Drought Severity, Disturbance Intensity and Wood Density of Dominant and Rare Tree Species in Brazilian Seasonally Dry Semideciduous Forests

Wood density is one of the main functional traits affecting demographic rates of tropical trees. However, little is known about how this translates into abundance of trees in local assemblages. I hypothesized that: (1) higher wood density would generally result in better adaptation to prevailing environmental conditions in seasonally dry semideciduous forests, which would translate into numerical dominance of denser-wood species; (2 and 3, respectively) as drought severity and disturbance intensity increase, the performance of individuals of denser-wood species would drop, so that the numerical dominance of those species would decline. A community-level functional measure (Delta MWD) was used to access the role played by wood density in shaping numerical dominance in assemblages. I obtained the value of Delta MWD for 30 local assemblages by computing the difference between wood density mean value of two species groups in each assemblage: dominant and rare. The proportion of Delta MWD positive values was significantly greater than 0.5, thus supporting hypothesis 1. This finding may be explained by the possible ability of denser-wood species to continue growing under drought conditions due to greater resistance to drought-induced cavitation of vessels associated with higher wood density. Pearson correlation coefficients between Delta MWD and drought severity and between Delta MWD and disturbance intensity were negative and statistically significant, supporting hypotheses 2 and 3, respectively. Also supporting these hypotheses, Glm fitting showed that drought severity and disturbance intensity had significant negative effects on Delta MWD; and the median values of Delta MWD of the subsets of assemblages showing higher levels of drought severity and disturbance intensity were significantly smaller than that of their respective counterparts. These findings may be explained by shifts of adaptive advantages from denser-wood drought tolerant strategists to softer-wood drought avoidance strategists, and from denser-wood resource conservative strategists to softer-wood resource acquisitive strategists, as drought severity and disturbance intensity increase, respectively.