Soil hydrothermal dynamics drive growth responses of Siberian larch forests at the dry distribution margin in the Eurasian interior to climate warming

: Under global warming, understanding forest growth dynamics and their environmental constraints at distributional margins is critical for assessing the stability of arid ecosystems. Using four tree-ring sites distributed along a longitudinal gradient in the southern margin of Larix sibirica forests within the arid interior of Eurasia, we systematically investigated radial growth patterns and the relative influences of atmospheric and soil hydrothermal conditions. Results showed that basal area increment generally increased from east to west, although this spatial pattern has recently deviated under rapid warming. Tree growth responses were primarily regulated by vertical environmental gradients and exhibited pronounced seasonal shifts in soil resource utilization. As the growing season progressed, water dependence shifted from middle to deeper soil layers, while responses to soil temperature at the high-elevation site transitioned from shallow to deep layers. Both linear and nonlinear models consistently demonstrated that soil hydrothermal dynamics exerted stronger controls on radial growth than atmospheric variables. However, under persistent warming, the limiting effects of soil conditions weakened, whereas atmospheric hydrothermal constraints intensified. Forest responses to environmental extremes also displayed strong spatial heterogeneity. Low-elevation sites were constrained by extreme drought and promoted by extreme wetness, whereas high-elevation sites were respectively limited by extreme cold and facilitated by extreme warmth. Notably, non-growing season drought at the treeline promoted growth by reducing snow cover and freeze-thaw stress, which accelerated spring soil thaw and expanded the growth window, while excessive moisture constrained growth. These findings suggest that future assessments of ecotonal forest stability should incorporate subsurface soil hydrothermal dynamics and the asymmetric effects of climatic extremes.