Long-term variation of tree growth and intrinsic water-use efficiency in Schrenk spruce with increasing CO2 concentration and climate warming in the western Tianshan Mountains, China

We combined dendrochronological methods and interannual delta C-13 measurements to investigate radial growth and physiological responses of Schrenk spruce (Picea schrenkiana) in response to rising atmospheric CO2 concentration (Ca) and changing climate in high-elevation forests in China's western Tianshan Mountains. The mean maximum temperature in May to August, reconstructed from delta C-13, revealed an overall warming trend, with persistent warm periods from 1910 to 1920, and from 1970 to 1980. Intrinsic water use efficiency (iWUE) increased by 28 % over the last 160 years; temporal trends in iWUE were calculated under three theoretical scenarios as a baseline for interpreting the observed gas-exchange at increasing Ca. Basal area increment (BAI) increased by 51.4 % since 1850 with two apparent increases and decreases. Trees exhibited sharp declines in BAI along with enhanced iWUE during the warmer periods; this was possibly due to a reduced stomatal conductance which prevented excessive water loss from trees. Conversely, BAI increased at reduced iWUE (-3.6 %, -7.4 %) during two cold-wet periods (e.g., 1880 to 1992, and 1945 to 1960), suggesting that a diminished water stress caused the observed growth pattern. However, BAI increased significantly (49.4 %) from 1965 to 1983 with constant intercellular atmospheric CO2 concentrations (Ci) response scenario under acute water limitations, indicating the CO2 stimulation of tree growth. These results showed that even at high elevations, increased iWUE may not lead to long-term enhancement of tree growth, and other factors may counteract CO2-fertilization effects, especially those related to a warming-induced drought. The results of this study suggest that the current models may overestimate the sink capacity of temperate forests, and indicate that multi-proxy records are needed to disentangle the role of a limiting factor in modulating the response of the Schrenk spruce forest to current climate change scenarios.