Recent strengthening of correlations between tree-ring δ¹³C and δ¹⁸O in mesic western China: Implications to climatic reconstruction and physiological responses

We examined the temporal relationships between stable isotopes of carbon (δ¹³C) and oxygen (δ¹⁸O) from annual tree-ring cellulose of Abies georgei on the moist Batang-Litang plateau, western China. Climatic response analysis reveals that tree-ring δ¹³C contains a strong moisture signal, and that δ¹³C was especially influenced by relative humidity and precipitation in May through August during the period 1960-2005. In addition, tree-ring δ¹³C and temperature in May to August are positively correlated because of the high-elevation tree growth sites. Drought is also recorded in tree-ring δ¹³C, reflecting the integrated influences of temperature and moisture on stomatal regulation and photosynthesis. Generally, correlations between δ¹³C and δ¹⁸O remained positive, but not significant, in most of the investigated period, and this correlation switched to highly significant from 1969 onwards. Changes in the δ¹³C-δ¹⁸O relationship reflect changes in water stress on carbon isotopic discrimination, suggesting an increase in stomatal control of photosynthesis in recent decades, which is consistent with the overall drying trend in the region. Spatial correlation maps confirm that during the past century the response of tree-ring δ¹³C to temperature and precipitation differs before and after 1969. Using previously reported δ¹⁸O results and new δ¹³C results on the same samples, we determined that the climatic signal recorded in tree-ring δ¹⁸O is more stable than that of δ¹³C. Analysis of temporal changes in the δ¹³C-δ¹⁸O correlations indicates that dominant processes and the controlling factors on tree-ring δ¹³C potentially shift over the long term under temporal fluctuations of dry-wet spells, especially under recent global warming, and stomatal control of photosynthesis may adjust to region-wide changes in climate in this region where water stress on forest was formerly minor.