Correlation between climate and growth for Florida scrub oaks: a dendroecology study

Background/Question/Methods

Florida scrub is a xeromorphic shrubland maintained by frequent fires and confined to inland sand ridges and coastal dunes.  The vegetation is dominated by clonal evergreen oaks, ericaceous shrubs, and palmetto, which resprout after fire.  Scrub is an upland habitat that occurs on nutrient poor and droughty soils and is home to several threatened and endangered species (e.g., Florida Scrub-Jay, gopher tortoise, and indigo snakes).  Much work has and is currently being conducted on understanding the scrub habitat, its historical state, and the habitat characteristics of importance for populations of threatened and endangered species.   The oak canopy, height and structure, is an important component that affects the suitability of the scrub as habitat for many of the threatened and endangered species.  However, little is known about the variables impacting oak growth.  Understanding how oak growth responds to climate is important for management decisions and for understanding how climatic changes may impact Florida scrub and the species that depend on it.   This research uses dendrochronology to examine the correlations between climate and growth for Florida scrub oaks.  Sections of myrtle oaks were collected from multiple scrub locations from the coastal barrier islands and mainland of Florida.  Some sections were collected from Kennedy Space Center/Merritt Island National Wildlife Refuge (KSC/MINWR) in the 1990’s as part of the initiation of scrub restoration.  Additional sections were collected from KSC/MINWR in 2007 and from mainland sites (Archbold Biological Station, Fox Lake Scrub Sanctuary, Indian Mound Sanctuary, Malabar West Sanctuary, and North Buck Lake Sanctuary) in early 2009.  Two sections from each tree were cut, sanded, and dated.  Ring widths were measured using a Velmex measuring stage, and a standardized tree ring chronology was created using the program ARSTAN.  This chronology was correlated with Palmer Drought Severity Index and regional temperature and precipitation data.

Results/Conclusions

The mean sensitivity and the series intercorrelation of the chronology are 0.41 and 0.474, respectively.  The mean sensitivity is lower than for many chronologies, however, the chronology includes young trees that do not have high correlation with the master chronology and the trees sampled are not near the edge of their range where sensitivity to climate should be maximized.   The growth of the oaks is strongly correlated with water availability (Palmer Drought Severity Index) thoughout the growing season (April – December) with increased growth during wetter years.  Precipitation alone has a strong positive correlation with growth only during the early growing season.