Environmental and biological controls on water and energy exchange in Florida scrub oak and pine flatwoods ecosystems

Scrub oak and pine flatwoods are two contrasting ecosystems common to the humid subtropical climate of Florida. Scrub oak forests are short in stature (<2 m) and occur on well-drained sandy soils, and pine flatwoods are much taller and occur in areas with poorly drained soils. Eddy covariance measurements were made from January 2001 to February 2003 over a scrub oak forest and from January 2002 to February 2003 over an adjacent pine flatwoods located on in central Florida, USA, and exposed to similar atmospheric conditions to evaluate how the dynamics of latent heat (lambda E) and sensible heat (H) exchanges are affected by environmental and biological variables. Annual evapotranspiration (Et) for the scrub oak was 737 and 713 mm in 2001 and 2002, respectively. Et was comparatively higher, 812 mm, in 2002 at the pine flatwoods due to higher soil moisture and leaf area. In both ecosystems, springtime increases in lambda E coincided with increasing leaf area and evaporative demand. However, H was the main energy-dissipating component in the spring due to the seasonal decrease in soil water content in the upper soil profile. In the spring, mean weekly Bowen ratio (beta, i.e. H/lambda E) values reached 1.6 and 1.2 in the scrub oak and pine flatwoods, respectively. With the onset of the summertime rainy season, lambda E became the dominant energy flux and beta fells to < 0.4. In both ecosystems, beta was strongly controlled by the interaction between leaf area and soil moisture. The lowest values of the decoupling coefficient (Omega, 0.2 and 0.25 scrub oak and pine flatwoods, respectively) also occurred during the dry springtime period indicating that surface conductance (g(s)) was the mechanism controlling energy partitioning causing high beta in both ecosystems. Et increases in the spring, when water in the upper soil profile was scarce and strongly retained by soil particles, indicated that plants in both ecosystems obtained water from deeper sources. The results from this research elucidate how energy partitioning differs and is regulated in contrasting ecosystems within the Florida landscape, which is important for refining regional hydrological and climate models.