NCCR Climate (Plant/Soil)

Keywords: climate change, drought, temperate grassland, stable isotopes

Context

With increasing clarity, results of different climate models show that the frequency of extremely dry summers like 2003 will increase in Switzerland. For 2070, models predict an average reduction in Swiss summer precipitation of around 20% and a maximal reduction of 50% compared to today. As Switzerland holds large water reserves in the Alps, drought has not been considered a problem for a long time. Additionally, due to the topographic diversity of the country, drought might be a regional rather than a national problem. However, there is no doubt that future summers will generally be drier. Since grasslands cover a large part of Switzerland at all elevations and are the base to agricultural milk and meat production the focus of Plant/Soil is on this ecosystem type. While the consequences a decrease in summer precipitation will have on temperate grassland ecosystems are not very well understood, we need a better understanding of the response of grasslands to drought to be able to develop viable adaptation strategies in advance. Besides the most obvious harvest reduction, drought can also considerably affect the carbon cycle.

Objectives

Assess the effect of experimental summer drought on temperate grassland systems at different altitudes, focusing on:

• above-ground biomass production
• below-ground biomass production
• species composition
• vegetation structure (height, LAI)
• C and N concentrations and C/N ratio in biomass
• mechanisms involved using stable isotopes
• water sourcing of species using ¹⁸O signature of root crown water

Experimental Design and Methods

The effects of drought on productivity and community structure of grasslands is studied in a field experiment at three different altitudes in Switzerland. At the ETH research stations rain shelters are built up to keep the rain out. These shelters are supposed to manipulate nothing but soil humidity. This is controlled with continuous measurements of soil and vegetation microclimate (temperature, humidity, evapotranspiration and PAR). Rainfall data are obtained from an external source. The vegetation structure is described with its height and LAI. Biomass is sampled throughout the growing season (corresponding to the normal mowing dates of the stations) and separated into species. Root biomass is estimated using ingrowth cores. The dried biomass is then weighed and analysed for C/N, ¹³C and ¹⁵N. Especially the analysis of the stable carbon isotope will give insight into the severity of the water stress for different species. Root crown and soil water are sampled in campaigns and analysed for ¹⁸O to learn more about the actual source of water for the plants.