Regeneration beyond the treeline: Seed and seedling performance of the Himalayan birch tree under climate warming

Abstract Predicting plant responses to climate change, such as shifting of mountain treelines, requires a mechanistic understanding of how abiotic and biotic factors interact to regulate tree regeneration. Although climate warming is anticipated to alleviate thermal constraints on tree growth beyond the current treeline limits, the interaction between biotic facilitation and climate in regulating tree regeneration remains poorly understood. In this study, we aimed to unravel the interactive role of microclimate, functional traits, soil properties and shrub facilitation on the recruitment of Himalayan birch tree, Betula utilis . For this, we combined controlled seed germination and seedling production with in‐situ transplant experiment at two sites along the treeline elevational gradient: below treeline (BTL), at treeline (TLN) and above treeline (ATL), supplemented with 3‐year field monitoring and multi‐driver modelling. We ran response‐surface and facilitation‐surface models to simulate the combined effects of soil warming and co‐varying environmental shifts on seedling survival and facilitation stability. The tree recruitment revealed a unimodal pattern, with both seed germination (23–31%) and seedling survival peaking at TLN. Seedling survival was predominantly dependent on biotic facilitation, with a 45% mean reduction in survival probability in open microsites compared to those beneath shrub canopies in TLN and ATL. Soil temperature emerged as the dominant abiotic driver, explaining 60–80% of the variation in modelled seedling survival. Our results reveal a novel warming‐facilitation paradox: while warming of +1.66°C could create thermal conditions in ATL analogous to the current optimum recruitment conditions of TLN, this same warming was predicted to cause a catastrophic decline in facilitation stability, with beneficial effects of shrubs rapidly reducing beyond a threshold of +1.5–1.7°C. Synthesis . Our findings demonstrate that the abiotic potential for treeline shift is nullified by the biotic collapse of critical facilitative recruitment niches. The current treeline represents a microclimatically modulated and biotically buffered optimum with no future analogue under climate warming. Our study provides a mechanistic explanation for widely reported treeline vegetation lag to climate and also posits that predictive models failing to incorporate the thermal sensitivity of biotic facilitation and its interaction with soil and resource‐acquisition strategies will overestimate climate warming‐induced treeline shifts.