In forests, competition for light and interspecific differences in shade tolerance drive vegetation structure and dynamics. In particular, the ability of tree seedlings to establish and persist in the shaded understory (i.e., shade tolerance) is a major demographic bottleneck, because seedling face unique challenges, such as smaller size, resource limitations, and low levels of defense. Shade tolerance is classically thought of as a fixed trait, defined by photosynthetic ability at low light; however, tree seedling shade tolerance may be a plastic trait, arising from interactions with soil microbes and light availability. Species differences in survival at low light may be associated with trade-offs between growth and defense against pathogens. Additionally, mycorrhizae may alter pathogen effects, switching from mutualistic to parasitic, depending upon light availability. Despite the importance shade tolerance in understanding forest community dynamics, it is rarely studied as a plastic trait in the context of biotic interactions with soil microbes. To investigate how soil microbes may mediate tree seedling shade tolerance, I conducted a greenhouse experiment, controlling pathogen/mycorrhizae presence and light level, for three temperate tree species.
Results/Conclusions
Using a Bayesian modeling approach, we were found that, in the absence of soil microbes, seedlings did not differ in predicted survival when grown in low versus high light. Additionally, species did not experience strong differences in low light survival, despite being categorized into opposite ends of the shade tolerance spectrum. When grown with pathogens and mycorrhizae, species experienced a significant reduction in survival at low light, and there were stronger differences in seedling shade tolerance. For the two species categorized as shade tolerant and intermediately-tolerant, the reduction in survival at low light was driven by pathogens, and mycorrhizae appeared to have no influence on seedling shade tolerance. For the species categorized as shade intolerant, the reduction in low light survival appears to be an interactive effect, mediated by both pathogens and mycorrhizae. This study demonstrates that tree seedling shade tolerance may be a plastic trait, arising from interactions with soil microbes and light availability. Understanding how biotic interactions cause greater variability in seedling responses to light could enhance our ability to predict future forest community dynamics.