Conspecific negative density dependence for seedlings is stronger in lower elevation forests

Ryan Grace, Ethan Torres, Aaron Nelson and Joseph LaManna, Department of Biological Sciences, Marquette University, Milwaukee, WI, Maggie Ross, Department of Biology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Relationships between species diversity and the environment, particularly elevation and latitude, have been at the forefront of ecological inquiry for decades. However, the contribution of local species interactions to diversity-environment relationships remains unresolved. Species interactions are thought to enhance diversity maintenance if they reduce performance when a species is at relatively high abundances and increase performance at relatively low abundances. This process reflects a negative feedback mechanism of local abundance on species performance, also known as conspecific negative density dependence (CNDD). Using data from along a 1,100 m elevational gradient, we measured the strength of CNDD by quantifying pairwise feedbacks between adult trees and seedlings for seven tree species. We used data on both seedling survival and growth across 244 1-m2 plots in old-growth forests in the Pacific Northwest. Each of these plots was paired with an adjacent adult neighbor to assess the impact of conspecific and heterospecific adults on seedling performance. We used this data to assess the hypothesis that CNDD, as measured by pairwise negative feedbacks, was stronger in warmer environments at lower elevations than in colder environments at higher elevations.
Results/Conclusions
Our findings support the hypothesis that CNDD is stronger in warmer environments at lower elevations than in colder environments at higher elevations. CNDD was stronger in lower elevation forests characterized by higher tree-species diversity than higher elevation forests in our system. Our results indicate that seedling performance was lower when growing in the vicinity of a conspecific adult than when growing in the vicinity of a heterospecific adult, and that these pairwise interactions may be an important contributor to changes in tree species diversity across elevations. Furthermore, we found that the direction and magnitude of interactions between seedlings and heterospecific adults varied widely among different heterospecific tree species. Thus, our results highlight the benefits of quantifying all pairwise feedbacks separately when evaluating CNDD. We also evaluated seedling performance away from any adult trees (but still within the forest stand) and found notably increased performance in the absence of nearby adult trees, emphasizing the overall negative impact of adult trees on seedling performance. As opposed to adults, numbers of conspecific and heterospecific seedlings in the plot did not generate CNDD, rather, seedling survival increased with total numbers of seedlings.