What causes variable response in tree growth to climate change at a single site? A case study of Picea crassifolia at the upper treeline, Qilian Mountains, China

Key message In situ observations indicate that different levels of micro-standing could induce divergent temperature responses in trees at similar elevations but different slope aspects, which is supported by physiological process model simulation of radial incremental growth. Abstract The unstable response of tree growth to climate change is a well-documented, yet poorly understood phenomenon. It has been reported that Picea crassifolia exhibits a systematically variable response to climate forcing at a single high-mountain study area located at the upper treeline in the Qilian Mountains, China. This study used in situ meteorological observations to force a simplified but nonlinear mechanistic model of tree-ring width to identify how variable conditions translate into differences in radial growth within this site. The simulations suggest no significant difference in soil moisture between sites, but disparate diurnal micro-site temperature maxima, apparently amplified by a uniform warming, drive the divergent growth trends between subsites. For tree-ring-based reconstructions of surface temperature, such divergent growth trends challenge the validity of the assumption that a linear growth response to environmental conditions within and across study sites. Therefore, we recommend the development of precise, high-resolution data sets for calibrating, validating and reconstructing climatic change, given the potential for emergent non-stable tree growth–climate relationships.