown closure and providing opportunity for understory manipulations.

Macroinvertebrate community structure in created wetlands of different successional stage

We analyzed biogeochemical characteristics of two temperate constructed wetlands in early and late spring to evaluate the utility of the macroinvertebrate community as an indicator of ecosystem development. The wetlands were similar in area, hydrologic regime, and influent water quality, but one was younger (15 y) and supported an early successional plant community while the older system (45 y) was forested. To characterize structural habitat in the two wetlands we analyzed water and soil quality, plant community characteristics, coarse woody debris, topographic variance and hydrologic regime. The correlation of macroinvertebrate community characteristics with structural variables was compared among wetlands, and macroinvertebrates were compared temporally within wetlands. Despite differences in nutrient concentration, substrate conditions, plant cover, and microtopography, the overall macroinvertebrate guild structure in the two systems was similar in terms of taxa richness, Simpson diversity, tolerance, and trophic distribution. Total abundance of macroinvertebrates was significantly correlated with a number of structural wetland characteristics, but trophic distribution was correlated only with nutrient availability. Temporal changes were apparent in both wetlands. The two wetlands supported proportionally different guild biomass in early spring, with the forested wetland dominated by grazing herbivores and shredders and the non-forested system by detritivores and predators. In late spring, however, there was no statistical difference in trophic distribution among the two wetlands, as both were dominated by grazing herbivores. These results suggest that macroinvertebrate community characteristics have limited utility as an indicator of successional development in created wetlands, as community characteristics are readily influenced by nutrient availability and temporal changes.

Heterogeneity in mycorrhizal inoculum potential of flood-deposited sediments

Riparian areas are diverse systems where flooding creates new sites for establishment of vegetation. Symbioses with soil microorganisms, such as mycorrhizal fungi, affect vascular plant growth and community composition. It is unknown, however, how mycorrhizal fungi are dispersed along rivers and what potential they have to inoculate roots of plants establishing on recently deposited sedimentary surfaces of flood plains. We measured AMF inocula in sediment deposited by an average spring flood along an expansive riverine flood plain in Montana, USA, to determine whether AMF inocula were present in sediments and what types of propagules (spores, hyphae, or colonized root fragments) contribute to AMF infectivity. Flood-deposited sediments contained sufficient inocula for AMF to colonize host plants (Sorghum sudanense) grown in a greenhouse, and both AMF hyphal lengths and spore densities were correlated with infectivity. Availability of mycorrhizal inocula, which is patchily distributed in this system, may lead to microsites that differ in ability to support establishment and growth of early-successional plants.

Fire Drives Transcontinental Variation in Tree Birch Defense against Browsing by Snowshoe Hares

Fire has been the dominant disturbance in boreal America since the Pleistocene, resulting in a spatial mosaic in which the most fire occurs in the continental northwest. Spatial variation in snowshoe hare (Lepus americanus) density reflects the fire mosaic. Because fire initiates secondary forest succession, a fire mosaic creates variation in the abundance of early successional plants that snowshoe hares eat in winter, leading to geographic variation in hare density. We hypothesize that fire is the template for a geographic mosaic of natural selection: where fire is greatest and hares are most abundant, hare browsing has most strongly selected juvenile-phase woody plants for defense. We tested the hypothesis at multiple spatial scales using Alaska birch (Betula neoalaskana) and white birch (Betula papyrifera). We also examined five alternative hypotheses for geographic variation in antibrowsing defense. The fire-hare-defense hypothesis was supported at transcontinental, regional, and local scales; alternative hypotheses were rejected. Our results link transcontinental variation in species interactions to an abiotic environmental driver, fire. Intakes of defense toxins by Alaskan hares exceed those by Wisconsin hares, suggesting that the proposed selection mosaic may coincide with a geographic mosaic of coevolution.

