Assessing dynamics in eco-hydrological processes during drought stress across scales in a grassland community by stable isotopes

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Hydrological extreme events as extended drought periods predicted by climate change scenarios impact terrestrial plant ecology, community structure and resistance, particularly in shallow rooted communities such as grasslands. However, the interplay between structural changes in plant communities and the underlying mechanistic ecohydrological responses (i.e., water-use, carbon uptake) of grassland communities to drought spells remain largely unclear and are hard to observe. At distinct scales (from single plants to the plant community) a multitude of processes are involved and strong feedbacks exist between the dynamics of ecohydrological processes. Plant transpiration, for example, is controlled by the availability of soil water and transpirational water loss through stomates during photosynthesis driven by the atmospheric demand (leaf-to-air water vapour deficit). Moreover, the capacity to utilize various soil water pools and sensitivity of stomatal control of water loss is species-specific. In this regard, stable isotope compositions of H2O and CO2 are insightful indicators of processes occurring in the soil-vegetation-atmosphere continuum, specifically since new technical developments now enable near continuous observations.The aim of this project is the detailed mechanistic analysis of the vegetation eco-hydrological response to extended drought periods using H2O and CO2 flux and isotopic measurements. This will be linked to community responses regarding structure and biomass production. We will study these aspects from the plant to community level in three interacting work packages (WP). WP1 focuses on root-water uptake (RWU) response to extended drought of single plants as well as observing shifts in the plant-plant interaction balance under controlled laboratory conditions. WP2 will link responses in RWU profiles and net ecosystem fluxes of H2O and CO2 (partitioned into transpiration and evaporation and gross carbon uptake and respiration) to community structural responses. A two years precipitation manipulation experiment will be conducted under field conditions integrated in an international drought manipulation network. In WP3, data acquired in the laboratory will be used to calibrate the detailed three-dimensional soil-root model R-SWMS. This will provide up-scaled root-growth, RWU and transpiration modules for the one-dimensional soil-vegetation-atmosphere transfer model SiSPAT-Isotope. Simulations with SiSPAT-Isotope will then be confronted to data acquired in the field for estimation of the hydrological response to drought periods. This will deliver a process-based understanding of the eco-hydrological soil-vegetation-atmosphere feedbacks in response to extended drought periods, which will help in the development of sustainability predictions of the climate change impacts on grassland communities.