Trait, phylogenetic, and β-diversity patterns reveal community assembly mechanisms on Mount St. Helens

Background/Question/Methods

How do traits, environmental factors, and stochasticity interact during community assembly?  Examining trait, phylogenetic, and β-diversity patterns following a large disturbance (e.g. primary succession) are invaluable for addressing this question. Overdispersion or clustering of traits and phylogenetic relationships in communities allow us to infer how competitive exclusion versus envirornmental filtering influences patterns, while patterns of β-diversity allow us to understand whether species turnover is more rapid early or late in succession. To address these questions, we use a 30-year dataset of plant community composition from Mount St. Helens following the 1980 volcanic eruption, in both primary and secondary plant succession sites. We examine community assembly from a trait and phylogenetic perspective by calculating functional trait diversity and phylogenetic net related index (NRI) and comparing to null model values. We also examine changes in species turnover across space (between sites) and over time (between years) by calculating β-diversity and comparing to null model values. Together, these analyses allow us to determine whether 1) diversity increases or decreases through time and 2) assess whether diversity in growth, dispersal, or nutrient-related traits explain community assembly patterns through time.

Results/Conclusions

For both primary and secondary succession sites on Mount St. Helens, we find significant phylogenetic overdispersion, which generally increases through time. While abundance patterns of different families and functional groups change over time, this pattern is primarily driven by new species colonizing that are distantly related to resident species. In contrast, we find varying patterns in multivariate functional trait diversity for different primary and secondary succession sites. Generally, primary succession sites exhibit some trait clustering early in succession but no clustering or dispersion as succession progresses.  Secondary succession sites exhibit trait dispersion early in succession but no clustering or dispersion as succession progresses. Growth traits (height and specific leaf area), dispersal traits (seed weight), and tissue nutrient content (C, N, P) exhibited varying patterns in trait diversity. Finally, we find greater β-diversity in secondary than primary succession sites and species turnover to increase through time. Differences in community assembly mechanisms between secondary and primary succession could be due to a larger initial resident species pool, greater dispersal, less stochasticity, and less stressful environmental conditions in secondary succession sites. Overall, these findings suggest evidence of competitive exclusion structuring communities over the course of plant succession and increasing diversity through time.