The effects of individual climate factors on ecosystems are usually considered in isolation; however, in reality, they occur simultaneously and influence each other. Increasing CO₂ concentrations in the atmosphere contribute to climate warming, which in turn fosters more frequent and intense drought periods. A study from Innsbruck shows that the consequences of drought on grasslands in a future climate will be more severe than previously assumed. The work of a team led by Maud Tissink and Michael Bahn from the Department of Ecology has been published in Science Advances.
Until now, it has been assumed that the effects of individual climate factors on ecosystems can largely be summed, meaning that the combined effect of increased CO₂ and warming approximately equals the sum of the individual effects. This assumption is practical for climate research because it simplifies model calculations. However, the new study provides evidence that this simplification is reaching its limits. In a multi-year field experiment in grassland, the research team systematically combined the three central climate factors-CO₂, warming, and drought-and examined their effects on key ecosystem functions: carbon uptake through photosynthesis, CO₂ release through respiration, and water balance.
A Time Machine for a Grassland Ecosystem
Lead author Maud Tissink conducted measurements over two years using special ecosystem chambers that allow for the capture of CO₂ and water flows under controlled conditions. The experiment acts like a time machine: it simulates the conditions expected in the coming decades and makes visible how the ecosystem responds. "Such three-factor experiments in ecosystems are rare worldwide because they are extremely labor-intensive. There are only a handful of such experiments systematically considering more than two factors," explains Michael Bahn , Professor at the Department of Ecology and head of the Functional Ecology research group .
Synergy Instead of Addition
The central finding is that when increased CO₂ and warming occur together, their effects can amplify each other, such that the combined effect is greater than the sum of the individual effects. When these two factors are combined with drought, the impact of drought on key ecosystem functions is disproportionately amplified. "It was actually expected that with increased CO₂, plants would conserve water, while warming would increase water demand, so the two effects would buffer each other in combination. However, the effect of warming was so strong that water conservation was insufficient. The negative effects of drought were thus intensified," explains Bahn. While drought under current conditions already significantly restricts carbon uptake, this effect under simulated future conditions with higher CO₂ levels and higher temperatures is much stronger: "In a future climate, grasslands reduce their overall carbon uptake due to drought four times more than today. The efficiency with which plants use water for carbon uptake also deteriorated significantly: for each unit of carbon absorbed, the ecosystem had to release more water, even though it was already drier," says Maud Tissink.
Consequences for Climate Models
These new findings are particularly relevant for modeling the interactions between ecosystems and the climate system. Ecosystem models based on the assumption of largely additive effects could systematically underestimate the consequences of future droughts. "Our data clearly show for Central European grasslands that the negative effects of drought in a future climate can be disproportionately amplified. This could also mean that the ability of these ecosystems to contribute to CO₂ uptake on land in a future climate could be lower than previously assumed," says Bahn. At the same time, he warns against generalizations: how far the findings can be directly applied to other climate zones and ecosystem types still needs to be clarified through similar experiments. It is clear that the effects of multiple combined climate factors on ecosystems must be systematically studied in future research to better assess the global consequences.
