Transpiration of forest trees and stands at different altitude: consistencies rather than contrasts?

Compared to lowland trees and forests, evidence on water relations at mountainous and sub-alpine sites with their climatic and topographic peculiarities is scarce. On such limited grounds, the outcome of three pilot studies is reviewed, intended to launch working hypotheses for initiating integrative research across the altitudinal gradient of the European Alps. Highlighted are tree transpiration and structural differentiation as well as stand water balance of mono-specific and/or mixed forests with Picea abies, Pinus cembra, Larix decidua and Fagus sylvatica at sub-alpine (timberline ecotone), mountainous and colline elevation. Given the preliminary evidence, tree-allometric structures of relevancy for water transport appeared to be independent of elevation and forest type, although timberline trees under open-canopy conditions did not buffer transpiration by internal water storage and had enhanced foliage/sapwood area ratios. Transpiration appeared to depend on site conditions rather than site-dependent adaptation. Canopy transpiration approached 20% of the high seasonal precipitation at the mountainous site, with about 60% being ascribed to run-off, whereas the water budget of the colline site was balanced during summer. The water balance of the subalpine site resembled, at lower precipitation, that of the mountainous site. The derived hypotheses focus on mixed-stand transpiration under altitude-specific topography and moisture regimes, hydraulic adaptation and water demand versus uptake capacity, as this latter ratio is crucial at high altitude in view of expected warming. The clarification of consistencies relative to contrasts indicated by the pilot studies will be challenging across altitudes in view of tree species, forest types and topography.