Papers Published

1. Phillips, N and Oren, R, Intra- and inter-annual variation in transpiration of a pine forest, ECOLOGICAL APPLICATIONS, vol. 11 no. 2 (April, 2001), pp. 385-396 [doi] .
   (last updated on 2012/01/03)

   Abstract:
   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 styractiflua), willow oak (Quercus phellos), 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 (E-C) and leaf area (E-L) bases, on daily and annual bases, and among sites. Potential sources of variation in E-L included: photosynthetic photon flux density (Q(p)), daylength-normalized vapor pressure deficit (D-Z), and volumetric soil moisture (theta) on daily time scales; and leaf area (L) dynamics within and among species on seasonal and interannual time scales. In pine and sweetgum, a saturating relationship of the form E-L = k(1)D(Z)/k(2)(Dz) best explained the majority of daily variability in E-L, except during severe soil moisture depletion (theta < 0.20 m(3)/m(3)). When accounting for reductions in canopy conductance when < 0.20 m(3)/ m(3), this relationship could be used for estimating E-C during one year for which was available (r(2) = 0.70). When excluding days in which theta < 0.20 m(3)/m(3), estimates of E-C improved (r(2) = 0.91). Annual changes in k(1) and k(2) in pine indicated reductions in E-L for a given D-Z, coinciding with stand L development and increasing self-shading. Yet species contributions to E-C tended to compensate, resulting in a coefficient of variation in annual E-C (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 E-C 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 E-C, 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 E-C as a conservative hydrological process.