Tree seedling functional traits mediate plant-soil feedback survival responses across a gradient of light availability.

1. Though not often examined together, both plant-soil feedbacks (PSFs) and functional traits have important influences on plant community dynamics and could interact. For example, seedling functional traits could impact seedling survivorship responses to soils cultured by conspecific versus heterospecific adults. Furthermore, levels of functional traits could vary with soil culturing source. In addition, these relationships might shift with light availability, which can affect trait values, microbe abundance, and whether mycorrhizal colonization is mutualistic or parasitic to seedlings. 2. To determine the extent to which functional traits mediate PSFs via seedling survival, we conducted a field experiment. We planted seedlings of four temperate tree species across a gradient of light availability and into soil cores collected beneath conspecific (sterilized and live) and heterospecific adults. We monitored seedling survival twice per week over one growing season, and we randomly selected subsets of seedlings to measure mycorrhizal colonization and phenolics, lignin, and NSC levels at three weeks. 3. Though evidence for PSFs was limited, Acer saccharum seedlings exhibited positive PSFs (i.e., higher survival in conspecific than heterospecific soils). In addition, soil microbes had a negative effect on A. saccharum and Prunus serotina seedling survival, with reduced survival in live versus sterilized conspecific soil. In general, we found higher trait values (measured amounts of a given trait) in conspecific than heterospecific soils and higher light availability. Additionally, A. saccharum survival increased with higher levels of phenolics, which were higher in conspecific soils and high light. Quercus alba survival decreased with higher AMF colonization. 4. We demonstrate that functional trait values in seedlings as young as three weeks vary in response to soil source and light availability. Moreover, seedling survivorship was associated with trait values for two species, despite both drought and heavy rainfall during the growing season that may have obscured survivorship-trait relationships. These results suggest that seedling traits could have an important role in mediating the effects of local soil source and light levels on seedling survivorship and thus plant traits could have an important role in PSFs.

Light availability and plant shade tolerance modify plant-microbial interactions and feedbacks in subtropical trees.

Plant-soil feedbacks (PSFs) are an important mechanism of species coexistence in forest communities. However, evidence remains limited for how light availability regulates PSFs in species with different shade tolerance via changes in plant-microbial interactions. Here we tested in a glasshouse experiment how PSFs changed as a function of light availability and tree shade tolerance. Soil bacterial and fungal communities were profiled using the 16S rRNA and ITS2 gene sequencing, respectively. Under low light, individual PSFs were positively related to shade tolerance, while the least shade-tolerant species produced the most positive PSFs under high light. Pairwise PSFs between species with contrasting shade tolerance were strongly positive under high light but negative under low light, thereby promoting the dominance of less shade-tolerant species in forest gaps and species coexistence under closed canopy, respectively. Under high light, PSFs were related to soil microbial composition and diversity, with the relative abundance of arbuscular mycorrhizal fungi being the primary driver of PSFs. Under low light, none of soil microbial properties were significantly related to PSFs. These findings indicate PSFs and plant shade tolerance interact to promote species coexistence and improve our understanding of how soil microbes contribute to variation in PSFs.

Globally, plant-soil feedbacks are weak predictors of plant abundance.

Plant-soil feedbacks (PSFs) have been shown to strongly affect plant performance under controlled conditions, and PSFs are thought to have far reaching consequences for plant population dynamics and the structuring of plant communities. However, thus far the relationship between PSF and plant species abundance in the field is not consistent. Here, we synthesize PSF experiments from tropical forests to semiarid grasslands, and test for a positive relationship between plant abundance in the field and PSFs estimated from controlled bioassays. We meta-analyzed results from 22 PSF experiments and found an overall positive correlation (0.12 ≤  r ¯  ≤ 0.32) between plant abundance in the field and PSFs across plant functional types (herbaceous and woody plants) but also variation by plant functional type. Thus, our analysis provides quantitative support that plant abundance has a general albeit weak positive relationship with PSFs across ecosystems. Overall, our results suggest that harmful soil biota tend to accumulate around and disproportionately impact species that are rare. However, data for the herbaceous species, which are most common in the literature, had no significant abundance-PSFs relationship. Therefore, we conclude that further work is needed within and across biomes, succession stages and plant types, both under controlled and field conditions, while separating PSF effects from other drivers (e.g., herbivory, competition, disturbance) of plant abundance to tease apart the role of soil biota in causing patterns of plant rarity versus commonness.

