Previous work in Florida Keys tropical hardwood hammocks showed declining dominance of deciduous species and increasing dominance of evergreen species with increasing stand age. In this study we examined species and functional trait changes in sites along a chronosequence. We determined functional traits of 21 tree species, and examined relationships among species and functional traits for sites and environmental variables proposed as drivers of succession. We also used traits to classify species into functional groups, and examined changes in functional group abundance over the chronosequence. We expected to see a strong change in species composition and community-weighted traits over the chronosequence. Similarly, we expected individual sites to move directionally along a successional gradient over a 19-year time span. Finally, we expected convergence in species composition and community-weighted traits from early to late succession. NMDS ordinations were conducted on species and community-weighted trait data for sites measured in 1994 and 2013. Then site age, soil depth and elevation environmental vectors were fit to the ordinations.
Results/Conclusions
We classified species into three functional groups based on leaf longevity and leaf seasonality: deciduous, leaf exchangers and evergreen. Deciduous species were most abundant early in succession, and steeply declined with increasing site age. Leaf exchanger abundance remained relatively steady throughout succession, and evergreen species abundance increased slowly with increasing site age. In all ordinations, NMDS1 was positively related to site age and soil depth (site age mean r2= 0.60, mean p = 0.004; soil depth mean r2= 0.57, mean p = 0.002), but not to elevation (elevation mean r2= 0.10, mean p± 0.035, t20= 2.74, one-tailed p± 0.005, t20= 1.70, one-tailed p= 0.052). We did not see convergence in either community-weighted functional traits or species composition over the course of succession. Although variance in ordination trait space was lowest in old growth forests (> 90 years), variance was high within both 30 – 50 and 71 – 90 year old stands. These peaks in trait space variance seem to coincide with the stochastic assembly of deciduous species with leaf exchanger and evergreen species early in succession, and the decline of deciduous species later in the successional process.