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Mycorrhizal fungi colonize tree seedlings shortly after germination, and the nature of this relationship (mutualistic to parasitic) has been reported to vary as a function of resources; however, this transition has rarely been quantified. Using a light gradient, we grew seedlings of eight tree species in soils that were cultivated by several co-existing species of trees. We used data on root mycorrhizal fungi to quantify colonization along the gradient of light. We then analyzed plant growth as a function of both the light gradient and the extent of mycorrhizal colonization. Mycorrhizal fungi colonization varied among species but was not correlated with the species’ seed sizes or shade tolerances. Within a species, colonization varied among soil sources, but those differences followed neither the conspecific–heterospecific dichotomy, nor the soil host’s arbuscular–ectomycorrhizal associations commonly reported. At high light levels, seedlings growth increased with increasing levels of colonization for seven species, and at low light levels, the effect of colonization was negative for five species. We also quantified the light threshold at which the plant – mycorrhizal fungi relationship shifted from neutral to positive (four species), from negative to neutral (one species), and from neutral to negative (one species), documenting differences among species that could exacerbate competitive interactions during recruitment.

Tree species coexistence has often been explained through either negative distance-dependent/density-dependent (NDD) mortality or resource-based niche partitioning. However, these two influential mechanisms for structuring community dynamics have rarely been investigated simultaneously. Negative conspecific effects, like those caused by plant–soil feedbacks (PSF), could link these two mechanisms by being restricted to low-light environments and determined by seedling shade intolerance. We conducted a 10 week long greenhouse experiment where we assessed survivorship of eight temperate tree species that varied in local adult abundance and seedling shade tolerance, to non-sterile vs. sterile soils collected under the canopy of conspecific vs. heterospecific adult trees grown at low and high light. We found that half of our species experienced strong negative conspecific feedbacks. For three out of four species, the PSF were biotic mediated. However, their effects on survival were often restricted to low-ligh...

Summary 1. The Janzen–Connell (J–C) Model proposes that host-specific enemies maintain high tree species diversity by reducing seedling performance near conspecific adults and promoting replacement by heterospecific seedlings. Support for this model often comes from decreased performance for a species at near versus far distances from conspecific adults. However, the relative success of conspecific versus heterospecific seedlings recruiting under a given tree species is a critical, but untested, component of the J–C Model. 2. In a shade-house experiment, we tested plant–soil feedbacks as a J–C mechanism in six tropical tree species. We assessed effects of conspecific versus heterospecific cultured soil extracts on seedling performance for each species, and we compared performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. Additionally, we tested whether soil microbes were creating these plant–soil feedbacks and whether low light increased species vulnerability to pathogens. 3. Among 30 potential comparisons of survival and mass for seedlings grown in conspecific versus heterospecific soil extracts, survival decreased in seven and increased in two, whereas mass decreased in 13 and increased in 1. To integrate survival and growth, we also examined seedling performance [(mean total mass × mean survival time)/(days of experiment)], which was lower in 16 and higher in 2 of 30 comparisons between seedlings grown with soil extract cultured by conspecific versus heterospecific individuals. Based on performance within a soil extract, conspecific seedlings were disadvantaged in 15 and favoured in 7 of 30 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance occurred regardless of sterilization, suggesting chemical mediation. Microbes lacked host-specificity and reduced performance regardless of extract source and irradiance. 5. Synthesis. These results, along with parallel research in temperate forests, suggest that plant–soil feedbacks are an important component of seedling dynamics in both ecosystems. However, negative conspecific feedbacks were more prevalent in tropical than temperate species. Thus, negative plant–soil feedbacks appear to facilitate species coexistence via negative distance-dependent processes in tropical but not temperate forests, but the feedbacks were mediated through chemical effects rather than through natural enemies as expected under the J–C Model.

Summary 1. The Janzen–Connell (J–C) Model proposes that host-specific enemies could maintain high tree species diversity by reducing seedling performance near conspecific adults. An implicit, but untested assumption of the J–C Model is that negative conspecific feedbacks would promote replacement by heterospecific seedlings. 2. In a glasshouse experiment, we tested plant–soil feedbacks as a J–C mechanism in four temperate tree species. We assessed effects of conspecific- relative to heterospecific-cultured soil extracts on seedling survival, total mass and performance for each focal species. To test the implicit assumption of replacement by heterospecific seedlings, we also compared relative performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. We also tested whether soil microbes caused these plant–soil feedbacks and whether low irradiance increased seedling vulnerability to pathogens. 3. When grown with conspecific versus heterospecific soil extract, Acer rubrum mass decreased, Quercus rubra mass increased and Fraxinus americana increased survival. Conspecific extract reduced Acer saccharum mass in low light but increased it in high light. To integrate survival and growth, we examined seedling performance [(mean total mass × mean survival time)/(days of experiment)] at low and high light. In conspecific versus heterospecific soil extract, seedling performance was lower in two, higher in four and neutral in 18 of 24 cases, suggesting no advantage to dispersing away from conspecifics. Based on relative seedling performance within a soil extract, conspecific seedlings were disadvantaged in two, favoured in three and neutral in 19 of 24 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance were chemically mediated, occurring regardless of sterilization. Microbes lacked host specificity and reduced performance regardless of extract source. Additionally, microbial factors reduced seedling performance for Q. rubra regardless of light availability, and for A. rubrum and F. americana only in high light. 5. Synthesis. These chemical-mediated plant–soil feedbacks probably influence community dynamics, but are inconsistent with the J–C Model. Even when a species’ seedlings responded more negatively to conspecific than heterospecific soil, heterospecific seedlings were not necessarily favoured in that species’ soil, precluding heterospecific replacement as an explanation for coexistence.

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.

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).