Effects of climate change and forest gap disturbance on the growth characteristics of Japanese cypress and Japanese cedar on Mount Lushan, subtropical China

Climate change, largely caused by elevated carbon dioxide (CO2) concentrations, is a driver of lasting disturbances that cause changes in forest ecosystem functioning. This study aimed to investigate how Japanese cypress (Chamaecyparis obtusa) and Japanese cedar (Cryptomeria japonica D. Don) plantations in the subtropical forests of China respond to disturbances in tree growth under climate change. In this study, 23 canopy gaps were selected from two species in the Lushan National Nature Reserve of Jiangxi Province, China. Increment cores were obtained from trees retained near the edges of gaps and within the forest. Tree-ring chronologies were established, and the basal area increment (BAI) was calculated. The growth averaging method was used to detect growth release and analyze two key parameters of tree radial growth: magnitude and time lag. Moving correlation analyses were used to assess the long-term relationship between tree growth and climate, and regression analyses were used to quantify the relationship between the BAI and atmospheric CO2 concentrations. Species characteristics, tree distance from the gap center, gap size, and elevation all influenced tree growth release which was greater for Japanese cedar than Japanese cypress, and decreased with increasing distance from the gap center. Diameter at breast height (DBH) and pre-release growth influenced the time lag in growth release. The time-lag effect was more significant with smaller DBH and pre-release growth and did not differ between the two species. The correlations among growth, temperature, and precipitation were altered by the microclimatic environment created by the gaps. The BAI of Japanese cypress and Japanese cedar responded quadratically with increasing CO2 concentration (Ca), and the BAI increased with rising Ca, peaking at 360–380 ppm, followed by a decreasing trend. Due to the effect of the disturbance, there was a BAI increase of approximately 400 ppm (2015) for trees at the gap edge. Growth characteristics were influenced by tree- and gap-level variables. Disturbance altered the link between tree growth and climate responses, increasing tree growth sensitivity to climatic influences, shifting the quadratic relationship between BAI and CO2 concentration, and providing growth potential to trees that crossed the CO2 tipping point.