Recent rising temperatures drive younger and southern Korean pine growth decline

The Earth has experienced an unequivocal warming, with the warmest period of the past 150 years occurring in the last three decades. Korean pine (Pinus koraiensis), a key tree species in northeast Asia, is predicted to be particularly vulnerable to climate change. Here, we use dendrochronological methods to test whether the observed growth decline of Korean pine in northeast China is related to climate warming and whether climate-growth responses varied with age. A total of 628 cores from 401 trees across 16 sites were sampled over the entire distribution area of Korean pine in China. Samples were divided into three age classes: younger (50-130 years), middle (131-210 years), and older trees (>210 years), and measured by the ring-width index and basal area increment (BAI). Results showed a significant decline in BAI in most sites coinciding with an increase of temperature in the growing season and a decrease in precipitation since the 1980s. Meanwhile, we found that temperature-induced growth decline was significantly related to tree age. The BAI of younger trees decreased significantly and sharply (0.44 cm² year^-1, P < 0.0001), while old trees either decreased slightly or stabilized (0.04 cm² year^-1, P = 0.33). Tree growth in the southernmost locations was more likely to decline, what was most likely a result of forest-stand age. The age-related growth decline induced by climate warming might be explained by tree species traits, differences in growth rates between age classes and their relation to stress, changes in root system, competition/stand structure or physiological mechanisms. Our results might also predict that early stand process-thinning is exacerbated by warming and drying. This research informs that the age effect of growth response to rising temperature should be considered in forest management under climate change, and particularly models of future carbon cycle patterns and forest dynamics.