▪ Abstract Feedback between plants and the soil is frequently invoked on the basis of evidence of mutual effects. Feedback can operate through pathways involving soil physical properties, chemical and biogeochemical properties and processes, and biological properties, including the community composition of the microbiota and soil fauna. For each pathway, we review the mechanistic basis and assess the evidence that feedback occurs. We suggest that several properties of feedback systems (for example, their complexity, specificity, and strength relative to other ecological factors, as well as the temporal and spatial scales over which they operate) be considered. We find that the evidence of feedback is strongest for plants growing in extreme environments and for plant-mutualist or plant-enemy interactions. We conclude with recommendations for a more critical appraisal of feedback and for new directions of research. Let us not make arbitrary conjectures about the greatest matters. Heraclitus (1)

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Feedback in the plant-soil system

Theoretical models predict that effects of dispersal on local biodiversity are influenced by the size and composition of the species pool, as well as ecological filters that limit local species membership. We tested these predictions by conducting a meta-analysis of 28 studies encompassing 62 experiments examining effects of propagule supply (seed arrival) on plant species richness under contrasting intensities of ecological filters (owing to disturbance and resource availability). Seed arrival increased local species richness in a wide range of communities (forest, grassland, montane, savanna, wetland), resulting in a positive mean effect size across experiments. Mean effect size was 70% higher in disturbed relative to undisturbed communities, suggesting that disturbance increases recruitment opportunities for immigrating species. In contrast, effect size was not significantly influenced by nutrient or water availability. Among seed-addition experiments, effect size was positively correlated with species and functional diversity within the pool of added seeds (species evenness and seed-size diversity), primarily in disturbed communities. Our analysis provides experimental support for the general hypothesis that species pools and local environmental heterogeneity interactively structure plant communities. We highlight empirical gaps that can be addressed by future experiments and discuss implications for community assembly, species coexistence, and the maintenance of biodiversity.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-0248.2009.01373.x

Seed arrival, ecological filters, and plant species richness: a meta-analysis.

SYNTHESIS Seed arrival, ecological filters, and plant species richness: a meta-analysis

Species diversity in communities of interacting organisms is thought to be enhanced by dispersal, yet mechanisms predicting this have little to say about what effects differing rates of dispersal have on diversity and how dispersal affects diversity at larger spatial scales. I performed meta‐analyses on 23 studies comprising 50 experiments that manipulated species migration and measured community richness or diversity to test three hypotheses: that dispersal increases local diversity; that this effect depends on the rate of dispersal, specifically, that local diversity should be maximized at intermediate dispersal rates or else linearly related to dispersal rate; and that regional diversity may be either unaffected or negatively impacted by dispersal because dispersal tends to homogenize local communities. I found that immigration increased local diversity. Further, in animal studies, diversity appears maximized at intermediate dispersal rates but not with plant studies; however, more standardiz...

https://www.jstor.org/stable/10.1086/504850

Dispersal and species diversity: a meta-analysis.

1 Local plant communities can only function within a metacommunity context if they are connected by appropriate dispersal vectors, accommodating the transport of propagules between sites. The capacity for long-distance dispersal may be a key factor in the survival of local populations, especially in fragmented landscapes, and hence may have a large impact on local species composition. Dispersal vectors with a large efficiency for long-distance dispersal included in this study are: water, wind, large mammals and birds. 2 We tested the hypothesis that variation in dispersal traits across plant communities is related to the position of the communities along major environmental gradients. This hypothesis was tested for (i) separate long-distance dispersal vectors and (ii) multiple dispersal vectors (the number of potential long-distance dispersal vectors per species). 3 To quantify linkages between dispersal traits and environmental gradients, we coupled a data base containing dispersal attributes with another data base, containing 40 000 local vegetation descriptions aggregated into 123 plant communities. For each dispersal vector, the proportions of species that have access to this vector per community (weighted trait scores) were projected along three major environmental gradients: soil moisture, nutrient availability and light availability. 4 The potential importance of individual dispersal vectors showed clear differences along the three environmental gradients, with the greatest differences along the light availability gradient. The differences in dispersal traits probably reflect environmental constraints on the availability or efficiency of individual dispersal vectors. 5 The ability to be dispersed by multiple dispersal vectors is a common phenomenon in most plant communities (an average of 2.15 vectors per species). The mean number of potential long-distance dispersal vectors per species increases with light availability. This probably implies that plant communities differ in their response to both habitat fragmentation and habitat restoration. 6 Despite differences in trait spectra among communities, all dispersal syndromes are represented in nearly all communities. An important consequence of this complementarity in dispersal traits is that species within the same community may experience different connectivity. 7 The results emphasize the need for dispersal models based upon multiple dispersal vectors that explicitly include parameters for habitat characteristics.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.0022-0477.2004.00916.x

Dispersal potential in plant communities depends on environmental conditions

Summary 1 We carried out a factorial experiment to examine how groundwater availability (low and high sites with intermediate or rare flooding), vegetation canopy, leaf litter and seed availability interacted to determine the species richness of a productive wet grassland community in Wicken Fen National Nature Reserve, Cambridgeshire, UK. Seeds of 18 species were added to half the plots in each of eight combinations of elevation, canopy and litter, and seedling emergence was observed for two growing seasons. 2 Both individual and interactive effects on plant diversity and colonization were determined for all four examined factors. Interactive effects explained 41‐63% of the total variation in both species richness and numbers of individuals growing from added seeds. 3 Neither elevation nor vegetation canopy had significant individual effects on total species richness, but their interaction was significant. Litter addition limited seedling emergence at the low elevation but favoured it at the high elevation. 4 The relative importance of vegetation canopy and plant litter in affecting plant community composition varied with the community parameter considered (species richness or number of seedlings), elevation and stage of vegetation development. In general, plant litter was more important in determining species richness, whereas the vegetation canopy was more important in determining seed germination and seedling emergence. Plant litter was also more important than vegetation canopy at an early stage of vegetation development and at low elevation. 5 Seed availability was the most important factor in determining overall species richness in the studied community. The influence of the local seed bank was very limited. Seedling emergence and seedling species richness were generally enhanced by lower elevation and seed addition, but depressed by vegetation and litter addition. 6 The complex relationships observed have considerable implications for ecological modelling and ecosystem restoration. Manipulation of one factor may produce unexpected effects on other factors, which may induce a series of consequences for the whole community. Further knowledge on how natural communities are organized and maintained is needed to guide the management of ecosystems.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2745.2003.00827.x

Shaojun Xiong

Papers

Interactive effects of soil moisture, vegetation canopy, plant litter and seed addition on plant diversity in a wetland community