A significant loss of plant diversity could result in grasslands worldwide, including meadows and pastures, absorbing 15 per cent less CO₂ through photosynthesis, a new study has revealed.
These reductions are estimated to occur by mid-century compared with a situation in which this diversity is maintained. The findings, published in PNAS by the Potsdam Institute for Climate Impact Research (PIK) and Kiel University (CAU), state that in some of the most affected parts of the world, CO₂ sequestration could drop by 50 per cent even if global warming is limited to 2 degrees Celsius.
“The strongest reduction in gross primary productivity (GPP) occurred around the equator, in Paraguay and India, where GPP was up to 50 per cent lower in the low-functional-diversity (FD−) scenario. GPP increased by up to 20 per cent in FD− in the African countries north and south of the equator and in Spain,” the study said.
“Climate change could reduce the productivity of plants that currently dominate pastures and meadows. But if plant diversity is maintained, other plants that cope better with climate impacts have the chance to become more prominent and stem the decline,” explained Stephen Wirth, a PIK researcher and lead author of the study, in a press statement.
He added: “It’s not just about lower rates of photosynthesis. Losing plant diversity also reduces forage supply for grazing animals, which, in turn, can threaten the livelihoods of people who depend on livestock grazing. Soils also store less carbon and contribute less to limiting global warming.”
The scientists used a model that represents grassland plants according to three broad ecological strategies: competitors, which grow rapidly and use resources efficiently; stress-tolerant plants, which invest in robust tissues and withstand difficult conditions; and ruderals, which grow quickly during favourable periods between disturbances.
The researchers used the model to examine two ecosystem services: gross primary productivity (GPP), the amount of carbon vegetation assimilates through photosynthesis, and soil organic carbon (SOC) stocks.
The study simulated grasslands from 1850 through 2100 using climate data from five general circulation models. It examined two emissions-and-development pathways: SSP1-2.6, a low-emissions scenario with average global warming below 2 °C, and SSP3-7.0, a high-emissions scenario with warming of around 4 °C.
In the high-diversity scenario, called high functional diversity (FD+), plant functional types already present at the end of the historical period were retained and new types were allowed to establish. In the low-diversity scenario, FD−, only the dominant types at the end of the historical period were retained, and the establishment of new types was prohibited.
High functional diversity (FD+) acted as an “ecological insurance policy”. In contrast, low-diversity communities (FD−) suffered severe breakdowns when their single remaining dominant species encountered bioclimatic limits.
The simulation found that, by the end of the century, global grassland gross primary productivity (GPP) in the FD− scenario was about 20 petagrams of carbon per year under the low-emissions scenario, compared with 25 petagrams per year under the high-emissions scenario.
The statement said that under 2 °C of warming, grasslands with strongly reduced diversity would take up around 11 billion tonnes less CO₂ per year through photosynthesis in 2050 than grasslands in which diversity was maintained.
“This reduction in uptake would be comparable to losing an area of grassland with average productivity roughly the size of the Arctic Tundra. The model also projects lower soil carbon stocks where diversity is reduced,” it said.
Under both climate pathways, the FD− simulations produced substantially lower GPP than the FD+ simulations.
“In some regions, remaining plants can compensate for the loss of other plant types, and productivity may even increase. Globally, however, the benefits of maintaining diversity prevail in both climate scenarios,” said Susanne Rolinski, a PIK researcher and co-author of the study. The researchers caution that the exact magnitude of these effects remains uncertain because the model simplifies plant traits and interactions.
The models showed that lower productivity in low-diversity grasslands was accompanied by a decrease in the dry matter available as forage for grazing livestock.
Removing non-dominant plant types did free up resources for the remaining dominant type, which often became more productive. However, that gain generally did not compensate for the productivity lost when stress-tolerant and ruderal types were removed, the researchers found.
The loss of functional diversity reduces soil organic carbon stocks, weakening the terrestrial carbon sink and threatening the availability of forage for livestock.
The authors conclude that preserving and actively supporting plant functional diversity in managed grasslands is crucial for climate change adaptation.
“Agricultural policy should support farmers in adapting their farms to climate change. This can help strengthen the livelihoods of people who depend on livestock grazing, particularly in the Global South,” said Friedhelm Taube, a researcher at Kiel University and co-author of the study, in the statement.