Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion

Under warmer springs at the alpine treeline, growth reactivation is advanced in trees, but delayed in shrubs. This phenological mismatch creates a competitive edge for trees and offers a novel explanation for the dynamics of alpine treeline in the face of climate change. Despite the importance of species interaction in modulating the range shifts of plants, little is known about the responses of coexisting life forms to a warmer climate. Here, we combine long-term monitoring of cambial phenology in sympatric trees and shrubs at two treelines of the Tibetan Plateau, with a meta-analysis of ring-width series from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Under a spring warming of +1 & DEG;C, xylem resumption advances by 2-4 days in trees, but delays by 3-8 days in shrubs. The divergent phenological response to warming was due to shrubs being 3.2 times more sensitive than trees to chilling accumulation. Warmer winters increased the thermal requirement for cambial reactivation in shrubs, leading to a delayed response to warmer springs. Our meta-analysis confirmed such a mechanism across continental scales. The warming-induced phenological mismatch may give a competitive advantage to trees over shrubs, which would provide a new explanation for increasing alpine treeline shifts under the context of climate change.

Automated summary

Under warmer springs, growth reactivation in trees is advanced while delayed in shrubs at the alpine treeline. This phenological mismatch creates a competitive advantage for trees and offers a new explanation for the dynamics of alpine treeline under climate change. A combination of long-term monitoring and meta-analysis revealed that shrubs are more sensitive to chilling accumulation, leading to delayed response in warmer winters and a shorter growing season. The findings suggest that the warming-induced phenological mismatch may contribute to the increasing shifts in alpine treeline.