Water Availability as the Driving Factor of Growth and Physiological Function of Co-occurring Scrub Species in Central Florida

Florida scrub is a xeromorphic upland shrub community dominated by evergreen oaks that resprout after fire, occurring on moderately to excessively well-drained nutrient-poor sand. Scrub is home to several threatened and endangered animal species (e.g., Florida scrub-jay, gopher tortoise, and indigo snake) and rare and endemic plants. Urban development and agriculture has greatly reduced and fragmented scrub habitat, and because of this Florida scrub is considered one of the most endangered ecosystems in the United States. Climate change is a further threat to Florida scrub. Under a warmer and drier climate, scrub species will be exposed to more frequent and more severe droughts. In order to understand how scrub will respond to a changing climate, we must first have a better understanding of how scrub species respond to climate and water availability. In this dissertation, I use a combination of dendrochronology and plant physiological techniques to examine species responses to climate and water availability. In chapter two, I use dendrochronology to examine the importance of seasonal climate on growth of three co-occurring species in Florida scrub, myrtle oak (Quercus myrtifolia Willd.), Chapman oak (Quercus chapmanii Sarg.), and south Florida slash pine (Pinus elliottii Engelmann var. densa Little & K.W. Dorman). This is the first dendrochronology study on co-occurring scrub species. Therefore, my first goal was to ask whether Chapman oak and myrtle oak put on distinct annual rings with year to year variability that would enable successful crossdating. Because I found that each species does have annual rings that can be crossdated, my second goal was to ask how growth in each species responds to climate and its extremes, drought and extreme precipitation. Growth in both oaks was positively correlated with spring precipitation, while slash pine total and latewood growth was positively correlated with April and September precipitation. Slash pine earlywood growth was positively correlated associated with increased winter precipitation. In all three species, growth was sensitive to both annual and spring droughts. However, only slash pine exhibited increased growth with precipitation associated with tropical weather during July-September. Earlywood growth of slash pine was positively correlated with the Ni[ntilde]o 3.4 index: colder, wetter winters tended to be associated with increased earlywood growth. The between-species differences in response to seasonal climate may be due to differences in growth phenology. For both oaks, the climatic limiting factor was water availability in the spring, which is when the majority of radial growth occurs. By contrast, for slash pine, growth was limited by precipitation in the spring and late summer, its peak growth period. During the period for which we have a dendrochronological record (1920 to present), precipitation in April has become increasingly correlated with growth for all three species; as it has become the driest spring month during this period. In chapter three, I use dendrochronology to determine the climate-growth relationships of the dominant oak in Florida scrub, myrtle oak, for five sites occurring on three different scrub ridges in central Florida. My goals were twofold: 1) to ask whether the climate-growth responses of myrtle oak were robust across sites and 2) to ask how myrtle oak growth was affected by droughts (determined on both seasonal and annual basis). Myrtle oak growth increased with increased spring precipitation; temperature had little effect on myrtle oak growth. The growth response of myrtle oak to moisture availability (measured by the standardized precipitation index (SPI)) in March, April, May, and June, was robust across sites, with the exception of Malabar West. Myrtle oaks at Malabar West grew on poorly drained soils and were less responsive to precipitation and drought than were oaks growing on the other sites. March, April, May, and June SPI explained between 21.5 to 58.3% of the variation in myrtle oak growth for each site. Seasonal droughts explained more of the variation in growth than annual droughts. Spring droughts decreased myrtle oak growth at all sites, and drought during the previous summer decreased growth at Malabar West. Drought impacted growth for a single year; growth was normal or greater than normal during the year after drought. These results suggest that the timing of drought is important for myrtle oak growth, and that seasonal measures of drought are more important than annual measures of drought for determining growth impacts. In chapter four, I examine how the distribution and physiological functioning of scrub species vary along the ridge-swale topography at the Kennedy Space Center. Climatic factors often limit species distributions and plant physiological functions over large elevation gradients. However, on small elevation gradients, hydrologic variation may have strong effects on the distribution of species and the physiological function within a species. I used point-intercept sampling along a ridge-swale gradient at Kennedy Space Center on Merritt Island, Florida, to study how species distribution varied over a 1.2 m elevation gradient. Data from water monitoring wells along the elevation gradient were used to ask whether elevation may serve as a proxy for depth to water table. Elevation served as a good proxy for depth to water table; water table depth increased with elevation. I focused especially on understanding how the distribution and physiological functioning of three co-occurring Florida scrub oak species (Chapman oak, sand live oak (Quercus geminata) and myrtle oak) varied along the elevation gradient. Cover of all three scrub oaks increased with increasing elevation. Only sand live oak exhibited differences in physiological functioning along the elevation gradient; individuals at lower elevations were more sensitive to drought than individuals on the ridge. All three oaks exhibited stomatal regulation of water use efficiency drought. Chapman oak did not exhibit decreased photosynthetic activity during drought. Leaf phenology may play a role in the different responses to drought exhibited