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Global study shows causes for the dominance of woody plants in drylands

Precipitation, temperature, and grazing have the greatest influence on whether woody or herbaceous species grow in dryland areas. This has been shown by a large global study that includes Potsdam researchers.

Their investigation of 92 locations in 25 countries on six continents revealed that drylands all over the world tend to "lignify" given certain climatic conditions and grazing with specific livestock species. This can lead to drylands losing some of their important ecological functions. The results of the study have now been in the journal Science Advances.

Deserts, grasslands, steppes, shrublands, and savannahs—drylands can be found all over the world and make up a substantial 41% of the land surface. At the same time, the way they are structured and function is shifting as a result of human-caused climate change. One of the most striking transformations that can be observed in the world's drylands is the increasing spread of woody plants, especially in ecosystems dominated by grass—making them less and less usable as pastures.

"In order to predict and steer the effects of global changes on ecosystems, it is therefore crucial to understand the factors that influence woody vegetation in drylands, which are expected to be among the ecosystems most affected by climate change," says Dr. Liana Kindermann, one of the Potsdam-based co-authors of the study.

In its large global field study, the international research team investigated numerous factors that influence the occurrence of woody species—including biotic interactions, spatial and temporal fluctuations in the availability of resources due to climate and soil factors, and disturbances of natural processes caused by fire or grazing.

In the process, they demonstrated that climate, soil properties, and grazing have the greatest influence on the woody plant population.

When the amount of rain per year increases in areas where the soil has a low water-holding capacity, woody species profit from this and spread. This is also true for temperature: As average temperatures rise, tree cover increases—up to an average annual temperature optimum of 15°C.

If temperatures continue to rise, tree cover tends to decrease again. Last but not least, the study showed that grazing with certain animal species advances the transformation of drylands: Where intensive grazing with horses or cattle takes place, woody cover increases; with increasing grazing pressure from goats, it decreases.

However, the study also found that the processes were highly complex, as Prof. Dr. Florian Jeltsch from the University of Potsdam emphasizes, "Our analysis shows that several influencing factors and their interactions must be considered simultaneously in order to accurately predict and, ideally, control vegetation changes."