ALTITUDINAL VARIATION AND CONSISTENCIES IN WATER RELATIONS
OF FOREST TREES AND STANDS
K.-H. Häberle1 *, G. Wieser2, K. Patzner1, H. Blaschke1, R. Matyssek1
1
Dept. of Ecology/Ecophysiology of Plants,Technische Universität München/Weihenstephan,
D – 85354 Freising, Germany
2 Dept. Alpine Timberline Ecophysiology, Federal Office and Research Centre for Forests, Rennweg 1,
A – 6020 Innsbruck, Austria
* corresponding author: haeberle@wzw.tum.de
Abstract: Compared to available information about water relations of forest trees and stands at low
altitude, respective knowledge is scarce about mountainous and sub-alpine locations with its climatic
and topographic peculiarities, limiting assessments of water budgets and risks of natural hazards at
such sites. In view of this deficit, the presentation elucidates the outcome of three case studies on tree
transpiration, stand water balance and related structural differentiation, conducted in southern Central
Europe at sub-alpine (1950 m a.s.l., timberline ecotone), mountainous (1220 m a.s.l.) and colline
elevation each (485 m a.s.l.). Picea abies, Pinus cembra, Larix decidua and Fagus sylvatica are
compared as growing under mono-specific and/or mixed stand conditions. The contribution is guided
through the three underlying questions as to whether (1) stand transpiration differs between mixed and
monoculture of tree species at mountainous altitude, (2) transpiration is determined by altitude-specific
climatic influences rather than altitude-specific ecophysiological tree differentiation, and (3) foliage
type (evergreeness vs. deciduousness) governs the water demand of trees and stands, irrespective of
altitude. Regarding (1), stand transpiration did significantly differ between spruce monoculture and
beech and spruce in mixture at the mountainous altitude, although the ground area-related
enhancement of the mixture was small. Canopy transpiration appeared to reach a site-specific
maximum independent of the stand type and structure, given the prevailing closed-canopy conditions
and high seasonal precipitation. (2) Tree-allometric structures of functional relevancy for water
transport were largely independent of elevation and forest type, although timberline trees under opencanopy stand conditions did not display a buffering effect of tree-internal water storage on water
transport and had an enhanced foliage/sapwood area ratio. Transpiration depended on site conditions
rather than site-dependent ecophysiological adaptation. (3) At the tree and stand level, foliage type did
not govern water demand, irrespective of altitude, which was the case, in particular, in beech and
spruce of the mountainous and colline sites, and in cembran pine and larch at the timberline. At the
stand level, canopy transpiration during the growing season approached 19 % of seasonal
precipitation at the mountainous site, while up to about 59 % was ascribed to run-off. The colline site
reflected transpiration to reach up to 73 % of seasonal transpiration, as the water budget was
approximately balanced during the growing season. The timberline site resembled the mountainous
site in terms of proportional transpiration (up to 62 %), while indicating run-off of up to 35 % of
seasonal precipitation. Overall, the comparison of tree and stand water relations as well as associated
structural features along altitude indicates higher hydraulic challenge in the timberline trees.
Consequences of climate change on the biohydrology of the research sites are discussed.
Keywords:
High altitude, timberline, water balance, tree/stand transpiration, mixed/mono
culture, canopy structure, foliage type, tree allometry