Mechanisms of ecological succession: insights from plant functional strategies

Successions are a central issue of ecological theory. They are governed by changes in community assembly processes that can be tracked by species traits. While single-trait-based approaches have been mostly promoted to address community assembly, ecological strategies actually encompass tradeoffs between multiple traits that are relevant to succession theory. We analyzed plant ecological strategies along a 140-year-long succession primary succession of 52 vertical outcrop communities after roadwork. We performed a RLQ analysis to relate six functional traits, associated with resource acquisition, competition, colonization ability and phenology, to the age of the outcrops. We found the prominence of two main axes of specialization, one related to resource acquisition and the other to reproduction and regeneration. We further examined the community-level variation in ecological strategies to assess the abiotic and biotic drivers of community assembly. Using trait-based statistics of functional richness, regularity and divergence, we found that different processes drove the variation in ecological strategies along the axes of specialization. In late succession, functional convergence was detected for the traits related to resource acquisition as a signature of habitat filtering, while the coexistence of contrasted strategies was found for the traits related to reproduction and regeneration as a result of spatial micro-heterogeneity. We observed a lack of niche differentiation along the succession, revealing a weak importance of biotic interactions for the regulation of community assembly in the outcrops. Overall, we highlight a prominent role of habitat filtering and spatial micro-heterogeneity in driving the primary succession governed by water and nutrient limitation.