The significance of cellulose-18O for understanding water-use efficiency of grassland: Evidence from experimental, observational and process-based modeling studies

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Intrinsic water use efficiency (iWUE) of temperate grassland has increased strongly in the last century, but it is unknown if this climate-change response originated from increased net assimilation rate or decreased stomatal conductance for water vapor. Such uncertainty hampers our understanding of climate-change effects on carbon and water cycles in temperate grasslands. It has been proposed that the oxygen isotope composition of plant cellulose (18O in cellulose) carries information from stomatal conductance, but the mechanistic relationship between stomatal conductance and 18O in cellulose is complex, and strongly influenced by vapor pressure deficit (VPD) and morpho-physiological vegetation parameters. The present project seeks to unravel that relationship by a combination of experimentation, simulation with an isotope-enabled soil-vegetation-atmosphere model (MuSICA), and comparison of modeled and detailed empirical data sets. The experiment will assess the direct and interactive effects of CO2 concentration (200, 400 and 800 micromol mol-1) and VPD (0.47 and 1.17 kPa) on stomatal conductance, 18O in cellulose, leaf- and stand-scale WUE and the key parameters that link the 18O signal (18O enrichment in leaf water and leaf growth zone water, and sucrose in source leaves and the leaf growth zone) and stomatal conductance in controlled environment studies with Lolium perenne, the most important forage grass of temperate grassland. The simulation modeling will predict stomatal conductance and 18O in cellulose, and track the 18O-signal from its sources (precipitation waters and atmospheric humidity) through soil, xylem and leaf water to photosynthetic products and cellulose. Model predictions are tested with (i) detailed data sets of diurnal, seasonal and multi-annual variations of the isotopic composition (18O, 2H) of soil, stem and leaf water, 18O in cellulose and iWUE of a temperate grassland ecosystem at Grünschwaige (Freising/Germany), (ii) the results of the controlled environment experiment (see above), and (iii) century-scale time series of 18O in cellulose and iWUE of a diverse range of grassland ecosystems at the Park Grass Experiment in Rothamsted/UK. In addition, the project will also explore effects of different co-dominant plant species, functional group composition (grasses/legumes/dicots) and management (cutting frequency and fertilizer supply) on community-level 18O in cellulose using species-specific 18O cellulose data from the Veitshof long-term grassland management experiment at Technische Universität München. It is expected that the work will significantly improve our understanding of how environmental and vegetation factors determine 18O in cellulose and how climate change has affected last-century carbon and water fluxes in temperate grassland.