樟子松树轮不同组分的稳定碳同位素分析

Analysis of stable carbon isotopes in different components of tree rings of Pinus sylvestris var. mongolica

对大兴安岭北部樟子松树轮中的全木、综纤维素和α纤维素3种组分按早晚材分别测定稳定碳同位素(δ13C)值,分析比较早晚材两种材质的3种组分δ13C值差异,探讨其对气候环境变化的响应。结果表明:从组分来看,樟子松树轮综纤维素的δ13C指标更接近于α纤维素;从材质来看,樟子松树轮晚材不同组分的稳定碳同位素信号对气候环境变化响应的一致性和敏感程度要大于早材。樟子松树轮晚材的综纤维素δ13C指标是研究过去气候或环境变化的理想载体,而α纤维素在提取过程中很可能丢失了部分气候信息。

Analysis of stable isotopes in tree rings is an important tool for global change research.There are two major advantages of using tree ring stable isotope data;they can be used for paleoclimate reconstructions with perfect annual resolution,and the confidence limits can be statistically defined.In addition,trees grow worldwide,and so it is possible to examine geographical climatic variations that have occurred in the past.Another advantage of analyses based on isotope ratios in tree rings is that the physiological mechanisms controlling their variations are reasonably well understood,and are relatively simple compared with the numerous factors that control the annual growth increment.Most research on stable isotopes in tree rings has focused on the stable isotopes of carbon,because they are the easiest to measure and show the most rapid development.However,to date,there is no consensus on which component of tree rings best reflects the climatic and environmental changes in terms of the carbon isotope ratio（δ13C）.In this study,we investigated the differences in δ13C among different tissue components of tree rings.Two tree disks of Pinus sylvestris var.mongolica（SZX01-08 and BZ4-10-1.2） were sampled from forest on the north slope of Yilehuli Mountain,Greater Khingan（approx.51°57′—52°00′N,124°13′—124°36′E）.This site is located in the exclusive cool temperature zone of China that is dominated by coniferous forest vegetation.Tree-ring samples of earlywood（EW） and latewood（LW） were obtained with different sculpturing and pooling programs to avoid interference from non-uniform aspect distributions,i.e.both samples were sculptured for no less than two aspects（E＋S＋W for SZX01-08 and ENE＋W for BZ4-10-1.2）.Based on measurement of ring widths and cross-dating,the periods analyzed were the maximum growth periods;AD1904—1908/1924—1928/1944—1948 for SZX01-08 and AD1930—1944 for BZ4-10-1.2,according to polynomial fitting of ring-width sequences.The holocellulose and α-cellulose fractions were extracted using the Soxhlet method,and the purity of the fractions was confirmed using a NEXUS670 Fourier transform infrared spectrometer.Stable carbon isotope ratios in different components（α-cellulose,holocellulose and wholewood） of samples were measured using a ThermoFinnigan-DeltaplusXP mass spectrometer and expressed as δ13C relative to the Vienna Pee Dee Belemnite（VPDB） standard.On the basis of relative analyses,the following results were obtained：（1） The δ13C values were highest for α-cellulose followed by holocellulose,and lowest for wholewood.In general,the differences in δ13C values among the different components were greater in LW than in EW,as demonstrated by the standard deviations.All differences were statistically significant at the 0.001 level as determined by ANOVA tests.（2） The scatter plots of α-cellulose vs.holocellulose and α-cellulose vs.wholewood showed that holocellulose was more similar than wholewood to α-cellulose,in terms of δ13C values.There were no significant correlations between α-cellulose and holocellulose or between holocellulose and wholewood for EW.In contrast,correlations between the δ13C values of the three components were all statistically significant at the 0.01 level for LW.This indicated that the isotope signals of LW in tree rings are more coherent and more sensitive to changes in the local climate and environment than EW.（3） To some extent,the climate or environment signals reflected by the δ13C ratios are more significant for holocellulose than for α-cellulose.This probably implies that the climate signals were impaired during extraction of α-cellulose.The relationship between δ13C of wholewood and climate or environment factors was statistically insignificant.The extraction of α-cellulose is time consuming and results in decreased accuracy;therefore,it is preferable to base palaeoclimatic or palaeoenvironmental reconstructions on data obtained from holocellulose in LW.