Temperature regulates the resumption of cambial activity, but little is known about the effect of water availability on the onset of xylogenesis. Ren et al. monitor the onset of xylogenesis in Juniperus przewalskii during 2009–2014 at its upper and lower treelines in the semi-arid forested area of the northeastern Tibetan Plateau.
They find that the onset of xylogenesis is driven by an interaction between thermal and precipitation thresholds, challenging the general opinion that temperature is the only driver of growth resumption. Climatic thresholds for the onset of xylogenesis are key indicators of forest resilience and tipping points under changing climates.