The resilience of a dry tropical forest to single-fire events: A case study in Guánica Forest, southwest Puerto Rico

Background/Question/Methods

Adaptations have developed in forest communities worldwide as mechanisms of resilience to natural disturbance events. How these different forest assemblages respond to an increasingly unpredictable and novel disturbance environment due to climate change and anthropogenic effects is still an open question. Guánica Forest is a tropical dry forest in Puerto Rico, highly resilient to local hurricane events but now being increasingly threatened by anthropogenically induced fire. Two fires that occurred in 2012 were compared with older burns (2, 14 and 29 years old) to establish survival success of native tree species to single-burn events. Regeneration of forest structural and compositional characteristics were measured and compared against mature forest values.  We also analyzed the association between post-fire basal resprouting and selected traits including bark thickness, specific leaf area and tree size (categorized between total basal area, largest stem and number of stems) to determine which species might persist in a system with introduced fire disturbance.

Results/Conclusions

Although almost all native species experienced resprouting rates in excess of 60% immediately after fire, results from old burn sites appeared to show a consistent failure to regain forest structural values found in undisturbed mature forest. This was coupled with a shift in species composition with an increase of non-native species (including Leuceana leucocephala and African pasture grasses) and a significant decrease in native-species richness. Although each old burn site had a different species composition, none of them attained more than 39% of the species richness of mature forest. Bark thickness and specific leaf area were not significantly correlated with post-fire resprouting ability. However, tree size was associated, with the number of stems in a tree proving to have the strongest correlation with post-fire resprout ability (ρ = 0.545). Furthermore, single-stemmed trees were around twice as likely to fail to resprout compared to multi-stemmed trees. In all, Guánica Forest does not possess high resilience to fire, with the associated effects possibly having negative implications for resilience towards hurricanes, the natural local disturbance, because fires change natural patterns of forest structure.