Allometric relations between DBH and sapwood area for predicting stand transpiration: lessons learned from the Quercus genus

Abstract

Catchment-scale transpiration is commonly determined by the use of sap-flow sensors, and its quantification, which is critical for water and forest management, relies crucially on the total catchment’s sapwood area (As). Species-specific allometric relationships between the trees As and the trees diameter at breast height (DBH) are widely used for determining stand or catchment As. However, substantial differences between studies challenge the robustness of these relationships between sites displaying various topographical and environmental characteristics. Our objectives for this study are to compare the parameters of these relationships between species of the Quercus genus from different sites across the globe and to test the role of topographical factors on the As-DBH relationship in Quercus petraea. Using 145 trees sampled within a 0.455 km2 catchment, we found that topography (slope, flow accumulation, aspect, curvature, and topographic wetness index) does not modulate the As-DBH relationship in Q. petraea, within our catchment. We compared our curve parameters with those from 16 studies on oak trees and found that the As-DBH relationship is not only species-specific, but depends on the site’s conditions. The use of species-specific parameters from other sites may lead to more than 100% difference in the calculation of As, and therefore in forest transpiration. In the light of these results, we recommend building site- and species-specific As-DBH relationships for determining stand or catchment transpiration, using a minimum of nine, randomly sampled trees, and different methods and azimuthal directions for determining sapwood depth.