It is widely believed that competition for light and interspecific differences in shade tolerance drive forest 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. Interspecific differences in shade tolerance may be associated with intrinsic differences in allocation to defense (e.g., lignin and phenolics) and recovery (e.g., nonstructural carbohydrates) traits. These traits may also differ within species, due to differences in their response to light availability, soil-borne pathogens, and mycorrhizae. Moreover, plant interactions with soil-borne microbes may be mediated by light availability, in addition to defense/recovery compounds that may confer resistance to both low light and pathogens. To investigate how soil-borne microbes and defense/recovery traits may mediate tree seedling survival at low light availability, we conducted a greenhouse experiment, controlling pathogen and mycorrhizae presence and light availability, and measuring defense/recovery traits, for three temperate tree species.
Results/Conclusions
Despite years of research emphasis on shade tolerance, seedling survivorship within species did not respond to light level, in the absence of soil microbes; differences only arose when soil microbes were present. Furthermore, species differences in low-light survivorship only appeared in the presence of soil-borne microbes and differed depending upon whether mycorrhizae and/or pathogens were present. Across species, tree seedlings allocated resources to an intrinsic amount of defense/recovery traits, according to their traditional shade tolerance classification. In particular, the most shade-tolerant species allocated more resources to lignin, phenolics, and nonstructural carbohydrates than the shade-intolerant species. Within species, tree seedlings allocated more resources to these defense/recovery traits in high light, compared to low light availability. Additionally, the presence of soil-borne microbes altered the amount of defense/recovery compounds that tree seedlings produced, and these differences were dependent upon light availability. This study demonstrates that tree seedling shade tolerance may arise from interactions with soil-borne microbes, defense/recovery traits, and light availability. Interactions between light availability and soil-borne microbes appear to modify tree seedling defense/recovery traits, leading to differences in tree seedling survivorship and shade tolerance. Understanding how biotic interactions cause greater variability in seedling responses to light could enhance our ability to predict future forest community dynamics.