Spruce Trees Have Stronger Drought Sensitivity at Low- Than High-Elevation Sites Across China’s Aridity Zones

The ecological relationships between tree growth and climate factors are important for the adaptation, growth prediction, and forest management of tree species against a background of global warming. Yet how these relationships for drought-sensitive conifer trees are changed and affected by elevation remains unclear. To investigate the effect of elevation on spruce tree growth–climate interactions, we collected a total of 1886 tree-ring cores (two cores per tree) from four spruce species, including Picea crassifolia Kom., Picea meyeri Rehd. et Wils., Picea schrenkiana Fisch. et Mey., and Picea wilsonii Mast., in 10 pure forests at a pair of high- and low-elevation alpine sites with the same slope across two aridity zones in China. We then constructed 20 stand tree-ring chronologies for the low- and high-elevation sites. Overall, the growth of spruce trees was more sensitive to drought variability at sites in the semi-arid zone whereas it was weakly limited by drought in the semi-humid zone. Further, spruce trees growing at high-elevation sites were sensitive to temperature unlike the stronger positive drought responses of those at low-elevation sites. In the latter half of the 20th century, the positive drought–growth relationships of spruce were enhanced at both high and low elevation across both aridity zones due to warming and increased drought intensity. Our dendrochronology-based findings suggest that spruce trees’ radial growth in stands at high elevation may have faced drought stress during three recent decades. Nevertheless, the low-elevation trees incurred stronger drought stress compared to the first half of the 20th century.