Boulders as functional microrefugia: Quantifying a continental-like microclimate supporting Pinus cembra at its oceanic range margin

Climatic microrefugia allow some forest tree species topersist outside their main distribution range bylocally decoupling site conditions from the regional climate. Atits western, oceanic range margin inthe Swiss Prealps, the Swiss stone pine (Pinus cembra L.) occurs onlarge boulders embedded within subalpine forests dominated byNorway spruce [Picea abies (L.) H.Karst.]. Wehypothesised that these landforms generate acontinental-like microclimate enabling P. cembra persistence under otherwise sub-oceanic conditions, and weaimed toquantify this phenomenon. Using high-resolution data loggers, wemeasured air and soil temperature, thermal amplitudes, and soil moisture onthe summits and atthe bases often limestone boulders over atwo-year period. Linear mixed-effects models revealed apronounced microclimatic decoupling between positions. Boulder summits were consistently warmer and drier during the growing season. Incontrast, during winter, summits were significantly colder, while boulder bases remained thermally stable due topersistent snow insulation. The strongest microclimatic divergence occurred inspring, when temperatures atthe bases remained stable near 0 C, whereas summit temperatures were markedly warmer and more variable. This continental-like microclimate likely promotes P. cembra persistence. Our results highlight the importance of topographic heterogeneity for maintaining marginal tree populations.