A Seasonally Varying Tree Physiological Response to Environmental Conditions: Results From Semi-Arid China

Drought is the primary limiting factor for tree growth in water-stressed regions through its effects on xylem development. In this study, we developed intra-annual series of tree-ring stable carbon (delta C-13) and oxygen (delta O-18) isotopes from Chinese pine (Pinus tabuliformis) to investigate the ecophysiological responses of trees to drought during the growing season in a climatic transition zone between arid and semi-arid areas in north-central China. Distinct intra-annual patterns of delta C-13 and delta O-18 were revealed, with average annual amplitude of 0.26 and 2.72%, respectively. Tree-ring delta C-13 values increased from the early- to mid-growing season, peaked in the mid-to-late-growing season, and decreased at the end of the growing season. In contrast, delta O-18 values continuously decreased throughout the growing season. As expected, drought significantly increased delta C-13 and altered its intra-annual pattern, particularly toward the end of the growing season. However, there was no significant difference in the average intra-annual pattern in delta O-18 between dry and wet years. The tree-ring delta O-18 at the end of the growing season was significantly correlated with the September to October average soil water delta O-18, as simulated by a global isotope-enabled climate model, which is consistent with the high proportion of exchangeable oxygen atoms predicted by previous studies for cellulose synthesis in the late growing season. Plain Language Summary Understanding the drought response mechanisms of trees is important for predicting, evaluating, and managing forest resources. Stable isotopes in tree-ring cellulose can be used to trace the effects of drought stress on tree growth. We investigated the impact of drought on trees through intra-annual variations and climate response of tree-ring stable carbon and oxygen isotope ratios. The carbon and oxygen isotope profiles showed distinctive patterns. Carbon isotopes recorded a stronger drought signal than oxygen isotopes, and this signal was mainly reflected in the late growing season. For the oxygen isotopic signature of drought, the results were consistent with a model which predicts that, as drought continues throughout the growing season, the signature of evaporation becomes less pronounced.