Energy exchange across a chronosequence of slash pine forests in Florida

We measured net atmospheric exchanges of energy and water vapor using eddy covariance along a chronosequence of Pinus elliottii plantations in north Florida: a recent clear-cut, a mid-rotation stand, and a 24-year-old, rotation-aged stand. Reflected energy averaged 0.26 of incoming solar radiation at the clear-cut and 0.18 at the closed-canopy stands. The sum of sensible (S), latent (LE) and soil heat fluxes accounted for 89 and 85% of net radiation (Rnet) at the clear-cut and mid-rotation age sites. Both S and LE were linearly related to Rnet at all sites. Seasonal differences occurred in the proportion of Rnet attributable to S and LE. S was a much smaller proportion of Rnet when the clear-cut and the mid-rotation age stands were flooded in the summer. LE was a greater proportion of Rnet during the summer/fall at all sites when LAI was greatest. Bowen ratios (S/LE) were 0.34, 0.50 and 0.59 in the summer/fall and 0.71, 0.77 and 1.00 in the winter/spring at the clear-cut, mid-rotation and rotation-aged stands, respectively. Maximum rates of evapotranspiration (ET) in the summer were 0.6 mm h−1 at all sites. Mean daily rates averaged 3.3 mm per day in the summer/fall and 2.0 mm per day in the winter/spring. Although, changes in LAI and canopy structure were large, annual ET estimates were similar and averaged 959, 951 and 1110 mm per year along the chronosequence. Results suggest that energy partitioning and annual ET in these pine forests are more sensitive to environmental fluctuations than to management activities.