Even individual years of drought can have extreme consequences

parched grass under a shelter at the experimental site in Colorado

Shelters in the shortgrass steppe at the Central Plains Experimental Range test site in Colorado, USA. © Melinda Smith

Extreme droughts are known to be bad for plants. But their consequences, especially for grasslands and bush landscapes, have so far been significantly underestimated, as a global study shows. Accordingly, dry periods have a greater influence on the ability of grasses and bushes to absorb carbon dioxide from the atmosphere than previously thought. This has consequences for the global carbon cycle. At the same time, the study shows which ecosystems are particularly resilient to drought and which regions will particularly suffer from drought in the future.

Extreme droughts were once considered rare events. However, with climate change and global warming, they are now becoming more common and will in the future affect many regions of the world every two to five years, as numerous studies predict. This has an impact on the vegetation in the affected areas as well as their animal and human residents. But how damaging is drought specifically for certain groups of plants? For the first time, a research group has now comprehensively and systematically examined the consequences of even a single year of moderate or extreme drought on grassland and bushland. To do this, the team led by Melinda Smith from Colorado State University visited 100 different ecosystems on six continents and simulated different drought conditions there. At each location, the researchers prepared test areas with transparent roofs so that the plants underneath received less rain but the same amount of light during the growing season.

Grasses and bushes suffer more from droughts than expected

The experiments showed that even moderately severe droughts had negative effects on grassland and bushland. The plants then not only absorbed less water, but also less carbon dioxide from the air and produced less biomass from it. This means that they grew less - on average by around 20 percent in moderate droughts and by around 35 percent in extreme droughts. In grassland and bushland, the loss of growth is much greater than previously assumed based on individual studies. “The observed reduction in a key carbon cycling process following a single extreme drought event far exceeds previously reported losses for grasslands and shrublands,” says Smith. Droughts have a massive impact on the global carbon cycle because these plants then remove significantly less carbon dioxide from the atmosphere. So far, grasses and bushes store around 30 percent of the world's carbon dioxide.

Biodiversity and adaptation can partially buffer effects

In general, the experiments showed that the plants had the biggest problems in tests with extreme and unfamiliar drought conditions. In places where less severe drought was simulated or where droughts were naturally more frequent, the drought was also noticeable through losses in growth, but these were less drastic. Even in locations with a high level of biodiversity, the plants coped better with drought than in those with only a few species. However, these plant communities also suffered during extreme droughts. Whether a location received a lot or little rain the previous year did not change the damage during the drought, as the team found.

The scientists conclude from their results that we have so far significantly underestimated the consequences of future dry periods. "Our data suggests greater losses in drier locations, but when you get to the extremes - which is what the forecasts are - we can generally expect significant losses no matter where you are in the world," says Smith. The data also makes it clear which ecosystems will suffer particularly from global warming. “Our study provides both an overview of the global impacts of climate change and an insight into the regions that will be most stressed or most resilient in the coming years,” says the biologist. But even moderate losses in plant growth due to drought could have major impacts on the global carbon cycle and on the people who rely on these systems, for example for their livestock. In the future, the researchers also want to investigate the effects of several years of drought in a row.

Source: Melinda Smith (Colorado State University) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2309881121

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