High-altitude water budgets: ecological and hydrological forest research in view of landscape modelling

Abstract

Modelling of water relations at the landscape and regionalscale has largely suffered from simplifications hardly val-idated or examined in their consequences for interpretingand quantifying interrelationships between system ecologyand hydrology. On such grounds, the overall aim of thestudies gathered under the special theme of this issue of‘‘European Journal for Forest Research’’ was to foster theprocess-related ecological and hydrological basis of eco-system and landscape modelling with particular attention tothe spatio-temporal variation in water fluxes. Researchobjects were mountainous forests with emphasis on theNorthern Alps of Central Europe, including comparisonswith colline and subalpine scenarios (e.g. Matyssek et al.2009) and with the specificities of high-altitude forests inthe tropics (Engelhardt et al. 2009).Regarding the Northern Alps, knowledge on the waterdemand of spruce/beech forests, in particular, with respectto mature stands, was rather limited, being in distinctcontrast with the ample evidence available on low-altitudeforests (e.g. Ko¨stner et al. 1998;Ko¨stner 2001). Such latterevidence can hardly be extrapolated, however, to moun-tainous scenarios, given the specific constraints at highaltitude imposed by climate and topography, their effectson stand structure and canopy exposure and the ultimatelyresulting consequences for system hydrology. Researchdemand is also given in more general terms in that thor-ough process understanding requires the analysis ofextremes in ecological gradients. In fact, mountainousforests represent such extremes—the more so as underlyingmechanisms of high-altitude hydrology still requireclarification.Emphasis on mountainous forests is also timely in viewof their increasing sensitivity to environmental impact asapproaching the timberline ecotone (Wieser and Tausz2007;Ko¨rner 2003). This tendency has become increas-ingly relevant in relation to climate change with non-clarified but probable effects on high-altitude ecosystems,and to efforts in forestry of re-establishing mixed stands(Dertz 1996), including at high elevation sites. In the caseof the Northern Alps, re-structuring at least comprisesbroadleaf European beech (Fagus sylvatica) and conifer-ous Norway spruce (Picea abies), although uncertaintyprevails about the water balance of such systems (Gesleret al. 2007). Conversely, mountainous forests are typicallyassociated with moist climatic and edaphic site conditions.In humid regions, vegetation becomes a crucial hydrolog-ical link, however, between soils and atmosphere, so thathigh-altitude ecology becomes a key determinant in achanging environment of evapotranspiration and run-off,and as a consequence, of hazardous risks by erosion,landslides and flooding, altogether arising from the scopeof site water balances. An exacerbating challenge forclarifying the linkage between water balance components