Eddy covariance was used to measure energy fluxes from July 2000 – June 2002 above the tree canopy and above the understory in a mature, naturally regenerated slash pine (Pinus elliottii Engelm. var. elliottii) – longleaf pine (Pinus palustris Mill.) flatwoods forest. Understory latent energy (λE) and sensible heat (H) fluxes accounted for 45% and 55% of whole-ecosystem fluxes, respectively, with strong seasonal variation in the proportion of λE attributable to the understory. The partitioning of net radiation (Rnet) to λE and H also changed seasonally, with half-hourly mean ecosystem H in the winter peaking at 175 W·m–2, almost twice as large as λE. In contrast, half-hourly ecosystem λE and H remained almost equal throughout the day in July and August, with mean midday peaks of approximately 200 W·m–2. Maximum hourly evapotranspiration (ET) in the months of July and August was 0.32 and 0.29 mm·h–1 for 2000 and 2001, respectively. For a variety of environmental conditions, mean daily ET was approximately 2.7 mm in the summer and 1.3 mm in the winter. Annual ET for the first year was 832 mm, or 87% of annual precipitation (956 mm). Although leaf area index was higher in the second year, annual ET was only 676 mm, which is considerably lower than that of the previous year, but it still accounted for approximately the same proportion (84%) of the much lower annual precipitation (811 mm). Canopy conductance declined as soils dried, changing patterns of partitioning of Rnet to λE.
