Climate extremes and grassland carbon dynamics

Mountain ecosystems have been considered as particularly vulnerable, and will likely be exposed to pronounced changes in climate. Climate extremes have been suggested to influence ecosystem processes more severely than gradual changes in temperature. The proposed project aims to study effects of summer drought and subsequent rainfall events, as well as consequences of reduced snow cover on the carbon (C) dynamics in mountain grasslands differing in land use. The set of questions addressed by the project is based on the hypotheses that 1) extreme climatic events have differential effects on the various components of the C cycle, 2) the response of C dynamics to extreme meteorological conditions are modified by land management and land-use change, 3) extreme events may induce memory effects that influence the susceptibility of ecosystem C dynamics to further climate extremes, 4) plant-soil interactions need to be considered explicitly for improving our understanding of the processes underlying ecosystem C dynamics as affected by climate extremes. In three mountain grasslands (meadow, pasture, abandoned) in the Austrian Central Alps a series of partly nested ecosystem manipulation experiments will be carried out, altering summer precipitation, snow cover and nutrient supply. C dynamics will be assessed by monitoring the net ecosystem exchange of CO 2 (NEE), soil respiration and its autotrophic and heterotrophic components, as well as above- and belowground net primary productivity and litter decomposition. Furthermore, implications of the climate manipulations for plant and soil carbon and nutrient status and for plant and microbial community composition will be assessed. Tracer experiments with stable C isotopes will be conducted to analyse the fate of fresh (versus older) C in the plant-soil system in response to summer drought and subsequent rainfall events. Thus, the proposed project will contribute to an understanding of patterns and processes of the terrestrial C cycle in response to changing environments.

Extreme climatic events have been suggested to increase in frequency and severity in the coming decades. There are still major gaps in our understanding of their effects on the carbon cycle, which may in turn exert critical feedbacks to the climate system. The project Climate extremes and grassland carbon dynamics analysed effects of summer drought and reduced snow cover on the carbon dynamics of mountain grasslands. It demonstrated that extreme summer drought can alter the short-term carbon balance by affecting ecosystem carbon uptake more strongly than carbon release. Summer drought reduced leaf area index and the transfer of carbon from photosynthesis to roots and microorganisms, and caused a change in the microbial community structure and an accumulation of labile carbon in the rhizosphere. Rewetting after drought caused a rapid loss of this accumulated recent carbon and a transient increase of soil CO2 emissions, and led to a fast recovery of microbial community composition. Abandonment of mountain grassland reduced water loss and thus the severity of drought effects. Recurrent drought over several subsequent years caused changes in the composition of plant species and of plant traits, and altered soil structure. It modified the drought response of grassland carbon uptake and soil CO2 emissions and their responses to rewetting after drought. Extreme winter events, i.e. strongly reduced snow cover, caused an increase in soil freezing events and exerted both immediate transient effects on microbial community composition and the turnover of labile C by microorganisms, as well as delayed responses on soil enzyme activities. The project findings therefore suggest that extreme summer drought, and to a lesser degree reduced snow cover, can affect the fate of carbon in the studied mountain grasslands and can alter plant and microbial community composition and functioning. Our research also indicates that land management and abandonment alter drought severity and drought responses of ecosystem processes and need to be considered when quantifying the resilience of mountain grasslands to extreme climatic events.