Integrated assessment of the direct and indirect effects of resource gradients on tree species recruitment.

Understanding the dynamics of tree establishment is critical to assess forests' composition, management practices, and current responses to global change. We carried out a field seedling transplant experiment to assess not only the direct effects of resources influencing recruitment of four tree species, but also their indirect and combined effects. Our analysis integrated first growing season demographic data together with estimates of mycorrhizal fungal colonization and resource availability (light, soil moisture, and soil nitrogen). Only by considering both the direct and indirect effects of resources we were able to account for most of the variability observed during seedling recruitment. Contrary to expectations, increasing light levels were not always beneficial for recruitment even in low light habitats, and soil moisture availability benefited seedling growth but not survival. In addition, mycorrhizal fungal colonization was not always favored by high light levels or by increasing soil moisture. Seedling survival for all species was lower in plots with higher arbuscular mycorrhizal fungi, while the association with ectomycorrhizal fungi varied from beneficial to detrimental. When integrating the direct, indirect, and interactive effects of resource availability and mycorrhizal fungal colonization on tree recruitment dynamics we found that species responded in a nonlinear fashion to increasing resource levels, and we also identified thresholds, i.e., shifts in the direction of the response, along the resource gradient. Our integrated assessment considerably outperformed a null model where only direct effects of resources were accounted for. These results illustrate how the combination of direct, indirect, and combined effects of driving variables better represents the complexity of the processes determining tree species recruitment than simple resource availability mechanisms.

Tree range expansion may be enhanced by escape from negative plant-soil feedbacks.

Many plant species are expected to shift their distributional ranges in response to global warming. As they arrive at new sites, migrant plant species may be released from their natural soil pathogens and/or deprived of key symbiotic organisms. Under such scenarios plant-soil feedbacks (PSF) will likely have an impact on plant species' ability to establish in new areas. In this study we evaluated the role that PSF may play on the migratory potential of dominant temperate tree species at the northern limit of their distributional range in the Great Lakes region of North America. To test their ability to expand their current range, we assessed seedling establishment, i.e., survival, of local and potential migrant tree species in a field transplant experiment. To test for the presence and strength of PSF, we also assessed seedling survival during establishment in a greenhouse experiment, where the potential migrant species were grown in soils collected within and beyond their distributional ranges. The combination of experiments provided us with a comprehensive understanding of the role of PSF in seedling establishment in new areas. In the field, we found that survival for most migrant species was similar to those of the local community, ensuring that these species could establish in areas beyond their current range. In the greenhouse, we found that the majority of species experienced strong negative conspecific feedbacks mediated by soil biota, but these responses occurred for most species only in low light conditions. Lastly, our combined results indicate that migrant tree species can colonize and may even have enhanced short-term recruitment beyond their ranges due to a lack of conspecific adults (and the resulting negative PSF from these adults).

Tolerance of soil pathogens co-varies with shade tolerance across species of tropical tree seedlings.

A negative feedback between local abundance and natural enemies could contribute to maintaining tree species diversity by constraining population growth of common species. Soil pathogens could be an important mechanism of such noncompetitive distance and density-dependent (NCDD) mortality, but susceptibility to local pathogens may be ameliorated by a life history strategy that favors survivorship. In a shade-house experiment (1% full sun), we tested seedling life span, growth, and mass allocation responses to microbial extract filtered from conspecific-cultured soil in 21 tree species that varied in abundance and shade tolerance in a wet tropical forest (La Selva Biological Station, Costa Rica). Forty-three percent of the species had significant reductions, and 10% of the species had significant increases in life span, growth, root length, or root surface area when inoculated with microbial extract; 10% of the species experienced opposing reductions and increases in these characteristics. Contrary to expectation, species' local abundance was not related to species-specific responses to microbial extracts from cultured soils. Across species, seedling shade tolerance (survival at 1% full sun) was negatively correlated with susceptibility to the microbial. treatment for both survival and total mass accumulation, thereby exaggerating shade tolerance differences among species. Thus, soil pathogens may contribute to species coexistence through heightening niche differentiation rather than through negative density dependence in common species.