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Partitioning of carbon, water, and energy fluxes between understory and canopy in a North Central Florida mature uneven aged pine flatwoods forest.
Starr, Gregory*,1, Powell, Thomas1, Martin, Timothy1, Gholz, Henry1,2, 1 School of Forestry Resources and Conservation, Gainesville, FL2 National Science Foundation, Arlington, VA
ABSTRACT- Of the 5.93 million ha of timberland within the state of Florida, only 19% are maintained under conditions classified as "natural" pine forest. While considerable attention has been focused on the carbon and water dynamics of managed pine plantations in the region, little attention has been given to these natural forests, which are an integral part of the complex mosaic of lands in the region. Based on this importance, it was our intention to quantify how environmental conditions affect ecosystem carbon, water, and energy exchange within a mature pine flatwoods forest and determine how specific components of the system, (i.e. leaf physiology, soil respiration, understory composition and canopy structure), contribute to these exchange rates. During the 2000-2001 field season, the natural pine flatwoods was a carbon sink with an annual net ecosystem exchange (NEE) of 183 g C m-2. The largest portion of this carbon was sequestered during the winter and spring when temperatures were cooler and canopy LAI was lowest. This could be attributed to a reduction in ecosystem respiration losses and a slight increase in understory photosynthetic rates caused by a larger portion of total radiation reaching the understory. Approximately 90% of the respiratory losses from the system could be accounted through soil respiration, which is highly correlated to soil water potential (r2=0.77, p<0.001). Comparison of eddy covariance measurements above and below the pine canopy indicated that 37% of the ecosystemís carbon, 51 % of its latent energy, and 56% of its sensible heat fluxes could be attributed to the understory. Preliminary results show that two species, Serenoa repens and Ilex glabra, comprise 95% of the understory vascular and both have relatively high leaf nitrogen concentration and photosynthetic capacities (~1.4% nitrogen and Amax ~ 4.0 mmol m-2 s-1) and account for ~ 90% of the carbon sequestered by the understory community. These initial assessments provide evidence that natural pine flatwood forest of Florida may be a small regional carbon sink.
KEY WORDS: pine flatwoods, carbon and water exchange, eddy covariance