Abstract
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 (δ13C) and oxygen (δ18O) 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 δ13CEW and δ13CLW in P. kesiya, while δ13CEW was significantly higher than δ13CLW in P. armandii. For both P. kesiya and P. armandii, δ13CEW was highly correlated with previous year’s δ13CLW, 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 δ13CEW and δ13CLW series were not correlated between the two pine species and could be influenced by local environmental factors. δ13CEW of P. kesiya was positively correlated with July to September monthly mean temperature (MMT), whereas δ13CEW of P. armandii was positively correlated with February to May MMT. Respective δ18OEW and δ18OLW in P. kesiya were positively correlated with those in P. armandii, indicating a strong common climatic forcing in δ18O for both pine species. δ18OEW of both pine species was negatively correlated with May relative humidity and δ18OEW in P. armandii was negatively correlated with May precipitation, whereas δ18OLW 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 δ18O analyses could provide seasonally distinct climate signals in southwestern China.
Highlights
Due to the intense human impact, including large scale forest harvesting and resultant land use changes, the area of natural forests in tropical and subtropical China is remarkably decreasing in favor of secondary forest or plantation (Fang et al, 2001; Piao et al, 2009)
There were significant correlations between δ13CEW and previous year’s δ13CLW in both pine species (Table 1). δ18OEW was positively correlated with previous year’s δ18OLW in P. armandii, while there were no significant correlations between δ18OEW and previous year’s δ18OLW in P. kesiya (Table 1). δ13CEW of P. kesiya vs. δ13CEW of P. armandii, and δ13CLW of P. kesiya vs. δ13CLW of P. armandii were not correlated between two pine species, whereas both δ18OEW and δ18OLW in P. kesiya were positively and significantly correlated with those of P. armandii, respectively (Table 2)
Our results showed that pine species from natural forest (P. armandii) had a slightly higher intrinsic water use efficiency (iWUE) compared to pine species from a secondary forest (P. kesiya) (Figures 4A,B), indicating that the former could have a more conservative water use than the secondary forest pine species, which is consistent with our first hypothesis
Summary
Due to the intense human impact, including large scale forest harvesting and resultant land use changes, the area of natural forests in tropical and subtropical China is remarkably decreasing in favor of secondary forest or plantation (Fang et al, 2001; Piao et al, 2009). The transformation of land use and vegetation types may have strong impacts on the local climate and the regional hydrological cycle (Li et al, 2009). It is of urgent importance to understand the responses of different vegetation types to climate factors and environmental change, especially in tropical and subtropical areas in southwestern China that are exposed to extreme summer monsoon precipitation events which may trigger devastating flood events. Δ13C is frequently correlated with air humidity or precipitation in dry environments (Gebrekirstos et al, 2009, 2011; Kress et al, 2009; Brienen et al, 2011), whereas it is associated with irradiance factors and growing season temperature in humid environments (McCarroll and Loader, 2004). Recent studies reported an increase of iWUE with elevated CO2 in sub-tropical and tropical regions (Brienen et al, 2011; Xu Y. et al, 2014)
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