<b>Data and code from: </b><b>Regeneration beyond the treeline: Seed and seedling performance of the Himalayan birch tree under climate warming</b>

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.</p><p dir="ltr">3. 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% variation in modelled seedling survival.</p><p dir="ltr">4. 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 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.</p><p dir="ltr">5. <i>Synthesis</i>.

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 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.</p>