Warming induced tree-growth decline of Toona ciliata in (sub-) tropical southwestern China

Tropical forests play an important role in the global carbon cycle and climate regulation. However, our understanding of how Asian tropical forest growth responds to climatic variations is still limited. We developed tree ring-width chronologies of Toona ciliata from 90 trees (139 cores) from two study regions in the tropical/subtropical forests in Yunnan, southwestern China. Bootstrapped correlation analysis revealed positive moisture sensitivity (precipitation, self-calibrated Palmer drought severity index, relative humidity, and soil moisture) and negative temperature sensitivity of T. ciliata, and the relationship was strongest during dry and/or dry-to-wet transition months, indicating that radial growth of T. ciliata is primarily limited by moisture availability during early growing season. Furthermore, radial growth of T. ciliata was significantly and negatively correlated with the vapor pressure deficit and potential evapotranspiration during dry and/or dry-to-wet transition months. We analyzed long-term growth trends of T. ciliata using ‘size class isolation' (SCI) and ‘generalized additive mixed models' (GAMM) approaches which remove the effects of tree size on tree growth. We detected decreasing growth trend for both approaches at both study regions, indicating that the growth decline of T. ciliata stands in southwestern China is likely due to global warming-induced moisture deficit. The growth of T. ciliata trees is likely to continually decline under projected warming and drying conditions. The observed growth declines of T. ciliata raised concerns about developing sustainable management and conservation programs for tropical/subtropical forests in China.