Hydrologic controls on nitrogen availability in a high-latitude, semi-arid floodplain

Past research shows a discrepancy between apparent nitrogen supply and the annual growth requirements for early successional plant communities along the Tanana River floodplain in interior Alaska. Because previous measurements of nitrogen fixation, mineralization, and deposition can only account for approximately 26% of these communities' nitrogen requirements, other mechanisms of nitrogen supply should be operating. This study examined the potential for subsurface hydrology to supply nitrogen to the rooting zones of early successional plant communities. Three transects of ground water wells were established along the floodplain, and hydrologic characteristics and soil/water chemistry were measured for 2 y. Hydrologic measurements were then used to model the potential nitrogen contributions from advective flow and capillary rise. Ion exchange membranes and stable isotope tracers were used to identify the use of hyporheic nitrogen by vegetation. Nitrogen accumulation on ion exchange membranes showed a large input of nitrogen during a period of high water in 2003. Plants showed uptake of hyporheic 15N-labelled nitrogen at 2 of 4 locations. Models of capillary rise and advective flow estimated that nitrogen supply from subsurface water equalled or exceeded total nitrogen supply from nitrogen mineralization and nitrogen fixation combined. These findings underscore the importance of hyporheic processes in controlling nutrient supply to early successional vegetation on the Tanana River floodplain.

Riverine organic matter and nutrients in southeast Alaska affected by glacial coverage

Dissolved organic matter and nutrients from high-latitude coastal watersheds stimulate microbial activity and primary productivity in near-shore ecosystems. A survey of southeast Alaskan watersheds suggests that the extent of glacial coverage may control the release of these nutrients to rivers and ultimately the oceans. The delivery of fresh water, carbon, nitrogen and phosphorous from high-latitude regional watersheds is important to the ecology and nutrient balance of coastal marine ecosystems in the Northern1 and Southern2 hemispheres. Bioavailable dissolved organic matter from rivers can support microbes in near-shore environments, and may also stimulate primary production3,4. Recent studies suggest that impacts of climate change, such as thawing permafrost, may affect nutrient yields in large northern rivers5. Here we analyse riverine dissolved organic matter and nutrient loads in three adjacent coastal watersheds along the Gulf of Alaska. We find that different levels of glacial coverage can alter the timing and magnitude of fresh water, dissolved organic matter and nutrient yields. Our results suggest that a lower extent of glacial coverage within a watershed can lead to higher amounts of dissolved organic matter, but decreased phosphorous yields. Moreover, an abundance of early successional plant species following deglaciation can increase riverine nitrogen levels. We conclude that changes in riverine yields of dissolved organic matter and nutrients due to reductions in glacier extent in coastal watersheds may affect the productivity and function of near-shore coastal ecosystems.

Effects of Artificial Roosts for Frugivorous Bats on Seed Dispersal in a Neotropical Forest Pasture Mosaic

In the Neotropics ongoing deforestation is producing open and heavily fragmented landscapes dominated by agriculture, mostly plantations and cattle pastures. After some time agriculture often becomes uneconomical and land is abandoned. Subsequent habitat regeneration may be slow because seed inputs are restricted by a lack of incentives--such as suitable roost sites--for seed dispersers to enter deforested areas. Increasing environmental awareness has fostered growing efforts to promote reforestation. Practical and cost-efficient methods for kick-starting forest regeneration are, however, lacking. We investigated whether artificial bat roosts for frugivorous bat species can attract these key seed dispersers to deforested areas, thereby increasing seed rain. We installed artificial bat roosts in a forest-pasture mosaic in the Costa Rican Atlantic lowlands and monitored bat colonization and seed dispersal. Colonization occurred within a few weeks of installation, and 10 species of bats occupied the artificial roosts. Five species of frugivorous or nectarivorous bats colonized artificial roosts permanently in both primary habitat and in deforested areas, in numbers similar to those found in natural roosts. Seed input around artificial roosts increased significantly. Sixty-nine different seed types, mostly of early-successional plant species, were transported by bats to artificial roosts in disturbed habitats. The installation of artificial bat roosts thus successfully attracted frugivorous bats and increased seed inputs into degraded sites. This method is likely to speed up early-vegetation succession, which in turn will attract additional seed dispersers, such as birds, and provide a microhabitat for seeds of mid- and late-successional plants. As well as supporting natural forest regeneration and bat conservation, this cost-efficient method can also increase environmental awareness among landowners.

