Extreme droughts threaten carbon storage in grasslands

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Shrubs and grasslands cover nearly half the planet and accumulate more than 30% of the world's carbon reserves. The study, published in Science, warns that a succession of extreme and prolonged droughts causes much greater losses in plant productivity—the creation of new organic matter through photosynthesis—in these ecosystems than a single year of intense drought or several years of moderate drought. The study analyzed experimental data from 74 grassland and shrubland ecosystems on six continents and reveals that not all regions of the planet will suffer the effects of extreme droughts equally. The results show that arid and semi-arid areas, such as the Mediterranean, the southwestern United States, southern Africa, and Central Asia, are the most vulnerable.

The work was led by Timothy Ohlert and Melinda D. Smith, researchers at Colorado State University, and has the participation of 120 international institutions, including the Center for Ecological Research and Forestry Applications (CREAF), the Institute for Global Change Research of the Rey Juan Carlos University (IICG-URJC) and the Pyrenean Institute of Ecology (IPE), a center of the Spanish National Research Council (CSIC), among other Spanish entities such as the Complutense University of Madrid, the University of Cádiz and the University of Alicante.

Specifically, the results show that, after four years of extreme drought, the average loss of plant productivity doubles compared to moderate-intensity droughts. This would jeopardize the global capacity of shrublands and grasslands to absorb and sequester carbon. Furthermore, essential activities such as livestock farming, since animals depend on grasslands for food, and agriculture, could be compromised because, among other things, shrublands and grasslands act as natural barriers to erosion and are reservoirs of biodiversity, for example, hosting pollinators and microorganisms beneficial to crops.

According to the team, the most damaging droughts are those that involve a lack of rainfall for several years and are extremely intense. "Historically rare, they occurred once every hundred years, but with climate change, it is estimated that they could occur more frequently and last longer," highlights Josep Peñuelas, a CSIC researcher at CREAF and co-author of the study. 

Biodiversity and humidity lose their protective effect

The study reveals that the most severe impacts of extreme droughts are concentrated in arid and semi-arid regions, such as the Mediterranean, where a lack of water can cause profound alterations in species dynamics. In these ecosystems, prolonged droughts can disrupt the emergence of new plants or drastically reduce their fertility, jeopardizing the survival of the most sensitive species. Added to this is greater variability in the rainfall cycle and high solar radiation, which increases temperatures and causes more water to evaporate.

In contrast, wetter grasslands and shrublands, such as those in northern Europe or the north-central United States, are more resilient to moderate droughts thanks to their greater diversity and water availability, among other factors. However, researchers warn that when extreme droughts occur for several consecutive years, even these ecosystems lose their resilience and see their productivity decline by up to 160% more than under moderate conditions—a risk that is increasingly likely under the current climate change scenario.

Decades of local study in a global framework

The research is part of the International Drought Experiment (IDE), the largest global experimental network dedicated to studying the effects of drought, with more than 170 researchers. The experimental design is unique in that it has been replicated in a similar way across the 74 study sites, encompassing different climate, soil, and species characteristics. To carry it out, the teams built rainfall manipulation structures that reduce each precipitation event by a predetermined amount. This demonstrates how widespread and globally significant the impacts of extreme droughts can be.

In the case of CREAF, the experimental plots studied are located in Garraf (Catalonia), are still active, and are among the oldest in the world: "Since the late 90s, we have been conducting experiments on the impact of drought on grasslands and shrublands. This provides us with very valuable and reliable data, which we have been able to contribute to this research," explains Romà Ogaya, CREAF researcher, co-author of the study, and coordinator of the field experiments. Meanwhile, the IICG-URJC has now accumulated 10 years of sampling in shrublands and grasslands in the southeast of the Community of Madrid, at the experimental facility set up on the El Espartal farm (Ciempozuelos): "Although the work published now focuses on four-year intervals, the study shows the importance of maintaining long-term experimental studies," notes Ana M. Sánchez, researcher at IICG-URJC. 

IICG-URJC researchers are working at the permanent experimental facility at Finca El Espartal, Ciempozuelos.

"The wealth of plant species in these systems, which are generally undervalued by society, performs key ecosystem services such as erosion control, runoff, and nutrient capture, which could be compromised if drought accelerated their disappearance," adds David Sánchez Pescador, a researcher at the IICG-URJC.

For its part, the IPE-CSIC was responsible for collecting the data obtained from subalpine pastures in the Aragonese Pyrenees. Six monitoring plots were established there, three of them with a drought treatment and three in which plant productivity was monitored for five years, in addition to other complementary climatic and soil variables. Plant productivity in the study area was less affected after the drought experiment compared to more arid areas or those that had suffered more extreme or prolonged droughts. "This highlights the great utility of coordinated studies to more deeply understand the expected impacts of droughts based on the specific conditions of each region," highlights Yolanda Pueyo, senior scientist at the IPE-CSIC.

“This large-scale, distributed team research effort provides a platform to quantify and further study how intensified drought impacts might manifest,” concludes Melinda D. Smith, one of the lead authors and a researcher at Colorado State University in the U.S.