Factors that reduce the success of tree species when their density is high, such as localized soil pathogens or species-specific predators, can help to maintain diverse local communities. While much attention has been paid to how these frequency-dependent interactions influence diversity in tropical forests, the existence, mechanisms, and importance of frequency-dependent feedbacks remain relatively untested for temperate tree species. The goal of this work is to test whether seedlings of Quercus rubra, an ecologically and increasingly economically important tree species in New England, exhibit frequency-dependent survival under field conditions, and to experimentally test whether intraspecific seedling competition or soil microbial communities are responsible for these patterns. To do this, we tracked the growth and survival of Q. rubra seedlings in a network of 1m2 plots at the Harvard Forest ForestGEO mapped plot from 2017-2018 that span a range of local conspecific adult densities. Additionally, we used acorns and soil inoculum from the same site for a greenhouse experiment to test whether soil microbial communities and/or intraspecific seedling competition was responsible for the patterns of mortality we observed in the field.
Results/Conclusions
The proportion of Q. rubra seedlings that survived from 2017 to 2018 under field conditions decreased with increasing density of adult Q. rubra but did not change with increasing density of adult heterospecifics. Survival of Q. rubra seedlings did not change with greater numbers of heterospecific or conspecific seedlings within a seedling plot, up to 65 seedlings/m2. In contrast, all Q. rubra seedlings under greenhouse conditions survived when they weren’t experiencing competition. However, of the greenhouse-grown seedlings with high competition, fewer survived when inoculated with soil from beneath a Q. rubra tree (67%) when compared to those inoculated with either sterilized soil (90%) or with fresh soil from beneath a heterospecific (91%). Taken together, this work suggests that Q. rubra seedlings exhibit conspecific frequency-dependent mortality in the field, and that the presence of conspecific soil microbial communities may be responsible for this pattern. This is important for predicting the future demographics of Q. rubra as well as understanding local diversity in temperate forests.