No abstract

The accumulation of species-specific enemies around adults is hypothesized to maintain plant diversity by limiting the recruitment of conspecific seedlings relative to heterospecific seedlings. Although previous studies in forested ecosystems have documented patterns consistent with the process of negative feedback, these studies are unable to address which classes of enemies (for example, pathogens, invertebrates, mammals) exhibit species-specific effects strong enough to generate negative feedback, and whether negative feedback at the level of the individual tree is sufficient to influence community-wide forest composition. Here we use fully reciprocal shade-house and field experiments to test whether the performance of conspecific tree seedlings (relative to heterospecific seedlings) is reduced when grown in the presence of enemies associated with adult trees. Both experiments provide strong evidence for negative plant-soil feedback mediated by soil biota. In contrast, above-ground enemies (mammals, foliar herbivores and foliar pathogens) contributed little to negative feedback observed in the field. In both experiments, we found that tree species that showed stronger negative feedback were less common as adults in the forest community, indicating that susceptibility to soil biota may determine species relative abundance in these tropical forests. Finally, our simulation models confirm that the strength of local negative feedback that we measured is sufficient to produce the observed community-wide patterns in tree-species relative abundance. Our findings indicate that plant-soil feedback is an important mechanism that can maintain species diversity and explain patterns of tree-species relative abundance in tropical forests.

Summary A growing body of empirical work suggests that soil organisms can exert a strong role in plant community dynamics and may contribute to the coexistence of plant species. Some of this evidence comes from examining the feedback on plant growth through changes in the composition of the soil community. Host specific changes in soil community composition can generate feedback on plant growth and this feedback can be positive or negative. Previous work has demonstrated that negative soil community feedback can contribute to the coexistence of equivalent competitors. In this paper, I show that negative soil community feedback can also contribute to the coexistence of strong competitors, maintaining plant species that would not coexist in the absence of soil community dynamics. I review the evidence for soil community feedback and find accumulating evidence that soil community feedback can be common, strongly negative, and generated by a variety of complementary soil microbial mechanisms, including host‐specific changes in the composition of the rhizosphere bacteria, nematodes, pathogenic fungi, and mycorrhizal fungi. Finally, I suggest topics needing further examination.

Mycorrhizal fungi influence plant growth, local biodiversity and ecosystem function. Effects of the symbiosis on plants span the continuum from mutualism to parasitism. We sought to understand this variation in symbiotic function using meta-analysis with information theory-based model selection to assess the relative importance of factors in five categories: (1) identity of the host plant and its functional characteristics, (2) identity and type of mycorrhizal fungi (arbuscular mycorrhizal vs. ectomycorrhizal), (3) soil fertility, (4) biotic complexity of the soil and (5) experimental location (laboratory vs. field). Across most subsets of the data, host plant functional group and N-fertilization were surprisingly much more important in predicting plant responses to mycorrhizal inoculation (Ôplant responseÕ) than other factors. Non-N-fixing forbs and woody plants and C 4 grasses responded more positively to mycorrhizal inoculation than plants with N-fixing bacterial symbionts and C 3 grasses. In laboratory studies of the arbuscular mycorrhizal symbiosis, plant response was more positive when the soil community was more complex. Univariate analyses supported the hypothesis that plant response is most positive when plants are P-limited rather than N-limited. These results emphasize that mycorrhizal function depends on both abiotic and biotic context, and have implications for plant community theory and restoration ecology.

Summary1. Plant-soil feedbacks is becoming an important concept for explaining vegetation dynamics, the invasiveness of introduced exotic species in new habitats and how terrestrial ecosystems respond to global land use and climate change. Using a new conceptual model, we show how critical alterations in plant-soil feedback interactions can change the assemblage of plant communities. We highlight recent advances, define terms and identify future challenges in this area of research and discuss how variations in strengths and directions of plant-soil feedbacks can explain succession, invasion, response to climate warming and diversity-productivity relationships. 2. While there has been a rapid increase in understanding the biological, chemical and physical mechanisms and their interdependencies underlying plant-soil feedback interactions, further progress is to be expected from applying new experimental techniques and technologies, linking empirical studies to modelling and field-based studies that can include plant-soil feedback interactions on longer time scales that also include long-term processes such as litter decomposition and mineralization. 3. Significant progress has also been made in analysing consequences of plant-soil feedbacks for biodiversity-functioning relationships, plant fitness and selection. 4. To further integrate plant-soil feedbacks into ecological theory, it will be important to determine where and how observed patterns may be generalized, and how they may influence evolution. 5. Synthesis. Gaining a greater understanding of plant-soil feedbacks and underlying mechanisms is improving our ability to predict consequences of these interactions for plant community composition and productivity under a variety of conditions. Future research will enable better prediction and mitigation of the consequences of human-induced global changes, improve efforts of restoration and conservation and promote sustainable provision of ecosystem services in a rapidly changing world.

