“Transforming Terrains: The Impact of Drought on Grassland Ecosystems at Universität Innsbruck”

Grasslands encompass nearly 40 percent of the Earth’s land area and are vital to the global hydrological cycle. Despite their importance, the repercussions of climate change on these ecosystems remain inadequately understood. A new study, conducted as part of an international initiative funded by the Austrian Academy of Sciences, led by Michael Bahn from the Functional Ecology research team at the University of Innsbruck, now sheds light on the future of grassland ecosystems. The findings reveal the impact of drought, climate warming, and elevated CO₂ levels on the availability of soil moisture and plant water consumption. ‘We examined the effects of warming, heightened atmospheric CO₂ levels, and drought not only separately but also in conjunction, as anticipated to happen in the foreseeable future,’ explains Michael Bahn. Employing rainwater tagged with stable isotopes, the team achieved a comprehensive understanding of the transport and retention of moisture in the soil’s pore spaces, as well as its loss from the system through seepage and evapotranspiration.

Short-term benefits

In conditions of heightened CO₂ concentrations, the primary rooting zone of plants retains more moisture, as plants utilize water more effectively. Conversely, rising temperatures result in decreased soil moisture levels. Recurrent drought, particularly in a future warmer climate, leads to significant alterations in soil structure, affecting water retention. ‘In this most extreme scenario, water in the soil is not mixed well, flowing mainly through larger, rapidly draining pores, resulting in reduced penetration into smaller, slower-draining ones. The older water remains trapped there longer,’ clarifies Jesse Radolinski. Consequently, repetitive drought in a future climate disrupts hydrological connectivity, essential for plant water availability. The researchers highlight that this limitation in soil water flow has profound implications for the overall functionality and resilience of grassland ecosystems. Plants are compelled to utilize water more efficiently, which in the long-term could restrict their growth and renewal capabilities. ‘Simultaneously, we also noted that increased CO₂ can yield favorable short-term effects, such as quicker recovery after drought periods. However, these advantages are outweighed by the adverse impacts of escalating warming and drought on overall water availability and hydrological connectivity,’ remarks Bahn.

Distinct experimental setup

‘Our experiment is unparalleled, as we have been able to experimentally replicate the conditions of a future climate since 2014, allowing us to examine long-term effects,’ emphasizes Bahn. The researchers utilized a distinctive experimental facility, which they engineered at the agricultural research center in Raumberg-Gumpenstein, Styria. It consists of 54 test plots equipped with infrared heaters and CO₂ fumigation systems, as well as automated roofs to block rainfall. This setup permitted the simulation of various realistic climate change scenarios and a detailed examination of interactions between soil water and vegetation.

Frequent drought hinders soil water mixing

The principal conclusion of the study is that hydrological connectivity within the soil pore structure is significantly disrupted by recurring droughts in a warmer climate with elevated CO₂ levels. ‘Typically, soil water is assumed to be well blended following rainfall; however, our findings indicate that this blending will be severely constrained after successive droughts in a future climate. This has important ramifications for plant water utilization,’ explains Radolinski.

‘Our investigation reveals that the relationships between soil and plants may be considerably more intricate than previously believed. This has significant consequences for ecosystems’ capacity to withstand and recuperate from drought periods,’ summarizes Bahn. The study’s outcomes once again underscore the necessity to formulate strategies that bolster ecosystem resilience to climate change and to enhance global climate protection efforts.

Publication: Drought in a warmer, CO₂-rich climate limits grassland water usage and soil water mixing. Jesse Radolinski, Michael Bahn et al. Science, 16 Jan 2025, Vol 387, Issue 6731, pp. 290-296. DOI: 10.1126/science.ado0734