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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. [ABSTRACT FROM AUTHOR]

• DBH and landscape characteristics induce tree-to-tree variability in sap velocity. • Variability of sap velocity depends on DBH for oak and on landscape characteristics for beech. • Relative importance of the drivers are species-specific and temporally dynamic. The substantial tree-to-tree variability of transpiration poses a major challenge to a reliable stand-scale quantification of transpiration. The diameter at breast height (DBH) and landscape characteristics have been identified as drivers of tree-to-tree variability, but it remains unclear if their control on sap velocity varies between species-specific water-use and environmental conditions. We hypothesized that their controls are specie-specific, such as the temporal dynamic of their relative importance. To test our hypotheses, we used a multi-species stand that include 37 trees equipped with sap-flow sensors from four species representing the dominant species in central Europe. We analysed the daily relative importance of DBH, landscape slope, aspect, flow accumulation, and topographical position. We found that tree-to-tree variability of sap velocity was mainly dependent on DBH for oak (twice higher relative importance than other species) and on landscape characteristics for beech (36% higher relative importance than other species) and conifers. The temporal dynamics of the relative importance of most tested drivers was found to be species-specific and linked to root-related aspects in response to hydro-meteorological conditions. During dry summer months, the daily relative importance of oak's DBH increased to almost 60% to be three times higher than the value for beech. In contrast, the relative importance of flow accumulation was always two to three times higher for beech trees than oak and conifer trees. This indicated that larger oak trees accessed deeper water sources than smaller oaks. However, the shallower root architecture of beech trees involved a higher dependence on shallow soil water because a larger DBH is seemingly not enhancing the tree's capacity to explore deeper soils. These new insights emphasize the critical importance of accounting for DBH and landscape characteristics through a species-specific and temporally dynamic correction in further approaches for upscaling of sap-flow data from individual tree to stand-scale. [ABSTRACT FROM AUTHOR]