Significance

Cold regions contain vast stores of permafrost carbon. Rapid warming will cause permafrost to thaw and plant respiration to accelerate, with a resultant loss of CO₂, but could also increase the fixation of CO₂ by plants. A network of 32 eddy covariance sites on the Tibetan Plateau, which has the largest store of alpine permafrost carbon on Earth, shows that this region functions as a net CO₂ sink. Our sensitivity analyses, experiments, and model simulations consistently showed that the fixation of CO₂ by plants outpaces the loss of CO₂ from permafrost and accelerates plant respiration. This indicates a plant-dominated CO₂ balance on the Tibetan Plateau, which could provide a negative feedback to climate warming.

Abstract

High-latitude and high-altitude regions contain vast stores of permafrost carbon. Climate warming may result in the release of CO₂ from both the thawing of permafrost and accelerated autotrophic respiration, but it may also increase the fixation of CO₂ by plants, which could relieve or even offset the CO₂ losses. The Tibetan Plateau contains the largest area of alpine permafrost on Earth. However, the current status of the net CO₂ balance and feedbacks to warming remain unclear, given that the region has recently experienced an atmospheric warming rate of over 0.3 °C decade⁻¹. We examined 32 eddy covariance sites and found an unexpected net CO₂ sink during 2002 to 2020 (26 of the sites yielded a net CO₂ sink) that was four times the amount previously estimated. The CO₂ sink peaked at an altitude of roughly 4,000 m, with the sink at lower and higher altitudes limited by a low carbon use efficiency and a cold, dry climate, respectively. The fixation of CO₂ in summer is more dependent on temperature than the loss of CO₂ than it is in the winter months, especially at higher altitudes. Consistently, 16 manipulative experiments and 18 model simulations showed that the fixation of CO₂ by plants will outpace the loss of CO₂ under a wetting–warming climate until the 2090s (178 to 318 Tg C y⁻¹). We therefore suggest that there is a plant-dominated negative feedback to climate warming on the Tibetan Plateau.

The full article is available on the PNAS website at https://www.pnas.org/content/118/33/e2015283118