Transpiration of conifers in contrasting environments

Future tree and forest transpiration as well as the potential role that forests will play in the protection against natural hazards in mitigating the impacts of climate change will depend not only on the response of trees to an altered atmospheric environment but also to their response to climate warming. A small temperature increase combined with an increase in the length of the growing season can alter the evapotranspiration regime in the European Alps. In inner alpine regions of the Tyrol the climax species Pinus sylvestris and Pinus cembra can be found in "xeric" inner-alpine valleys and on "mesic" sites at timberline. In timberline associated forests ample precipitation and moderate evaporative demand generally cause soil water content to be sufficiently high for the trees` water demand. In these latter terms, timberline sites contrast significantly from low elevation sites in dry inner alpine valleys where high evaporative demand and low soil water content cause stomatal closure in most tree species and thus limiting transpiration. Water uptake by the roots and the control of transpiration by the foliage are the two key factors influencing the tree s water balance. These both components along the soil-plant-atmosphere continuum may be affected by climate warming. In addition, rising temperatures are also expected to enhance the severity and frequency of drought which may affect the water balance of trees and stands and hence may probably also lead to changes in the distribution of individual species. The latter might be especially true at montane xeric sites where the water supply of trees might be limited due to a thin soil layer. For that reason, daily and seasonal tree water loss will be compared at both sites by measuring of the mass flow of xylem sap through tree trunks by means of the heat balance approach which offers a direct and quantitative assessment of crown transpiration. These measurements will then be compared to measurements of microclimate and soil water availability. Therefore, the goals of this project are to improve our understanding on tree and stand transpiration with respect to different climatic conditions in the Central Austrian Alps. Analysis of crown transpiration throughout the growing season will lead to a more comprehensive understanding on how abiotic factors will influence tree transpiration at different altitudes. Scaling individual tree transpiration to the stands canopy level will allow the assessment of the overall water balance, which finally may contribute to risk assessment against natural hazards like landslides and flooding.

The project Transpiration of conifers in contrasting environments aimed to improve our understanding on tree transpiration in an inner-alpine valley and within the treeline ecotone, and to understand how abiotic factors will influence tree transpiration. Currently, ample precipitation and moderate evaporative demand generally cause soil water content to be sufficiently high for the trees water demand within the treeline ecotone. In these latter terms, treeline sites contrast significantly from low elevation sites where high evaporative demand and low soil water content cause stomatal closure in most tree species and thus limiting transpiration. Water uptake by the roots and the control of transpiration by the foliage are the two key factors influencing the trees water balance. These both components along the soil-plant-atmosphere continuum may be affected by climate warming. Rising temperatures are expected to enhance the severity and frequency of drought which may affect the water balance of trees and stands and hence may probably also lead to changes in the distribution of individual species. Beside monitoring tree water loss under current climatic conditions we also performed ecosystem manipulation experiments. Our measurements confirmed that under current climatic conditions transpiration of conifers is mainly controlled by site prevailing conditions on evaporative demand in terms of irradiance and vapour pressure deficit. Even rainless periods of up to 30 days hardly affect tree water loss of conifers in an inner-alpine valley in the Tyrol, Austria. At our dry inner-alpine valley site, withholding precipitation throughout three growing seasons however, significantly reduced water loss of Scots pine and Norway spruce while transpiration of European larch was hardly affected. In all the three species investigated, artificial soil drought caused a decline in stem increment. Conversely, at treeline artificial soil warming caused root water uptake and transpiration of P. cembra to increase significantly above levels in control trees, while stem growth was not affected.Based on these results it might be possible that the water balance of the treeline ecotone in the Central Austrian Alps may change in a future warming environment, suggesting that run- off may decline, probably also diminishing latent risks of natural hazards. On the other side, increasing soil drought may change the distribution of individual tree species in inner alpine dry sites. Decreasing soil water availability in inner alpine dry valleys might be a selective advantage for P. sylvestris and P. abies, but be critical for L. decidua.