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Stable isotopes in wood cellulose of tree rings provide a high-resolution record of environmental conditions, yet intra-annual analysis of carbon and oxygen isotopes and their associations with physiological responses to seasonal environmental changes are still lacking. We analyzed tree-ring stable carbon (δ 13 C) and oxygen (δ 18 O) isotope variations in the earlywood (EW) and latewood (LW) of pines from a secondary forest ( Pinus kesiya ) and from a natural forest ( Pinus armandii ) in southwestern China. There was no significant difference between δ 13 C EW  and δ 13 C LW  in  P. kesiya , while δ 13 C EW  was significantly higher than δ 13 C LW  in  P. armandii . For both  P. kesiya  and  P. armandii , δ 13 C EW  was highly correlated with previous year’s δ 13 C LW , indicating a strong carbon carry-over effect for both pines. The intrinsic water use efficiency (iWUE) in the earlywood of  P. armandii  was slightly higher than that of  P. kesiya , and iWUE of both pine species showed an increasing trend, but at a considerably higher rate in  P. kesiya . Respective δ 13 C EW  and δ 13 C LW  series were not correlated between the two pine species and could be influenced by local environmental factors. δ 13 C EW  of  P. kesiya  was positively correlated with July to September monthly mean temperature (MMT), whereas δ 13 C EW  of  P. armandii  was positively correlated with February to May MMT. Respective δ 18 O EW  and δ 18 O LW  in  P. kesiya  were positively correlated with those in  P. armandii , indicating a strong common climatic forcing in δ 18 O for both pine species. δ 18 O EW  of both pine species was negatively correlated with May relative humidity and δ 18 O EW  in  P. armandii  was negatively correlated with May precipitation, whereas δ 18 O LW  in both pine species was negatively correlated with precipitation during autumn months, showing a high potential for climate reconstruction. Our results reveal slightly higher iWUE in natural forest pine species than in secondary forest pine species, and separating earlywood and latewood of for δ 18 O analyses could provide seasonally distinct climate signals in southwestern China.

Earlywood and Latewood Stable Carbon and Oxygen Isotope Variations in Two Pine Species in Southwestern China during the Recent Decades