Diversity drives higher tree seedling performance due to associational resistance

Background/Question/Methods

Decades of experimentation show that loss of biodiversity can negatively impact ecosystems. While most of these experiments occurred in grassland systems and/or manipulated a single trophic level, the majority of terrestrial producer diversity resides in forests and exists in trophically complex environments. Here we conducted two experiments to examine interactions among tree diversity, density, and herbivory. The first was a two-year field manipulation that factorially manipulated tree seedling diversity (one versus 15 species) and herbivory (presence/absence of white-tailed deer, Odocoileus virginianus) to examine how plant diversity impacted herbivory, and conversely how deer browsing and attack by other, non-manipulated enemies altered patterns of plant diversity. We also conducted a second, concurrent experiment that varied monoculture density (three versus 15 individuals) to explore the impacts of negative density dependence in this system. We tested three hypotheses: (1) tree diversity and deer herbivory will interact to affect seedling performance, (2) enemies will differentially attack less diverse plots, and (3) increasing density will increase negative intraspecific interactions.

Results/Conclusions

When deer were absent, plant growth and survival were actually higher in monoculture, but in the presence of deer, the pattern was reversed. Thus, we found a diversity effect only when we manipulated a higher trophic level. Higher performance in polyculture resulted from highly browsed species gaining associational resistance from heterospecific neighbors. The associational resistance only applied to deer, however, as insects and foliar fungi were largely unaffected by the diversity manipulations. We also found no evidence to suggest that higher performance in polyculture was due to reductions in conspecific density. Counter to expectations from negative density dependence theory, we found that increasing density in monocultures had no effect on growth and survival and actually decreased some enemy attack. Taken together, these results highlight the importance of plant diversity in buffering damage from vertebrate herbivores, and also show that herbivory itself can instigate a diversity effect. This suggests a positive feedback loop among diversity and enemies, whereby diverse plant patches help to maintain diversity by limiting damage and enemies promote diversity by leveling competitive interactions among primary producers.