Red oak seedling leaf damage is higher nearby mature red oak trees and influences both leaf and stem biomass ratios

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Dayna De La Cruz and Jaclyn Matthes, Biological Sciences, Wellesley College, Wellesley, MA, Fiona Jevon, School of the Environment, Yale University, New Haven, CT

Background/Question/Methods
Northern red oak (Quercus rubra) is an ecologically important species in forests of the northeastern U.S. Red oak seedlings are critical to study as their life stage is the toughest age for survival. Red oak seedling leaf damage and allocation are potentially negatively impacted by nearby mature red oak trees due to feedbacks with pathogens and herbivores, or by the abundance of other red oak seedlings with whom they might compete. We used a field experiment to investigate these two biotic feedbacks and the influence of canopy light on red oak seedling leaf damage and biomass allocation. In June 2019 at Harvard Forest, we selected 30 mature trees evenly split across red oak, white pine (Pinus strobus), or red pine (P. resinosa). Beneath each mature tree, we transplanted 40 seedlings of four species into two seedling plots with even (5 of each species) or uneven (11 that matched mature species) seedling densities. We measured seedling survival and leaf damage three times during the summer and removed seedlings in September to measure biomass allocation. Each seedling was separated into aboveground and belowground tissues, dried and weighed. From this, we calculated the aboveground-to-belowground biomass ratio, and aboveground, belowground, leaf, and stem biomass ratios.
Results/Conclusions
Ninety-four percent of red oak seedlings survived to the end our study and the mature tree species did not influence red oak seedling survival, although red oak seedling leaf damage was significantly higher beneath mature red oak trees. Red oak seedlings also experienced lower survival in plots with higher red oak seedling densities, suggesting an effect of intraspecific competition. As leaf damage increased, the seedling leaf biomass ratio decreased and stem biomass ratio increased, suggesting that this was not only due to the leaf damage itself. Leaf and stem biomass ratios were not influenced by the presence of conspecific mature trees, conspecific seedling densities, nor variation in canopy light. Together, these results suggest that the presence of nearby mature red oak trees increased the herbivory stress to red oak seedlings, which may impact longer-term survival and growth allocation. This study provided useful insights for abiotic and biotic factors that influence red oak seedling survival in northeastern forest communities which could be of great use within a changing environment. Knowing how environmental factors affect seedling biomass allocation could help scientist better understand their responses across structurally diverse ecosystems and find more effective ways to predict future compositions of forests.