Species-specific control of DBH and landscape characteristics on tree-to-tree variability of sap velocity

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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.