Early monsoon drought and mid-summer vapor pressure deficit induce growth cessation of lower margin Picea crassifolia

The response of tree growth to climate can vary over time and geographical space. This study aims to evaluate and compare the climatic drivers of Qinghai spruce (Picea crassifolia Kom.), a species endemic to northwest China, over an elevational gradient, from its lower distributional margin to its upper distributional margin. We sampled pure Qinghai spruce stands and obtained time-series of growth from 25 trees each in three populations across it distributional range in the Helan Mountains. We found that trees at the lower distributional margin have lower first-order autocorrelation coefficients in annual growth and have experienced an increase in locally-absent rings (stem-growth cessation) since 2001. While all populations have a similar growth response to climate, trees at lower distributional margin are more sensitive to variations in June precipitation and July vapor pressure deficit (VPD) than populations at higher elevations. Early-monsoonal precipitation, June, appears to be the primary diver of stem-growth cessation events in trees at the lower distributional margin. Evidence indicates that July VPD exacerbates the effect of low June rainfall enough to increase stem-growth cessation in the lower distribution population. Our results find that precipitation is a major driver of Qinghai spruce growth, trees at lower distribution limit are more sensitive to precipitation than at higher elevations, and that high VPD exacerbates stress such that it increases the rate of stem-growth cessation. It seems that the increase in severity and frequency of stem-growth cessation could be a signal of an ecological tipping point in the lower population. Increased vapor pressure deficit and warming in the region would likely exacerbate the stress of low earlymonsoonal rainfall on Qinghai spruce growth. If this occurs, it could potentially increase the mortality rate of lower distributional margin trees, especially those that are already experiencing events of temporary growth cessation.