Can the seed bank be used for ecological restoration? An overview of seed bank characteristics in European communities

AbstractQuestion:Can seeds in the seed bank be considered as a potential source of material for the restoration of European plant communities including forest, marsh, grassland and heathland?Methods:This study reviews seed bank studies (1990–2006) to determine if they provide useful and reliable results to predict restoration success. We formally selected 102 seed bank studies and analyzed differences between four plant community types in several seed bank characteristics, such as seed density, species richness and similarity between seed bank and vegetation. We also assessed the dominant genera present in the seed bank in each plant community.Results:We observed remarkably consistent trends when comparing seed bank characteristics among community types. Seed density was lowest for grassland and forest communities and highest in marshes, whereas species richness, diversity and evenness of the seed bank community was lowest in heathland and highest in grassland. Similarity between seed bank and vegetation was low in forest, and high in grassland. There was a lot of overlap of the dominant genera of seed bank communities in all studies.Conclusions:The absence of target species and the high dominance of early successional species, in particular Juncus spp., indicate that restoration of target plant communities relying only on seed germination from the seed bank is in most cases not feasible. The exceptions are heathland and early successional plant communities occurring after temporally recurring disturbances. Restoration of plant communities composed of late successional species, such as woody species or herbaceous species typical of woodland or forest rely mainly on seed dispersal and not on in situ germination.

The Role of Frugivorous Bats in Tropical Forest Succession

Discussion of successional change has traditionally focused on plants. The role of animals in producing and responding to successional change has received far less attention. Dispersal of plant propagules by animals is a fundamental part of successional change in the tropics. Here we review the role played by frugivorous bats in successional change in tropical forests. We explore the similarities and differences of this ecological service provided by New and Old World seed-dispersing bats and conclude with a discussion of their current economic and conservation implications. Our review suggests that frugivorous New World phyllostomid bats play a more important role in early plant succession than their Old World pteropodid counterparts. We propose that phyllostomid bats have shared a long evolutionary history with small-seeded early successional shrubs and treelets while pteropodid bats are principally dispersers of the seeds of later successional canopy fruits. When species of figs (Ficus) are involved in the early stages of primary succession (e.g. in the river meander system in Amazonia and on Krakatau, Indonesia), both groups of bats are important contributors of propagules. Because they disperse and sometimes pollinate canopy trees, pteropodid bats have a considerable impact on the economic value of Old World tropical forests; phyllostomid bats appear to make a more modest direct contribution to the economic value of New World tropical forests. Nonetheless, because they critically influence forest regeneration, phyllostomid bats make an important indirect contribution to the economic value of these forests. Overall, fruit-eating bats play important roles in forest regeneration throughout the tropics, making their conservation highly desirable.

Effects of forest floor disturbances by mechanical site preparation on floristic diversity in a central Ontario clearcut

