Biodiversity and climate change | Grasslands are an important carbon sink

Diverse meadows can better withstand climate change. Photo: imago/Zoonar Around a quarter of the earth's ice-free surface is covered by grassland. This includes savannahs, steppes, meadows and pastures. These ecosystems have long been underestimated in science and climate protection; they are not only home to a large number of plant and animal species, but also regulate the water balance and store around a third of terrestrial carbon.

A study recently published in the journal “PNAS” by the Potsdam Institute for Climate Impact Research (PIK) and the Christian Albrechts University in Kiel comes to the conclusion that the productivity of grassland and its climate protection function depend significantly on its plant diversity. A sharp decline in these could mean that meadows and pastures worldwide will be able to bind around 15 percent less carbon by 2050 than they are today. In the tropics and subtropics it could even be 50 percent less, even if global warming remains below two degrees.

»Compared to forests, relatively little carbon is stored above ground in grasslands. The largest part is in the ground," explains Stephen Wirth, lead author of the study and vegetation modeler at PIK. Plants get carbon from the atmosphere through photosynthesis. "The entry into the soil occurs largely through the roots, which, when they die, are decomposed by the microorganisms." In doing so, they use up some of the carbon, the rest remains in the soil.

Tropical and subtropical grassland ecosystems store particularly large amounts of CO2. The fact that this ability decreases sharply when there is a significant loss of species is due to the fact that, in contrast to other climate zones, non-dominant plant species make a major contribution to the productivity of the grassland. If they are lost, this becomes much more significant. At the same time, the adaptability of ecosystems decreases: “If previously dominant plants grow less under climate change, other, better-adapted plants can no longer compensate for this loss,” warns Wirth. Biodiversity is seen as a kind of insurance against climate change.

The fact that plant diversity promotes the productivity of grasslands is already known from local experiments such as the Jena experiment. Since 2002, various meadow plant species have been combined in around 150 plots on a 10 hectare area on the Saale in order to test how species diversity affects ecosystem processes. It was shown that more species-rich meadows use natural resources such as light, water and nutrients more efficiently and store more carbon. We are currently testing how they behave during extreme climate events in terms of plant biomass, soil temperature and nutrient content. It has already been proven in some cases that species-rich meadows are more stable. For example, it was shown that particularly wet or dry weather affects the growth of species-rich meadows less than that of species-poor meadows.

“Compared to forests, relatively little carbon is stored above ground in grasslands.”

The present study by PIK and the University of Kiel is now using a global vegetation model to examine for the first time how the loss of species under climate change affects productivity and thus CO2 uptake and carbon storage in the soil worldwide. In this sense, the Federal Agency for Nature Conservation (BfN) describes it as “groundbreaking”.

According to the BfN, grassland soils in our latitudes are one of the most important carbon stores. A particularly large amount of carbon is stored in old permanent grassland. The roots there reach deeper and there is more root mass overall than in species-poor commercial grassland. »If it is plowed and converted into land, greenhouse gases that have an impact on the climate are quickly released. That's why the protection of permanent grassland is so important not only with regard to biodiversity, but also to climate protection," explains BfN spokeswoman Katrin Hof. In general, different properties and survival strategies of individual grassland plants made the plant community as a whole more resilient.

Wirth cites land use change as the most important current driver of species loss - when extensive or unused grassland is converted into intensively managed grassland or arable land. If too many animals graze on an area, if they stay there too often or for too long, overgrazing occurs. »By removing large amounts of above-ground biomass (…) less can be invested in underground biomass. “The soil then becomes impoverished over time due to the lower inputs,” says Wirth.

With regard to Germany, the BfN explains that intensive grazing is still cheaper than stable keeping with intensive use of grassland. “As part of the intensification of forage production, large parts of the grassland were developed into very productive stands dominated by a few grass species,” explains Hof.

However, these areas require appropriate amounts of fertilizer to achieve high yields. In addition to the loss of species, this is also problematic for the climate, groundwater and surface water - nitrogen additions can even harm the productivity and resilience of grassland vegetation by primarily promoting rapid linear growth rather than the stability of the stalks.

However, the lower productivity of species-poor grassland types is not only problematic with regard to climate protection or their ability to adapt to climate. This also reduces the amount of feed available for grazing animals. This is particularly important for countries where pasture farming still serves as a source of livelihood for many people. The authors of the new grassland study are therefore committed to sustainable pasture management.

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