Reduced terrestrial ecosystem carbon uptake under future climate

Abstract:

Elevated atmospheric carbon dioxide stimulates plant productivity and ecosystem carbon gain but may also stimulate respiratory processes and thereby ecosystem carbon loss with the net balance being generally uncertain. In addition, climate driven warming and altered precipitation regimes under future climate also affects both uptake and release of carbon from terrestrial ecosystems making the net effect of climate change on ecosystem carbon budgets highly uncertain. In order to understand the response of these climate change driven changes, a large number of ecosystem experiments with single climate change factors have been conducted providing insight into the response of processes as well as ecosystems. However, ecosystems may respond in a complex and interactive way when all drivers of biological activity change in concert, which may not be well covered by past experiments nor reflected in existing Earth System Models causing potential over-prediction of future ecosystem carbon storage. It is therefore critical for future climate projections to understand better how changes in climate will interact with the effects of elevated CO₂.

In a Danish climate change experiment, CLIMAITE, a shrubland ecosystem was exposed to all three main climate change factors, elevated CO₂ and temperature and altered precipitation and the impacts on a range of ecosystem processes as well as the overall feedback to the atmosphere were studied and quantified. The measurements include direct measurements of carbon feedback from each experimental plot, which is almost never measured in elevated CO₂ experiments for practical reasons. Our unique results show that long-term (seven years) simultaneous exposure to all climate change factors reduced the carbon storage of the shrubland ecosystem while in contrast, exposure to single factors individually led to either no change or increased carbon storage. This demonstrates significant interactions among climate change factors, especially when CO₂ is involved leading to reduced sink strength in all combinations with elevated CO₂ because ecosystem respiration was stimulated more than plant carbon uptake by photosynthesis. This may have major implications for ecosystem carbon feedback under future climate. Models seem not to reflect these interactions very well.