Tree seedling functional traits influence survivorship responses to light availability and soil-borne microbes

Background/Question/Methods

The maintenance of species diversity is a central issue in plant community ecology. In tree communities, seedling survivorship is a major demographic bottleneck. Thus, interspecific differences in seedling survivorship – arising from light limitation and harmful soil microbes in plant-soil feedbacks – are critical processes in species coexistence. These survival patterns may be associated with intrinsic differences in allocation to mycorrhizal fungi, defense traits (e.g., phenolics and lignin/hemicellulose) and recovery traits (e.g., nonstructural carbohydrates). Traits may also differ within species, due to variation in their response to light availability and soil-borne microbes from soil cultured by conspecific versus heterospecific adults. We conducted a 17-week field transplant experiment at Alma College’s Ecological Preserve, Michigan, USA, in which we assessed seedling survivorship and functional traits for four temperate tree species (Acer saccharum, Prunus serotina, Quercus alba, and Q. rubra) that varied that in local adult abundance and shade tolerance. These seedlings were grown in intact soil cores collected under the canopy of conspecific versus heterospecific adult trees, and along a light gradient (5-15% light availability).

Results/Conclusions

We found that mycorrhizal colonization and defense/recovery traits (phenolics, lignin/hemicellulose, and nonstructural carbohydrates) generally increased with light availability and were higher in Q. alba and Q. rubra seedlings. However, this relationship was dependent upon whether seedlings were grown in conspecific versus heterospecific soil and varied with seedling species. Mycorrhizal colonization covaried with phenolics and lignin/hemicellulose. Furthermore, species aligned along a tradeoff between phenolics and lignin/hemicellulose. Combining all species and light levels, seedling survivorship decreased with arbuscular mycorrhizal colonization and increase with ectomycorrhizal colonization. Additionally, seedling survivorship increased with greater defense (phenolics, lignin/hemicellulose) and recovery traits (NSC). Our results indicate that multiple alternative strategies in allocation to defense/recovery traits could enhance tree seedling survivorship across light gradients and soil source.