No slowdown of growing season extension with warming in a permafrost-affected meadow on the Tibetan Plateau

The Tibetan Plateau holds the world's largest alpine permafrost and is undergoing an acceleration of warming. Phenological shifts over alpine permafrost in a warmer world have been little studied and are greatly underrepresented in current syntheses. Here, we conducted seasonal and gradient temperature-controlled experiments in a permafrost-affected meadow to evaluate how warming drives shifts in spring and autumn phenology, and associated growing-season length at both community and species levels. We found that there is no sign of slowdown in spring advance with warming under a higher year-around warming treatment, aligning with a future medium warming scenario. Although spring advance led to an advance in autumn senescence according to spring-only warming experiments, the advance could not offset the delay due to concurrent warming. As a result, year-around warming significantly delayed autumn senescence, although there was a deceleration in delay with warming under high temperature treatment than under the low one. This finding can be attributed to the possibility that winter warming is insufficient to reduce chilling accumulation, which would not delay spring phenology and then lead to a non-slowdown in spring phenological advancement. Taken together, there is no slowdown in an extension of growing season length with warming under a higher year-around warming treatment, with an increase of length by 9 and 21 days at the end of this century under a CO₂ stabilization and medium warming scenarios, respectively. Synthesis. Our results suggest that a continued growing season extension at least under the medium warming scenario would help permafrost-affected meadow ecosystems to mitigate permafrost carbon release on the Tibetan Plateau.