The nature of the interaction between plants and their soil community was investigated by testing for differential responses of four old-field perennial plant species to inocula derived from soil communities that had been grown with ("cultured by") one of these four plant species. The differentiation of the soil communities was evident in measurements of plant survival, phenology, growth, and root-shoot ratios. Effects on survival and growth suggest negative feedbacks between these species and the soil communities that they culture. Survival rates of Krigia dandelion were significantly reduced when grown with their "own" soil community. Considered as a whole, the three other species (all grasses) had significantly lower growth and root-shoot ratios when grown with soil communities started with their own inocula compared to soil communities started with the inocula of other species. However, the significance of this effect on growth rate and root-shoot ratios was due primarily to the pairwise comparison of Anthoxanthum odoratum and Danthonia spicata and of Anthoxanthum and Panicum sphaerocarpon, respectively. Pairwise comparisons of Danthonia and Panicum do not suggest differential responses to each other's soils in growth rate or root-shoot ratios nor do soil communities appear to affect the relative competitive ability of these two species. While the components of the soil community responsible for these effects have not been identified, similar although less pronounced patterns were observed in experiments using inocula consisting of washed live root segments as compared to experiments using whole soil as inoculum, suggesting that root pathogens are one important agent.

Introduced plant populations lose interactions with enemies, mutualists and competitors from their native ranges, and gain interactions with new species, under new abiotic conditions. From a biogeographical perspective, differences in the assemblage of interacting species, as well as in abiotic conditions, may explain the demographic success of the introduced plant populations relative to conspecifics in their native range. Within invaded communities, the new interactions and conditions experienced by the invader may influence both its demographic success and its effects on native biodiversity. Here, we examine indirect effects involving enemies, mutualists and competitors of introduced plants, and effects of abiotic conditions on biotic interactions. We then synthesize ideas building on Darwin's idea that the kinds of new interactions gained by an introduced population will depend on its relatedness to native populations. This yields a heuristic framework to explain how biotic interactions and abiotic conditions influence invader success. We conclude that species introductions generally alter plantsÕ interactions with enemies, mutualists and competitors, and that there is increasing evidence that these altered interactions jointly influence the success of introduced populations.Ecology Letters (2006) 9: 726-740

Predominant frameworks for understanding plant ecology have an aboveground bias that neglects soil micro-organisms. This is inconsistent with recent work illustrating the importance of soil microbes in terrestrial ecology. Microbial effects have been incorporated into plant community dynamics using ideas of niche modification and plant-soil community feedbacks. Here, we expand and integrate qualitative conceptual models of plant niche and feedback to explore implications of microbial interactions for understanding plant community ecology. At the same time we review the empirical evidence for these processes. We also consider common mycorrhizal networks, and suggest these are best interpreted within the feedback framework. Finally, we apply our integrated model of niche and feedback to understanding plant coexistence, monodominance, and invasion ecology. Plant Community Ecology Models Overlook Soil Microbial InteractionsCommunities of competing plant species are stabilized by stronger negative intraspecific interactions relative to interspecific interactions [1]. Traditionally, strong negative intraspecific interactions have been thought to result from high resource use overlap [2,3]. These models of resource partitioning have been developed into an influential framework for understanding plant community dynamics, but the empirical evidence supporting them is still limited. Plant competition experiments have not shown unequivocally that the strength of intraspecific competition exceeds that of interspecific competition [4] and the empirical evidence of coexistence of competing plant species through resource partitioning remains mixed [5][6][7].In response to the perceived limitations of explaining species coexistence through resource partitioning, plant ecologists have increasingly looked for mechanisms that might limit the negative effect of competition on inferior competitors and thereby slow competitive exclusion. For instance, competition-colonization tradeoffs can allow inferior competitors to persist through their greater likelihood of establishing in transient gaps in vegetation [8].© 2010 Elsevier Ltd. All rights reserved.Corresponding author: Bever, J. D. (jbever@indiana.edu). Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Current theory neglects the less visible organisms in the soil and this might be one reason for the limited success in finding a mechanism to explain the coexistence of competing plant species. The presence and composition of soil microbial communities has been shown to have large impacts on plant-plant interactions [14][15][16] and con...