Sap flow characteristics and whole-tree water use of Pinus cembra across the treeline ecotone of the central Tyrolean Alps

Whole-tree water use of nine Pinus cembra trees was estimated in the treeline ecotone of the Central Austrian Alps. Sap flow density using Granier-type thermal dissipation probes and environmental parameters was monitored along an elevational gradient from the forest limit up to treeline and finally mediating the krummholz limit throughout two growing seasons. Normalized sap flow density (Qs) was significantly correlated with solar radiation (Rs) and vapor pressure deficit (D) throughout the treeline ecotone. Multiple regression analysis indicated that at the forest limit and at treeline, D had a similar effect on Qs than Rs. At the krummholz limit by contrast, D had a greater effect on Qs than Rs due to partially stomatal closure and wind-induced clustering of the needles, which impaired their response to available irradiance. Whole-tree water use scaled to crown surface area estimated for an entire growing (172 days) declined from 449 mm at the forest limit to 274 mm at treeline and was 251 mm at the krummholz limit, which is within the values estimated for other European forest ecosystems. Nevertheless, the observation above the forest limit in the central Tyrolean Alps tree transpiration scaled to crown surface area is comparable to the water use of adjacent low-stature vegetation, which should also be taken into account when forecasting potential effects of global change on the water balance of the treeline ecotone.