The effects of forest floor disturbance by six mechanical site preparation (MSP) treatments on structural and compositional diversity of early successional communities were examined in a clearcut in central Ontario. Periodic assessment of floristic diversity and abundance was performed to quantify treatment effects on species richness and abundance of three discrete plant life forms (cryptogam, herbaceous and woody) in the first five growing seasons after MSP. Botanical surveys of the surrounding uncut forest provided an estimate of pre-harvest forest species composition of the clearcut study site. Intensity of forest floor disturbance differed significantly among MSP treatments, creating a variety of microsites. The percentage of mineral soil exposure and abundance of downed woody material ranged from 7% to 52% and 18% to 1%, respectively, from the least to most severe treatments. Five years after MSP, 134 species (50 cryptogams, 47 herbaceous, 37 woody) were present in the clearcut study site, compared with 102 species (54 cryptogams, 25 herbaceous, 23 woody) in the uncut forest. Species richness and abundance of cryptogam, herbaceous and woody plant species increased with time after MSP, except for herbaceous species richness. MSP treatment significantly influenced species richness of cryptogams, herbaceous and woody plants, and abundance (percent cover) of woody plants. Treatment differences in richness and abundance were strongly correlated with intensity of forest floor disturbance. Scalping treatments that created the most mineral soil exposure and removed the most downed woody material typically supported the lowest species diversity and lacked several native forest species. Cryptogams were most affected by MSP, with higher richness and abundance where forest floor disturbance was relatively low. MSP did not affect the incidence of alien plant species: only two alien species were observed on the study site, representing less than 1% of total cover. Evaluation of MSP effects on early successional plant communities based on beta diversity in an ordination analysis revealed several distinct species groups associated with specific treatments. The relative amount of forest floor disturbance and downed woody material among treatments were strong predictors of species groupings within the ordination. Plant communities in slash piles were distinct from those resulting from MSP and contained several remnant forest species. Examination of species’ autecology provided supporting evidence that the displacement of downed woody material and forest floor organic layers was the leading cause of changes in species richness and abundance on this site.

CONTRASTING STRUCTURE AND COMPOSITION OF THE UNDERSTORY IN SPECIES-RICH TROPICAL RAIN FORESTS

In large samples of trees > or = 1 cm dbh (more than 1 million trees and 3000 species), in six lowland tropical forests on three continents, we assigned species with >30 individuals to one of six classes of stature at maturity (SAM). We then compared the proportional representation of understory trees (1-2 cm dbh) among these classes. The understory of the three Asian sites was predominantly composed of the saplings of large-canopy trees whereas the African and American sites were more richly stocked with trees of the smaller SAM classes. Differences in class representation were related to taxonomic families that were present exclusively in one continent or another. Families found in the Asian plots but not in the American plot (e.g., Dipterocarpaceae, Fagaceae) were predominantly species of the largest SAM classes, whereas families exclusive to the American plots (e.g., Melastomataceae sensu stricto, Piperaceae, and Malvaceae [Bombacacoidea]) were predominantly species of small classes. The African plot was similar to Asia in the absence of those American families rich in understory species, while similar to America in lacking the Asian families rich in canopy species. The numerous understory species of Africa were chiefly derived from families shared with Asia and/or America. The ratio of saplings (1-2 cm dbh) to conspecific canopy trees (>40 cm dbh) was lower in American plots than in the Asian plots. Possible explanations for these differences include phenology, moisture and soil fertility regimes, phyletic constraints, and the role of early successional plants in forest development. These results demonstrate that tropical forests that appear similar in tree number, basal area, and the family taxonomy of canopy trees nonetheless differ in ecological structure in ways that may impact the ecology of pollinators, dispersers, and herbivores and might reflect fundamental differences in canopy tree regeneration.

Temporal variation in plant–soil feedback controls succession

Soil abiotic and biotic factors play key roles in plant community dynamics. However, little is known about how soil biota influence vegetation changes over time. Here, we show that the effects of soil organisms may depend on both the successional development of ecosystems and on the successional position of the plants involved. In model systems of plants and soils from different successional stages, we observed negative plant-soil feedback for early-successional plant species, neutral feedback for mid-successional species, and positive feedback for late-successional species. The negative feedback of early-successional plants was independent of soil origin, while late-successional plants performed best in late- and worst in early-successional soil. Increased performance of the subordinate, late-successional plants resulted in enhanced plant community diversity. Observed feedback effects were more related to soil biota than to abiotic conditions. Our results show that temporal variations in plant-soil interactions profoundly contribute to plant community assemblage and ecosystem development.

