Global Warming, Local Responses: Why Treelines Do Not Respond Uniformly to Climate Change in the French Pyrenees

ABSTRACT Aim Treeline ecotones are key indicators of environmental change, yet their responses to climate warming vary markedly across landscapes. This study investigates how elevation and local environmental contexts moderate warming effects on treeline dynamics. Location French Pyrenees. Time Period 1950s to 2015. Major Taxa Studied Montane and alpine treeline ecotones are dominated, respectively, by Fagus sylvatica and Pinus uncinata . Methods We analysed 626 treelines using two metrics of long‐term ecotone dynamics: elevational shift and forest infilling. We tested three hypotheses: (i) whether elevation moderates warming effects on treeline dynamics; (ii) whether treeline responses remain spatially heterogeneous within elevational zones; and (iii) whether local environmental factors explain this heterogeneity. Climate change was quantified as absolute maximum temperature change (Δ T max ) between 1950–1984 and 1985–2020. Sites were objectively stratified into montane treeline ecotones (MTE; < 1942 m) and alpine treeline ecotones (ATE; ≥ 1942 m). Results Elevation strongly moderated climate change effects, with (i) montane treelines showing larger elevational shifts but weaker relationships with warming, likely reflecting forest rebound following pastoral abandonment, while (ii) alpine treelines showed smaller shifts but stronger warming relationships, likely constrained by slow demographic processes near tree physiological limits. We also found support for spatial heterogeneity persistence within both montane and alpine belts. Local environmental factors largely explained this heterogeneity, with well‐drained, steep sites exhibiting positive warming responses and poorly‐drained sites and crystalline substrates showing negative or neutral responses. Main Conclusions Treeline responses to warming are context‐dependent, with local environmental conditions moderating and even reversing climate effects. The spatial heterogeneity observed reconciles apparently contradictory patterns in global treeline literature by demonstrating that warming effects interact with local filters to produce heterogeneous outcomes even within elevational zones. These findings highlight the need for spatially explicit models of treeline dynamics that integrate local environmental contexts, improving our ability to monitor and conserve these sensitive ecosystems.