© 2007 Heron Publishing—Victoria, Canada
Variation in the radial patterns of sap flux density in pubescent oak (Quercus pubescens) and its implications for tree and stand transpiration measurements
Rafael Poyatos (1, 2), Jan Cermák (3) and Pilar Llorens (1)
1. Institute of Earth Sciences ‘Jaume Almera’ (CSIC), Lluís Solé Sabarís s/n, 08208 Barcelona, Spain / 2. Corresponding author (rpoyatos@ija.csic.es) / 3. Institute of Forest Ecology, Mendel University of Agriculture and Forestry, Zemedelská 3 , 61300 Brno, Czech Republic / Received January 26, 2006; accepted June 27, 2006; published online January 2, 2007
Summary
Radial variation in sap flux density across the sapwood was assessed by the heat field deformation method in several trees
of Quercus pubescens Wild., a ring-porous species. Sapwood depths were delimited by identifying the point of zero flow in radial patterns of sap
flow, yielding tree sapwood areas that were 1.5–2 times larger than assumed based on visual examinations of wood cores. The
patterns of sap flow varied both among trees and diurnally. Rates of sap flow were higher close to the cambium, although there
was a significant contribution from the inner sapwood, which was greater (up to 60% of total flow) during the early morning
and late in the day. Accordingly, the normalized difference between outer and inner sapwood flow was stable during the middle
of the day, but showed a general decline in the afternoon. The distribution of sap flux density across the sapwood allowed
us to derive correction coefficients for single-point heat dissipation sap flow measurements. We used daytime-averaged coefficients
that depended on the particular shape of the radial profile and ranged between 0.45 and 1.28. Stand transpiration calculated
using the new method of estimating sapwood areas and the radial correction coefficients was similar to (Year 2003), or about
25% higher than (Year 2004), previous uncorrected values, and was 20–30% of reference evapotranspiration. We demonstrated
how inaccuracies in determining sapwood depths and mean sap flux density across the sapwood of ring-porous species could affect
tree and stand transpiration estimates.
Keywords:
diurnal variability, radial patterns, ring-porous species, scaling, transpiration.
