Phylogenetic trajectories during secondary succession in a Neotropical dry forest: Assembly processes, ENSO effects and the role of legumes

Phylogenetic analysis of plant communities is useful for inferring ecological mechanisms driving forest succession. However, such analysis has scarcely been undertaken in tropical dry environments, especially for the dynamics of demographic components (i.e., recruited, surviving and dead plants) affecting the successional process. Here, we combine chronosequence and dynamic data to study the role of habitat filtering and limiting similarity in the old-field succession of a seasonal tropical dry forest subjected to strong climatic events (e.g., the El Nino Southern Oscillation (ENSO) and heavy rains). We documented successional changes in the phylogenetic structure and phylodiversity of regenerative communities (shrubs and trees <= 1 m in height), assessed phylogenetic changes in demographic components during succession, and explored the effects of interannual rainfall variation on such changes. Over five years, we monitored the dynamics of regenerative plant communities in a chronosequence of nine abandoned pastures (fallow age of 0-12 years) and three old-growth sites. For each year and site, we quantified changes in the standardized mean phylogenetic distance (MPD.OBS.Z) and phylodiversity (PD) of entire regenerative communities and recruited, surviving and dead plants. For each phylogenetic metric and demographic component, we assessed the effect of successional age, time (years), and interannual rainfall variation. Large variation in MPD.OBS.Z and PD was observed among sites and years, especially in the recently abandoned pastures. Overall, the phylogenetic community structure was clustered early in succession and became more random as succession advanced. Species from the order Fabales made especially strong contributions to the phylogenetic clustering, especially during the first years of succession. In dead plants, the MPD.OBS.Z and PD increased in the drier year due to the high mortality of plants from several clades (orders). The high prevalence of clustered and random phylogenetic structure suggests that abiotic filtering and stochastic processes are major structuring mechanisms in this successional system. Due the strong contribution of Fabales in the phylogenetic structure of regenerative communities of tropical dry forest, the effects of strong climatic events (e.g., ENSO) on Fabales could influence the old-field succession of tropical dry forests.