Carbon exchange of a mature, naturally regenerated pine forest in north Florida. Global Change Biol

Global Change Biology (Impact Factor: 8.04). 08/2008; 14(11). DOI: 10.1111/j.1365-2486.2008.01675.x
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We used eddy covariance and biomass measurements to quantify the carbon (C) dynamics of a naturally regenerated longleaf pine/slash pine flatwoods ecosystem in north Florida for 4 years, July 2000 to June 2002 and 2004 to 2005, to quantify how forest type, silvicultural intensity and environment influence stand-level C balance. Precipitation over the study periods ranged from extreme drought (July 2000-June 2002) to above-average precipitation (2004 and 2005). After photosynthetic photon flux density (PPFD), vapor pressure deficit (VPD) >1.5 kPa and air temperature <10 °C were important constraints on daytime half-hourly net CO₂ exchange (NEEday) and reduced the magnitude of midday CO₂ exchange by >5 μmol CO₂ m⁻² s⁻¹. Analysis of water use efficiency indicated that stomatal closure at VPD>1.5 kPa moderated transpiration similarly in both drought and wet years. Night-time exchange (NEEnight) was an exponential function of air temperature, with rates further modulated by soil moisture. Estimated annual net ecosystem production (NEP) was remarkably consistent among the four measurement years (range: 158-192 g C m⁻² yr⁻¹). In comparison, annual ecosystem C assimilation estimates from biomass measurements between 2000 and 2002 ranged from 77 to 136 g C m⁻² yr⁻¹. Understory fluxes accounted for approximately 25-35% of above-canopy NEE over 24-h periods, and 85% and 27% of whole-ecosystem fluxes during night and midday (11:00-15:00 hours) periods, respectively. Concurrent measurements of a nearby intensively managed slash pine plantation showed that annual NEP was three to four times greater than that of the Austin Cary Memorial Forest, highlighting the importance of silviculture and management in regulating stand-level C budgets.

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    • "Soils are of the Alderwood series (Dystric Entic Durochrept) from glacial origin. And (4) a pine forest located near Gainesville, Florida (Lat: 29.74°N; Long: 82.22°W), with annual precipitation of 1228 mm and mean annual temperature of 22 °C, dominated by longleaf pine (Pinus palustris Miller) and slash pine (Pinus elliottii Engelm.; Powell et al., 2008). Soils are poorly drained ultic alaquods. "
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    • "These models suggest different carbon-cycling responses depending on the climate regime, where humid sites (such as in the eastern United States) were generally less responsive to lower precipitation than drier sites (such as in the western United States) (Gerten et al., 2008). In the southeast United States, Powell et al. (2008) reported little change in the net ecosystem production (NEP = gross ecosystem productivity minus ecosystem respiration) during drought in Pinus elliotti due to the counteracting effects of decreased canopy photosynthesis and soil respiration . Elsewhere in the Southeast, NEP in a Pinus taeda plantation was reduced by drought, primarily through decreased canopy and whole-tree leaf conductances (Noormets et al., 2010), without a corresponding decrease in ecosystem respiration. "
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    • "In comparison, winter and summer mean daily ET rates were similar to values reported for pine dominated ecosystems on the Atlantic Coastal Plain, which ranged from 0.5 mm in the winter to 3.9 mm in the summer (Powell et al. 2005, Sun et al. 2010, Clark et al. 2012). Particularly, Powell et al. (2008) reported winter mean daily ET rates of 1.3 mm (per 30-min average), which correspond well to our findings, and summer mean daily ET rates of 2.7 mm, which were slightly lower than summertime ET rates in this study. Higher ET during the summertime can largely be explained by increased physiological activity in the plant community. "

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