COS 279-4 - Long-term patterns of tree growth in South Florida

Abstract: South Florida ecosystems, particularly pine rocklands and hardwood hammocks, are threatened by land use change and exotic species. The 2% of this ecosystem that remains is now threatened by climate change. It is critical to understand the responses of pine rocklands and hardwood hammocks to disturbance if we hope to conserve what’s left of South Florida’s natural subtropical forests.

Using dendrochronology, we measured and characterized long-term growth patterns in three dominant native tree species: Bursera simaruba, Swietenia mahagoni, and Pinus elliottii. Core samples were collected from approximately 30 individuals of each species in hardwood hammocks (B. Simaruba and S. mahagoni) and pine rocklands (P. elliottii) in South Florida parks to examine growth patterns. To account for ontogeny, we transformed the tree ring widths to basal area increment (BAI) and then into a size-standardized annual tree growth rate metric by fitting a smoothing spline. Lastly, we performed correlations and linear regressions to determine the relationships between tree growth and climatic variables to address two questions: Q1. What climatic factors are important for driving tree growth rates in South Florida? And Q2. Have tree growth rates changed through time due to climate change? Given that South Florida is in the hotter, southern extent of P. elliottii’s range but at the cooler, northern extent of S. mahagoni’s and B. simaruba’s ranges, we predicted growth rates to be decreasing in P. elliottii and increasing in S. mahagoni and B. simaruba as they respond to increasing temperatures.

We found significant negative correlations between tree growth and spring precipitation in P. elliottii and B. simaruba, whereas S. mahagoni growth was driven by precipitation in the fall. There was no relationship between temperature and growth of B simaruba. Growth of P. elliottii was negatively correlated with March temperatures and growth of S. mahagoni was negatively correlated with December temperatures. None of the three tree species significantly changed their growth rates through time in South Florida, but trees in more urbanized parks were found to be growing slower than conspecifics in more natural parks.

These results show that as climate changes, the foundational species of the endangered pine rocklands and hardwood hammocks may be vulnerable to changes in precipitation and temperature as well as other environmental effects associated with urbanization. Being home to several endemic and threatened species, we must continue to study the effects of the changing environments if we hope to conserve Florida’s remaining subtropical forests.