The consequences of elevated CO² and land use in alpine ecosystems

The consequences of elevated CO2 and land use in alpine ecosystems
This PhD thesis addresses two main aspects of Global Change and their impacts on alpine vegetation and eco-hydrology, (1) the steadily increasing concentration of CO2 in the atmosphere as well as (2) land use and its current decline across the Alps. Current and future rises of atmospheric CO2 concentration are commonly expected to stimulate photosynthesis and to reduce carbon limitation of plant growth. Whether this hypothesis holds for high-elevation pioneer plants in glacier forefields, which were suggested to be particularly responsive, was explored in a CO2 enrichment experiment in the Swiss central Alps. As a second direct biological effect, elevated CO2 often diminishes the water consumption of vegetation by a reduction in stomatal opening. These water savings should translate into temporally increased soil water contents and greater total runoff (the sum of all drainage processes). This question was combined with a second driver of evapotranspiration (ET), the density and height of vegetation. Biomass removal by land use likely reduces water vapour losses in alpine grassland, which should lead to increased runoff as well. Since land use is continuously declining across the Alps, the consequences of the subsequent vegetation transformation on the water balance of alpine grassland and finally, on water yield of alpine catchments were explored over an East to West gradient across the Alpine Arc together with partners in France and Austria.
Three seasons of free-air CO2 enrichment (FACE) at 2440 m a.s.l. revealed that none of the nine glacier forefield species grown in assemblages were stimulated by elevated CO2 throughout the course of the experiment, irrespective of any fertilizer addition, which by itself significantly enhanced growth. With a great robustness across species and growth conditions, our results thus suggest that glacier forefield pioneers, growing under harsh climatic conditions are not carbon limited at current atmospheric CO2 concentration. To quantify the partly opposing effects of elevated CO2 (more runoff) and land use abandonment (less runoff) on the water balance of alpine grassland we used intact monoliths of montane and alpine pastures and meadows in more than 250 weighing and non-weighing lysimeters at three sites in the French, Swiss and Austrian Alps. The experimental CO2 enrichment reduced grassland ET and significantly increased soil moisture but not runoff. Land use simulation by midseason clipping yielded significantly increased runoff sums due to reduced ET, with the highest increases observed in tall grassland of the lower alpine belt in Austria, and the lowest in naturally short vegetation of the upper alpine belt in Switzerland. Future elevated CO2 slightly counteracts the land use effects at canopy level, however, the net effect of declining land use and elevated CO2 is still clearly negative for catchment water yield and thus, for fresh water supply and potential hydroelectric power production. Although these economic costs of water loss due to declining land use are moderate per hectare of alpine grassland, the sums are substantial when scaled to the vast area potentially affected in the Alps. Our results underline the significance of continued land care for catchment value in different alpine regions across the Alps.