Sap flow characteristics of Pinus mugo and Alnus alnobetula within the treeline ecotone of the Central Tyrolean Alps: consistencies rather than differences

Land-use change and climate warming favour the expansion of shrubs in alpine environments, with implications for ecosystem functioning and biogeochemical cycles. Thus, we estimated canopy transpiration and canopy conductance of a Pinus mugo and an adjacent Alnus alnobetula stand on a south-facing steep slope in 2,130 m a.s.l. within the treeline ecotone of the Central Austrian Alps throughout one investigation period. For each species, canopy transpiration was estimated by means of sap flow measurements which were scaled up to the stand canopy level. Independent of species, soil water content hardly affected sap flow density, indicating that P. mugo and A. alnobetula are extremely tolerant to limited topsoil water availability. By contrast, sap flow density was strongly correlated with solar radiation and vapour pressure deficit (VPD), while canopy conductance declined with increasing VPD. Our estimated canopy transpiration averaged 2.4 ± 0.1 mm d −1 in P. mugo and 3.0 ± 0.1 mm d −1 in A. alnobetula . The corresponding values for canopy conductance were 37 ± 2 and 42 ± 2 mm s −1 , respectively. The observation that, within the treeline ecotone on Mt. Patscherkofel, canopy transpiration of P. mugo and A. alnobetula stands is noticeably higher than that of adjacent low-stature vegetation and isolated trees should be considered when forecasting potential effects of climate and land-use change on the water balance of the treeline ecotone.