Intra- and inter-annual variation in transpiration of a pine forest
A study on the long-term water use of stands dominated by loblolly pine (Pinus taeda) and associated species was conducted in Duke Forest, North Carolina, USA. Associated species included sweetgum (Liquidambar styraciflua), willow oak (Quercus phettos), red maple (Acer rubrum), and eastern redcedar (Juniperus virginiana). Sap flux and climate data were collected from 1992 through 1996. Our objectives were to quantify the major sources of both environmental and forest structural variation influencing forest transpiration on ground (Ec) and leaf area (EL) bases, on daily and annual bases, and among sites. Potential sources of variation in EL included: Photosynthetic photon flux density (Qp), daylength-normalized vapor pressure deficit (Dz), and volumetric soil moisture (θ) on daily time scales; and leaf area (L) dynamics within and among species on seasonal and inter-annual time scales. In pine and sweetgum, a saturating relationship of the form EL = k1Dz/k2Dz best explained the majority of daily variability in EL, except during severe soil moisture depletion (θ < 0.20 m3/m3). When accounting for reductions in canopy conductance when θ < 0.20 m3/m3, this relationship could be used for estimating Ec during one year for which θ was available (r2 = 0.70). When excluding days in which θ < 0.20 m3/m3, estimates of Ec improved (r2 = 0.91). Annual changes in k1 and k2 in pine indicated reductions in EL for a given Dz, coinciding with stand L development and increasing self-shading. Yet species contributions to Ec tended to compensate, resulting in a coefficient of variation in annual Ec (9.6%) that was less than that of either annual precipitation (11.9%) or annual mean vapor pressure deficit (13.4%). The lower coefficient of variation in Ec over five years, in comparison to environmental variability, resulted from underlying divergent trajectories in water use among species, with pine maintaining and sweetgum increasing contributions to Ec, while red maple and eastern redcedar decreased theirs. This reflects canopy development in which pine and sweetgum are becoming dominant while red maple and eastern redcedar are increasingly suppressed. The results from this study are consistent with, and provide a temporal dimension to, the thesis of forest Ec as a conservative hydrological process.
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