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Elevated CO2 level in the atmosphere is expected to improve the tree growth rates and intrinsic water-use efficiency (iWUE). Although current results inferring from tree rings found the tree growth decline in water-limited area, it is still unclear whether spruce trees in humid southwestern China benefit from the increasing CO2. In this study, tree-ring width data were used to investigate the tree radial growth rate of Chuanxi spruce (Picea likiangensis var. balfouriana). Moreover, combining with the tree-ring carbon isotope date, we analyzed the physiological responses of Chuanxi spruce to rising CO2 concentrations in the atmosphere (Ca) associated with climatic change in southwestern China. From 1851 to 2009, iWUE of Chuanxi spruce rose by approximately 30.4%, and the ratio of atmospheric CO2 to leaf intercellular CO2 concentration (Ci/Ca) showed no significant trend in the study area. The result suggested that Chuanxi spruce used an active response strategy when Ca was significantly increased. iWUE showed a significant increasing trend in parallel with tree radial growth, indicating that the increasing iWUE resulted in an increase in radial growth. These results suggest that spruce forests in southwestern China have not shown declining trends under increasing Ca and climate change scenarios, in contrast to trees growing in water-limited areas. Therefore, spruce forests benefit from the increasing CO2 in the atmosphere in the humid areas of southwestern China.

There are differing views regarding the climatic factors that limit radial growth of Qilian juniper (Juniperus przewalskii Kom.) at the upper treelines on the northeastern Tibetan Plateau. In this study, trees from an upper treeline site in the Anyemaqen Mountains of the northeastern Tibetan Plateau were selected to present new evidence that low temperature limits the radial growth of Qilian juniper. Using a signal-free regional curve standardization (SF-RCS) method, a ring-width chronology for Qilian juniper was developed extending from AD 1082 to 2010. The results of correlation analysis between tree-ring index and instrumental climatic variables showed that both winter (December in the previous year and January in the current year) and summer (July and August in the current year) temperatures were significantly and positively correlated with the tree-ring index. Precipitation in June was also significantly and positively correlated with the tree-ring index, but was not as important as low temperature in controlling tree growth. To test the key limiting climatic factor for tree radial growth at different altitudes, an indicator termed the “relative distance to upper treeline” (RDUT) was developed to quantify the representativeness of collected samples for the forest’s upper treeline. The RDUT showed that the upper 20% of the forest belt may be an important boundary in terms of capturing the temperature signal from tree-ring width at the upper treelines on the northeastern Tibetan Plateau. Our findings enhance the existing understanding that temperature is the limiting factor at upper treelines on the northeastern Tibetan Plateau, and will be useful in the reconstruction of past temperature in future studies.

Having analyzed the tree ring width and maximum latewood density of Pinus densata from west Sichuan, we obtained different climate information from tree-ring width and maximum latewood density chronology. The growth of tree ring width was responded principally to the precipitation in current May, which might be influenced by the activity of southwest monsoon, whereas the maximum latewood density reflected summer temperature (June-September). According to the correlation relationship, a transfer function had been used to reconstruct summer temperature for the study area. The explained variance of reconstruction is 51% (F=52.099, p