Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit

Abstract

Carbon and oxygen isotope compositions (δ¹³C, δ¹⁸O) in tree rings have been shown to bear relevant climatic signals. However, little is known about the interrelationship between both isotopes in wood constituents for species from other than relatively wet climates. We hypothesized that in a species adapted to temporary droughts (e.g. Pinus halepensis Mill.) the signal derived from δ¹⁸O in precipitation would be hidden by the strong variability in leaf transpirative enrichment. To test this assumption, we compared the effect of precipitation, temperature and vapour pressure deficit (VPD) on δ¹⁸O and δ¹³C along 23 sites covering the ecological range for this species. We extracted the cores from the south side of four to six adult dominant trees per aspect (north/south) within each site. For each aspect and site, fragments of the period 1975–1999 were pooled and milled to a fine powder. To further test the postulated need for cellulose purification in the assessment of climatic information, we studied these relationships in whole and extracted wood, holocellulose and lignin. In all wood fractions, δ¹³C was related to annual precipitation [r=−0.58 (P < 0.01) to −0.78 (P < 0.001)] and VPD [r= 0.53 (P < 0.01) to 0.57 (P < 0.01)]. In contrast, for δ¹⁸O only holocellulose showed consistent relationships with climatic data, being strongly significant for VPD [r= 0.66 (P < 0.001)]. However, it was unrelated to modelled δ¹⁸O in precipitation, confirming that transpirative enrichment (driven by VPD) dampened the source signal in P. halepensis. The relationships between δ¹³C and δ¹⁸O were generally poor, regardless of the wood constituent, suggesting that although both variables were somewhat related to transpirative demand, they were relatively independent. This was further confirmed by building stepwise models using both isotopes to predict annual and seasonal precipitation [r²= 0.34 (P < 0.01) to 0.68 (P < 0.001)], temperature [r²= 0.15 (P < 0.05) to 0.37 (P < 0.01)] and VPD [r²= 0.31 (P < 0.01) to 0.55 (P < 0.001)]. We concluded that, even when partially describing the same climate variables, the information underlying the two isotopes can be regarded as complementary.