Response of water fluxes and biomass production to climate change in permanent grassland soil ecosystems

Abstract

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.

Effects of dry spells on soil moisture and yield anomalies at a montane managed grassland site: A lysimeter climate experiment

“…This suggests that warming can cause a shift from energy‐limited to water‐limited conditions in such montane grasslands. As shown by Forstner et al (2021) shifts from energy‐limited to water‐limited conditions affected not only the soil water storage but also evapotranspiration and seepage. This complicates the assessment of drought impacts, because the impact of precipitation deficits on the individual components of the soil water budget depends on a variety of factors.…”

“…The lysimeter weight and the seepage weight provide information about the soil water balance components precipitation ( P ), actual evapotranspiration ( ET a ) and seepage rate ( NetQ ), that is, the water flux across the lysimeter bottom. To quantify P , ET a and NetQ , data processing with manual and technical correction was necessary to compensate disturbances due to external forces on the lysimeter such as wind, manual operations (cutting events, maintenance) or animals (Forstner et al, 2021). The changes in soil water storage (Δ S ) obtained from the lysimeter water budget (Equation ) were used to determine the amount of available moisture within a plant's root zone: …”

“…Furthermore, the 2018 drought led to ongoing water deficits through the year 2019 and into 2020 (Kleine et al, 2021). Recent studies in nonwater‐limited ecosystems showed that under heated (+3°C) compared to ambient conditions, soil water storage and aboveground biomass production tended to decrease in the drought year 2018 (Forstner et al, 2021). Therefore, understanding how nonwater‐limited ecosystems in a humid montane environment will be affected by more frequent and severe droughts under the current climate and future conditions is important to maintain grassland agriculture's economic and ecological benefits.…”

Same soil, different climate: Crop model intercomparison on translocated lysimeters

“…In addition to the space-for-time substitution approach, one possibility to deal with a limited site-specific range of observations for model calibration could be to use data from manipulative experiments to broaden the training dataset for model calibration. For example, Forstner et al (2021) showed that only the combined use of observational and manipulative climate change experiments was able to identify how altering individual and multiple drivers (i.e., reduced R, elevated CO 2 , and surface temperatures, increase in ET 0 ) affect ecosystem functioning of grassland ecosystems on changing climate conditions. Still, many challenges remain in addition to those discussed above, such as consideration of changes in soil properties due to climatic changes and the effects of these changes on model parameters or hints on missing model structures, depending on changing climatic conditions (e.g., Robinson et al, 2016Robinson et al, , 2019.…”

Evaluation of precipitation measurement methods using data from precision lysimeter network

“…This indicates a possibly small impact of NREs on the results of this study. However, NRE (i.e., dew, hoar frost and fog) can contribute a substantial amount of water for the ecosystem at the annual scale (Forstner et al, 2021;Groh et al, 2019), which demonstrates clearly that high-precision lysimeters are ideal tools measuring the different NRE of ecosystems. A higher temporal resolution of the data, possibly 10-min intervals, could help to temporally delineate precipitation events and NRE, as used by Groh et al (2018b).…”