Understanding the mechanisms that maintain species diversity is a fundamental goal of ecology. While recent studies have examined the role of biotic interactions in the maintenance of plant diversity, how abiotic factors might mediate the role of biotic interactions in diversity maintenance remains uncertain. Conspecific feedbacks, generated by host-specific pathogens, mycorrhizae, and/or intraspecific competition, may maintain tree-species diversity by limiting growth and survival of species at high densities and facilitating species at lower densities. Some evidence suggests that conspecific feedbacks are less negative and more faciliatory in abiotically stressful environments, following the stress-gradient hypothesis, but tests are rare. Greater facilitation in stressful environments may arise because detrimental effects of host-specific pathogens and intraspecific competition no longer overwhelm the beneficial effects of host-specific mutualists such as mycorrhizae. We tested for differences in conspecific feedbacks along stress gradients at the NSF Andrews long-term ecological research site (LTER) in Oregon, USA. Using three years of growth and survival data from 375 seedling plots across gradients of elevation, humidity, and nutrient availability, we predicted that feedbacks would be more negative in benign environments (low elevation, high humidity, high nutrient availability), and more positive in abiotically stressful environments (high elevation, low humidity, low nutrient availability).
Results/Conclusions
We found that conspecific feedbacks switch from negative in benign environments to positive in abiotically-stressful environments. Across the elevation gradient at the Andrews LTER, conditions become much drier and nutrients become less accessible to trees as elevation increases. At lower elevations, seedling survival was suppressed by neighboring conspecific adult trees relative to neighboring heterospecific adult trees (i.e. negative conspecific feedbacks). At more stressful higher elevations, conspecific feedbacks switched to being faciliatory between adults and seedlings. Our findings indicate that conspecific feedbacks follow a similar pattern put forth by the stress-gradient hypothesis, where host-specific biotic interactions that generate conspecific feedbacks tend to be more positive as abiotic stress increases. Our findings also have important implications for how forest diversity will be affected by increasingly stressful abiotic conditions anticipated with climate change. Specifically, our results suggest that increasingly faciliatory (positive) conspecific feedbacks with drying and other abiotic stress associated with climate change will tend to benefit dominant species at the expense of relatively less abundant and rare species.
