Indikatoren zur Schätzung des Trockenheitsrisikos in Buchen- und Fichtenwäldern

Abstract

Indicators for quantitative assessment of drought risks in beech and Norway spruce forests The application of climate models to forecast future forest development asks for quantitative drought-response relationships, with the term “drought” first needing a definition. The long-term data series of an intercantonal forest observation program allowed to test various drought indicators, to compare them and to derive quantitative relationships for beech and Norway spruce. For basal area increment of both tree species indicators of the site water balance (SWB) performed best. For beech mortality site water balance and several indicators basing on the ratio between actual and potential evapo-transpiration (ETa/ETp) were equivalent, whereas for spruce mortality ETa/ETp during the first 80 days of the season was the best indicator. With these indicators the average drought related growth reduction after 2003 – a year with extreme drought – was estimated to amount to 32% for beech and 37% for Norway spruce. Mortality of Norway spruce increased by 130%, whereas the estimates for the increase of beech mortality vary between 54 and 110%, depending on the indicator. The observed quantitative relationships for growth were applied to map drought responses of growth for Switzerland. The maps clearly show the dry regions of Switzerland (northern Switzerland, southern Jura foothills, Lemanic region, Valais and Rhine valley around Chur), where basal area increment of beech and Norway spruce was reduced by more than 40%.