Topographic Heterogeneity Drives the Functional Traits and Stoichiometry of Abies georgei var. smithii Bark in the Sygera Mountains, Southeast Tibet

Bark is a multifunctional organ critical for tree survival, yet its functional plasticity in response to micro-environmental heterogeneity at alpine timberlines remains poorly understood. Here, we investigated the variations in bark physical traits (thickness, density), allometric scaling, and stoichiometric characteristics (C, N, P) of Abies georgei var. smithii (Viguie & Gaussen) W. C. Cheng & L. K. Fu on contrasting sunny and shady slopes in the Sygera Mountains, southeastern Tibetan Plateau. Despite the relative homogeneity of soil physicochemical properties between slope aspects, bark traits exhibited remarkable phenotypic plasticity. Trees on the shady slope possessed significantly thicker bark with higher nitrogen concentrations, adopting a “resource-acquisitive strategy”. Standardized Major Axis (SMA) regression indicated isometric scaling (b≈1.03) for trees on the shady slope, reflecting a sustained investment in bark thickness to provide thermal insulation against cold stress. Conversely, trees on the sunny slope exhibited negative allometry (b ≈0.87), characterized by denser tissues and elevated C/N ratios. This shift represents a conservative strategy geared toward hydraulic safety and resistance to high radiation and evaporative loss. Crucially, our results show that bark traits are largely decoupled from soil nutrient gradients, being shaped instead by microclimate. The distinct trade-off—prioritizing insulation on shady slopes versus conservation on sunny slopes—underscores the importance of phenotypic plasticity for the persistence of timberline species in a changing climate.