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.
Highlights
Current and future climate change is expected to alter air temperature, CO2 concentration in the atmosphere, and precipitation (P ; Abbott et al, 2019; IPCC, 2018). Changes in these conditions will alter hydrological processes and affect the soil water availability, which is of critical importance for the agricultural sector in terms of plant development and food production (Thornton et al, 2014)
The climate change experiment Lysi-T-FACE is an experimental concept that has been designed to enable the warming of grassland plots, using an infrared heating system (Kimball et al, 2008), and enriching the CO2 content of the air using a Mini-FACE system (T-FACE; Miglietta et al, 2001)
The water use efficiency (WUE) at the wetter and colder climate was, on average, 1.53 g m−3 and, clearly higher than under warm and dry conditions (1.29 g m−3; Fig. 3d). These results show that decreasing P and increasing ET0 led to a decrease in WUE by 22 %
Summary
Current and future climate change is expected to alter air temperature, CO2 concentration in the atmosphere, and precipitation (P ; Abbott et al, 2019; IPCC, 2018). Since the late 19th century, air temperatures in the Alpine region have risen about twice as much as the global or Northern Hemispheric average (Auer et al, 2007) These strongly changing climatic conditions, in particular the expected increase in frequency and magnitude of extreme events such as droughts and heavy rainfall, potentially have adverse effects on the soil water balance and biomass production of grasslands, especially in mountainous regions. A shift in the temperature regime will prolong the growing season, which might change the vegetation composition, water use efficiency (WUE) of grasslands, enable a more intensified use of grassland sites (more frequent mowing), and increase the biomass production (Eitzinger et al, 2009; Tello-García et al, 2020) This might largely depend on the hydrological status of the ecosystem, which can be characterised by the Budyko framework as energy limited or water limited (Budyko and Miller, 1974). The combined effect of higher temperatures, causing increased ETa and the expected decrease in summer P , suggests more frequent and more severe occurrences of droughts (e.g. the 2018 European drought; Peters et al, 2020), which may lead to a lower water availability in the soil and, adversely affect the AGB production of grassland, as well as the quantity and quality of the drainage water (Herndl et al, 2019)
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