Decline in tree-ring growth of Picea mongolica and its intra-annual eco-physiological responses to drought and CO2 enrichment in semi-arid China

Abstract

Episodes of drought-induced decline in tree growth and mortality are becoming more frequent as a result of climate warming and enhanced water stress in semi-arid areas. However, the ecophysiological mechanisms underlying the impact of drought on tree growth remains unresolved. In this study, earlywood and latewood tree-ring growth, δ¹³C, and δ¹⁸O chronologies of Picea mongolica from 1900 to 2013 were developed to clarify the intra- and inter-annual tree-ring growth responses to increasingly frequent droughts. The results indicate that annual basal area increment residuals (BAI_res), which removed tree age and size effects, have significantly decreased since 1960. However, the decreasing trend of earlywood BAI_res was higher than that of latewood. Climate response analysis suggests that the dominant parameters for earlywood and latewood proxies (BAI_res, δ¹³C and δ¹⁸O) were drought-related climate variables (Palmer drought severity index, temperature, relative humidity, and vapor pressure deficit). The most significant period of earlywood and latewood proxies’ responses to climate variables were focused on June–July and July–August, respectively. BAI_res, and δ¹³C were significantly affected by temperature and moisture conditions, whereas δ¹⁸O was slightly affected. Decreasing stomatal conductance due to drought outweighed the influence of increasing CO₂ on intrinsic water use efficiency (iWUE), and ultimately led to a decline in BAI_res. Compared to latewood, the faster decreasing BAI_res and smaller increasing iWUE of earlywood suggested trees were more vulnerable to water stress in the early growing season. Our study provides insights into the inter- and intra-annual mechanisms of tree-ring growth in semi-arid regions under rising CO₂ and climate change.