Abstract. Effects of climate change on the ecosystem productivity and water fluxes
have been studied in various types of experiments. However, it is still
largely unknown whether and how the experimental approach itself affects the results of such studies. We employed two contrasting experimental approaches, using high-precision weighable monolithic lysimeters, over a period of 4 years to identify and compare the responses of water fluxes and
aboveground biomass to climate change in permanent grassland. The first,
manipulative, approach is based on controlled increases of atmospheric
CO2 concentration and surface temperature. The second, observational,
approach uses data from a space-for-time substitution along a gradient of
climatic conditions. The Budyko framework was used to identify if the soil
ecosystem is energy limited or water limited. Elevated temperature reduced the amount of non-rainfall water, particularly
during the growing season in both approaches. In energy-limited grassland
ecosystems, elevated temperature increased the actual evapotranspiration and decreased aboveground biomass. As a consequence, elevated temperature led to decreasing seepage rates in energy-limited systems. Under water-limited conditions in dry periods, elevated temperature aggravated water stress and, thus, resulted in reduced actual evapotranspiration. The already small seepage rates of the drier soils remained almost unaffected under these conditions compared to soils under wetter conditions. Elevated atmospheric CO2 reduced both actual evapotranspiration and aboveground biomass in the manipulative experiment and, therefore, led to a clear increase and change in seasonality of seepage. As expected, the aboveground biomass productivity and ecosystem efficiency indicators of the water-limited ecosystems were negatively correlated with an increase in aridity, while the trend was unclear for the energy-limited ecosystems. In both experimental approaches, the responses of soil water fluxes and
biomass production mainly depend on the ecosystems' status with respect to
energy or water limitation. To thoroughly understand the ecosystem response
to climate change and be able to identify tipping points, experiments need
to embrace sufficiently extreme boundary conditions and explore
responses to individual and multiple drivers, such as temperature, CO2
concentration, and precipitation, including non-rainfall water. In this
regard, manipulative and observational climate change experiments complement one another and, thus, should be combined in the investigation of climate change effects on grassland.
Response of water fluxes and biomass production to climate change in permanent grassland soil ecosystems
