Positive interactions are prevalent in a long-term plant community

Deyi Yin, Department of Ecology and Environment, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China, Marc Cadotte, Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada and Scott Meiners, Biological Sciences, Eastern Illinois University, Charleston, IL

Background/Question/Methods
Positive interactions fundamentally influence populations, communities and ecosystem processes, however they have received far less attention than negative biotic interactions. General understanding of the prevalence of positive interactions, as well as the factors influencing them remains unknown for plant community assembly. In this work, we assessed the pairwise biotic interactions using 60 years of successional data from the Buell-Small Succession Study by modeling species occurrence, survival, colonization and population growth rate of each focal species as a function of the percentage cover of a second species over time. We then evaluated the effects of species characteristics on the frequency of positive interactions each beneficiary received, and each facilitator provided as well as the effects of species phylogenetic and functional dissimilarity on the strengths of positive interactions.

Results/Conclusions
We found that positive interactions are commonly observed across species and the relative importance of positive and negative interactions varies with different population processes and species origin of the interacting species pairs. Specifically, positive interactions are more frequently observed for processes that are associated with species establishment (e.g., population occurrence, survival and colonization), while less frequently observed for population growth, compared to negative interactions. In addition, the frequency of positive interactions and the net biotic effects among native species are consistently greater than that among non-native species across different population processes. We also found that beneficiaries that have higher peak cover and lower leaf nitrogen concentration are likely to receive more facilitation, and facilitators can provide more positive associations when they reach their peak cover earlier and have lower leaf nitrogen concentration. The strengths of positive interactions are influenced by phylogenetic distance and species dissimilarity (e.g., in seedmass and leaf nitrogen concentration), however the effects are not consistent across different population processes. Overall, this study provides fundamental insights into the prevalence of positive interactions in plant communities and the roles of species origin and species characteristics in the frequency and strength of the positive interactions.