FLORISTIC CONSERVATION VALUE, NESTED UNDERSTORY FLORAS, AND THE DEVELOPMENT OF SECOND-GROWTH FOREST

Nestedness analysis can reveal patterns of plant composition and diversity among forest patches. For nested floral assemblages, the plants occupying any one patch are a nested subset of the plants present in successively more speciose patches. Elimination of sensitive understory plants with human disturbance is one of several mechanisms hypothesized to generate nonrandom, nested floral distributions. Hypotheses explaining distributions of understory plants remain unsubstantiated across broad landscapes of varying forest types and disturbance histories. We sampled the vegetation of 51 floodplain and 55 upland forests across Illinois (USA) to examine how the diversity, composition, and nestedness of understory floras related to their overstory growth and structure (basal area), and their overall floristic conservation value (mean C). We found that plant assemblages were nested with respect to site species richness, such that rare plants indicated diverse forests. Floras were also nested with respect to site mean C and basal area (BA). However, in an opposite pattern from what we had expected, floras of high-BA stands were nested subsets of those of low-BA stands. A set of early-successional plants restricted to low-BA stands, and more importantly, the absence of a set of true forest plants in high-BA stands, accounted for this pattern. Additionally, we observed a decrease in species richness with increasing BA. These results are consistent with the hypothesis that recovery of true forest plants does not occur concurrently with overstory regeneration following massive anthropogenic disturbance. Nestedness by site mean C indicates that high conservation value (conservative) plants co-occur in highly diverse stands; these forests are assumed to be less disturbed historically. Because site mean C was uncorrelated with BA, BA-neutral disturbances such as livestock usage are suggested as accounting for between-site differences in mean C. When considered individually, conservative plants were actually more likely to be found in low-BA stands (uplands only). This suggests that floras of historically more open-canopied oak-hickory uplands are being degraded by canopy closure from increasing density of "mesophytic, nonpyrogenic" trees. It also indirectly suggests that recent moderate logging is uncorrelated with floristic conservation values.

Diet composition and foraging ecology of Asian elephants in Shangyong, Xishuangbanna, China

The composition of the diet and the foraging ecology of the Asian elephant in its natural habitat were studied from 1998 to 2000 in the Shangyong National Natural Reserve located at Xishuangbanna, China, using field observation and dung analysis. A total of 106 plant species were recorded as being eaten by Asian elephants, 83 of which were identified in the elephants' dung. The plant families that contributed to a major proportion of the elephants' diet in the study area were: Gramineae (8 spp., 10.0%), Moraceae (7 spp., 9.9%), Papilionaceae (4 spp., 8.4%), Araliaceae (3 spp., 6.6%), Vitaceae (3 spp., 5.7%), Apocynaceae (3 spp., 4.6%), Musaceae (1 spp., 4.2%), Zingiberaceae (3 spp., 3.7%), Myrsinaceae (3 spp., 3.6%), Rosaceae (3 spp., 3.6%), Euphorbiaceae (5 spp., 3.3%), Ulmaceae (2 spp., 3.0%) and Mimosaceae (4 spp., 2.9%). The most important plants in the elephants' diet were Ficus spp. (Moraceae, 9.0%), Dendrocalamus spp. (Gramineae, 4.5%), Musa acuminata (Musaceae, 4.2%), Microstegium ciliatum (Gramineae, 3.5%) and Amalocalyx yunnanensis (Apocynaceae, 3.1%). Asian elephants consumed a variety of plants in terms of life forms, including trees, vines, shrubs and herbs. Early successional plant species constitute a higher proportion of the diet than late successional plants (42 spp. taking 59% vs. 32 spp. taking 37%). Browsing species accounted for a larger proportion of the diet compared to grazing species (77 spp. taking 91% vs. 6 spp. taking 9%). The number of plant taxa (species, genus, family) in elephants' diet each month negatively correlated with monthly rainfall and mean temperature. The study may help to develop proper strategies for wildlife management especially with regard to the human-elephant conflict, which is now a serious issue in the conservation of Asian elephants in this area.