Treeline stress amplifies divergence in carbon-water responses between evergreen and deciduous conifers.

As alpine forests face increasingly intensified thermal and hydric constraints under climate change, understanding species-specific ecophysiological responses to environmental drivers across altitudinal habitats is essential. Yet whether coexisting evergreen and deciduous conifers adopt contrasting carbon-water regulation strategies under altitudinal stress remains unclear. We examined seasonal gas exchange dynamics in mature spruce (Picea meyeri) and larch (Larix principis-rupprechtii) across contrasting altitudes in the Luya Mountain, northern China. Net photosynthetic rate (Pn), stomatal conductance (Gsw), and leaf water potential (Ψl) were monitored throughout the growing season to quantify how stomatal behavior and environmental drivers regulate carbon-water coordination. At lower altitudes, both species exhibited similar seasonal dynamics, with vapor pressure deficit emerging as the primary constraint, indicating convergence in carbon-water coordination under relatively favorable conditions. In contrast, environmental controls on carbon-water regulation differed between species at the treeline: spruce was predominantly temperature-driven and weakly responsive to soil moisture, whereas larch maintained higher Pn and Gsw and responded mainly to soil water availability. These contrasting controls indicate distinct stomatal regulation strategies under treeline stress, with spruce may adopt a more conservative carbon-water balance and larch may exhibit greater regulatory flexibility. Overall, altitudinal stress shapes species-specific carbon-water coordination, promoting convergence at lower altitudes but amplifying functional divergence at the treeline. This mechanistic insight improves predictions of species-specific responses of alpine conifers to ongoing climate change.