Hydrological variability and long-term floristic-structural modifications in different habitats of a tropical semi-deciduous forest

Shifts in hydrological regimes alter river flow rates and flood pulses, decrease environmental heterogeneity and the floristic-structural complexity of associated plant communities. We tested the hypothesis that drought events affect plant community composition and structure at a small-scale within a riparian fragment towards a reduction in floristic-structural complexity. The tree community was sampled in three habitats (wet, transitional and dry) and monitored in seven inventories carried out between 1991 and 2018. Hydrological variations were evaluated through annual rainfalls, river flow rates and water level data. The species richness and the detrended correspondence analysis axes were used to characterise the temporal modifications in floristic composition. Community structure was described in terms of biomass: accumulated, growth of survivors, mortality and recruitment. Generalised linear mixed models were fitted to evaluate the effects of time and environment in community. It was concluded that the climate has become drier in recent years due to declining precipitation that has affected flow rates and water levels. The floristic-structural complexity of the study fragment was maintained during the monitoring period. However, prolonged and extreme drought events displayed the potential to impact floristic-structural patterns.

Disentangling spatial,environmental and historical effects on tropical forest tree species turnover

Aims We aimed at disentangling the effects of spatial distance,current and past environmental dissimilarity,and their combinations on tree community taxonomic and phylogenetic turnover by addressing the following questions:(i)Is tree community taxonomic and phylogenetic turnover related to the indirect effects of spatial distance via environmental dissimilarity?(ii)Does tree community taxonomic and phylogenetic turnover respond to paleoclimate(Last Glacial Maximum and Mid-Holocene)?Methods The study was carried out in 14 Atlantic rainforest sites in Brazil(20.4 ha sampled)containing 615 tree species from 83 plant families.We obtained plot-level geographic coordinates and soil variables and site-level bioclimatic variables in the current,Mid-Holocene and Last Glacial Maximum.We used structural equation models with a distance-based approach to(i)test the direct effects of spatial distance and environmental dissimilarity and(ii)test the indirect effects of spatial distance via environmental dissimilarity on taxonomic(Bray–Curtis distance)and phylogenetic turnover(Comdist and Comdistnt distances).Important Findings Our results suggest a weak indirect effect of spatial distance via environmental dissimilarity on taxonomic and phylogenetic turnover.Tree community turnover was driven by the direct effects of neutral,niche-based and historical processes.Thus,we inferred that the paleoclimate(historical processes)promoted the selection of the clades that gave rise to the current flora,while spatial distances(neutral processes)limited the dispersal range of species from the regional pool and environmental conditions(niche-based processes)locally selected the taxa that are able to persist.