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

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.