Previous work along a chronosequence of Florida Keys tropical hardwood hammocks described a trend of declining dominance of deciduous species and increasing dominance of evergreen species with increasing site age. In this study we more closely examined functional changes in sites along the same chronosequence. We measured functional traits of 21 species of trees present in the sites, and examined relationships among community-weighted trait values for sites and environmental variables that we proposed as putative drivers of succession. We expected that traits associated with earlier successional species, such as leaf seasonality and high SLA, would be associated with low site age; and traits associated with later successional species, such as high leaf longevity and high wood density, would be associated with high site age. Twelve functional traits were measured for each species: leaf longevity, leaf seasonality, SLA, wood density, frequency of multiple stems, carbon stable isotope ratio (i.e., δ13C) total leaf phosphorus, total leaf nitrogen, leaf N:P, tree height:DBH, crown area:DBH and maximum height. Community-weighted mean trait values were calculated for each site based on the relative basal areas of species present in the site. Environmental variables measured for each site were site age, soil depth and elevation. NMDS ordination was conducted on community-weighted mean trait values for 27 sites measured in 1992, 1994 and 2013. Then site age, soil depth and elevation environmental vectors were fit to the ordination.
Results/Conclusions
Vector-fitting to the ordination of community-weighted mean trait values for sites showed that site age and soil depth were both associated with the ordination (site age r2= 0.54, p = 0.001; soil depth r2= 0.50, p = 0.001). Traits associated with increasing site age were wood density, leaf longevity and total leaf phosphorus. Traits negatively associated with site age were SLA, total leaf nitrogen, leaf N:P and leaf seasonality. Increasing site age was correlated with increasing soil depth; and leaf nutrient traits associated with increasing site age might be a response to soil development during site succession. Leaf seasonality and leaf longevity were associated with younger and older sites, respectively. Whereas leaf seasonality is a response to a seasonal rainfall pattern, leaf longevity most likely is a response to resource limitation.