Accelerated succession in alpine treelines under climatic warming

Discerning how climate change drives species succession for forecasting future forest composition is a fundamental challenge. Controlled experiments have suggested that climatic warming accelerates species succession, putting pioneer species at a disadvantage. Warmers conditions would thus be expected to accelerate successional dynamics in populations at the limits of their thermal ranges, such as alpine treelines where conditions are harsh. We tested this hypothesis by reconstructing the spatiotemporal patterns of two tree species, the early- and late-successional Himalayan birch and Himalayan fir, respectively, at alpine treelines. We also examined how species interactions and successional strategies affect treeline dynamics using plot data and by fitting an individually based treeline model. Fir showed increasing recruitment and a higher rate of upslope shift (0.11 ± 0.02 m y-1) compared to birch (0.06 ± 0.03 m y-1) over the last 200 years. Spatial analyses evidenced that strong competition between these two species, particularly when the trees were young. Following an initial colonization by birch, fir started to establish 21 years later. Model outputs from various warming scenarios indicate that fir will likely accelerate its upslope movement in response to higher temperatures. By contrast, birch recruitment tends to decline with warming, forming stable treelines. Our findings point to accelerating successional dynamics in mixed treelines with late-successional species rapidly outcompeting pioneer species. Our findings provide strong evidence that climatic warming is affecting forest composition at climatically controlled range limits, offering a crucial insight into future changes and their threat on ecosystem services and conservation of bioresources.