South Florida slash pine (Pinus elliottii var. densa) is the dominant woody species and sole canopy tree in pine rocklands of the Lower Florida Keys. These globally endangered communities support a diverse subcanopy of West Indian hardwoods and palms as well as a variety of endemic herbs that are alternately encouraged or held back by fire. This doctoral dissertation research project will use dendrochronology to investigate the structure and disturbance history of declining pine rockland communities in the Lower Florida Keys. The ecological and climatological factors contributing to the distribution of South Florida slash pine at the southern extent of its range will be examined. In addition, the reconstruction of spatial and temporal variability of past fire regimes will provide a better understanding of how fire affects the structure and extent of these communities. This project also will investigate the interaction between regional climate oscillations (e.g. the El Nino-Southern Oscillation) and wildfire activity. Quantitative information on the historical fire occurrence and climate response of slash pine communities is essential to the long-term management and restoration of pine rockland communities.

This project will increase basic understanding of how human-related and natural disturbance events have decreased the spatial distribution of South Florida slash pine in the Lower Florida Keys. This project will apply techniques of dendrochronology in extreme southern Florida, in a subtropical region where tree?]ring science has never been applied, and it will produce climate proxy datasets that will add to the growing global network of low?]latitude climate proxy data. Results from this research will contribute to the understanding of how climate oscillations affect the region, particularly South Florida, and they will advance understanding of the complex dynamics of pine rocklands that will aid management efforts aimed at preserving these declining communities. The vegetation data produced by this project will provide valuable information regarding stand age distribution and composition in relation to natural and human-related disturbance regimes, and data on the association between pre-settlement wildfire and climate mechanisms will provide a better understanding of how fires shaped pine rocklands and the extent to which climate oscillations affected fire occurrence. As a Doctoral Dissertation Research Improvement award, this award will provide support for a doctoral student to establish a strong independent research career.

Project Report

The primary goals of this research were to (1) determine if the southernmost distributed native tree species in the USA, south Florida slash pine, forms annual growth rings and therby could be used in research related to ecology, biology, and climatology; (2) determine the seasonal growth dynamics of slash pine using cambial phenology techniques; (3) reconstruct historical wildfires in the lower Florida Keys using slash pine to determine the climatic drivers of fire; and (4) determine how various disturbances (i.e. fire, hurricanes, habitat fragmentation) might contribute to the further reduction of the globally-endangered pine rockland ecosystem in the Florida Keys. FIrst, as a result from Federal funding from the NSF, we found that the annual growth rings of slash pine can provide valuable information for various Earth science research in the Florida Keys, and should be used by researchers that focus on biology, biogeography, climatology, ecology, or dendrochronology to enhance project objectives. Second, we determined that the annual cellular growth of slash pine in the Florida Keys is primarily driven by variations in solar radiation. Third, we discovered that before the mid-20th century, wildfires burned large areas of the island, and these wildfires were likely controlled by dry climate conditions brought on by phase shifts in El-Nino and La Nina and the Pacific Decadal Oscillation. Finally, by comparing fire histories and forest structure between two adjacent islands (Big Pine Key and No Name Key), we found that fire suppression efforts during the late 20th century will likely contribute to the reduction of pine rockland habitat in the region if fire is not reintroduced. In the absence of fire, some large areas of pine rocklands are being taken over by tropical hardwood tree and shrub species. Overall, our results are important because information generated by this research is currently aiding land managers in their application of prescribed fires to increase the distirbution of the globally-endangered pine rockland ecosystem in the Florida Keys.

National Science Foundation (NSF)
Division of Behavioral and Cognitive Sciences (BCS)
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Thomas Baerwald
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University of Tennessee Knoxville
United States
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