Sap flow characteristics and canopy transpiration of Alnus alnobetula within the treeline ecotone of the Central Tyrolean Alps

Abstract Throughout two investigation periods, we estimated the canopy transpiration, understory evapotranspiration and total stand water use of Alnus alnobetula at three stands within the treeline ecotone of the Central Austrian Alps. Our study included one site at the lower edge of the treeline ecotone and two plots at the tree limit: one north-facing leeward and one south-east facing windward. Canopy transpiration ( T c ) was estimated at each site by taking sap flow measurements on six stems and scaling them up to stand canopy level. Understory and soil evapotranspiration ( ET u ) were derived using the soil water budget method. Throughout the treeline ecotone, normalized sap flow density was significantly correlated with solar radiation and vapour pressure deficit. By contrast, soil water content had no effect on normalised sap flow density, suggesting that A. alnobetula is highly tolerant of the limited soil water availability in the topsoil. Our estimated total stand evapotranspiration ( ET to t = T c + ET u ) for the treeline ecotone on Mt. Patscherkofel averaged 4.3 ± 0.6 mm per day, while T c averaged 3.6 ± 0.5 mm per day. These values considerably exceed the means reported for the growing season of adjacent isolated Pinus cembra trees, dwarf shrub communities, and grasslands, and should be taken into account when forecasting the potential effects of shrub encroachment on the water balance of the treeline ecotone.