Forest succession in tropical hardwood hammocks of the Florida Keys: Post-hurricane seedlings

Background/Question/Methods

Although natural disturbances often retard or reverse successional trajectories of woody plant communities, a previous study in hardwood hammocks in the Florida Keys showed that differential tree species mortality from Hurricane Andrew in 1992 potentially accelerated successional replacement of species.  In this study we continued examination of effects of Hurricane Andrew on succession by comparing seedling species composition measured three years after the hurricane to pre-hurricane tree species composition.  Tree and seedling data were collected along a chronosequence of 19 of the previous study’s 23 hammock sites.  Sites ranged in successional age, defined as time since clearing due to agriculture or other anthropogenic disturbance, from 14 to 100 years.  We conducted weighted averaging regression on pre-hurricane tree species from all sites to calculate a successional age optimum and tolerance for each species.  We then used species successional age optima and tolerances in weighted averaging calibration to calculate inferred site ages based on pre-hurricane tree species, and on post-hurricane seedling species.  To detect any potential successional change due to post-hurricane seedling recruitment, we compared inferred site ages based on post-hurricane seedling species to inferred site ages based on pre-hurricane tree species. 

 Results/Conclusions

Inferred site age based on seedling species was greater than inferred site age based on pre-hurricane tree species for 15 of 19 sites, and magnitude of the difference between inferred ages increased with decreasing site age.  In most of the younger sites, at least one of the top three most abundant seedling species was a late-successional species not present in trees at the site; and deciduous canopy or sub-canopy tree species abundant at these sites were not abundant as seedlings.  Although 31 of 32 tree species also were present as seedlings in sites, only 8 species, with an abundance-weighted mean successional age optimum of 82 years, comprised the most abundant species across all sites.  Abundant seedling species in all sites tended to be evergreen, sub- to mid-canopy species with fleshy fruits and animal-dispersed seeds.  Whereas direct mortality of early-successional trees from Hurricane Andrew initiated an accelerated successional trajectory, subsequent seedling establishment likely continued the trajectory by increasing the probability of recruitment by late-successional species.  Because seeds of this suite of species are largely animal-dispersed, we emphasize that bird and mammal seed dispersers are an integral part of post-hurricane succession in Florida Keys tropical hammocks.