天山山区树轮气候研究若干进展

由于分布广泛、分辨率高、定年准确和气候敏感性好等原因,树木年轮在重建过去区域、半球甚至全球气候环境变化中扮演着重要角色。天山地处中亚干旱区,气候变化波动大,对全球变化响应敏感,植物生长的干旱胁迫作用强烈,天山山区分布有大量雪岭云杉和西伯利亚落叶松等长龄且对气候敏感的针叶树种,因此天山山区是树轮气候研究的理想区域。天山山区树轮气候研究始于20世纪70年代,尤其是近10 a有了长足的进步,有关天山山区树轮气候研究已经在国际上有一定影响。本文通过综述国内外对天山山区树轮气候研究的现状和进展,总结了近200 a基于树轮资料的天山山区较为一致的气候变化规律,并为进一步开展天山山区树轮气候研究提出建议。天山山区未来树木年轮气候学研究应在开展大量不同区域树木年轮气候学重建基础上,尝试理解树木径向生长对气候的响应机理研究,同时选用不同数理方法和多树木年轮指标进行长时间尺度和大空间范围重建工作,并讨论中亚干旱区过去千年气候变化的影响机制。     

Stable isotope coherence in the earlywood and latewood of tree-line conifers

Annually resolved and replicated tree-ring stable isotope series have the potential to reconstruct growing season environmental parameters over multi-millennial timescales. As this archive may require only minimal statistical detrending, it has the potential to preserve a large portion of low frequency climate signals. To date, many studies have utilised only the latewood portion of the tree ring, in an attempt to minimize carry-over effects from previous year reserves and maximise the annual nature of the climate signal preserved. However, the old trees from tree-line locations, necessary to build long chronologies, often display narrow ring-widths (< 0.5 mm), making accurate earlywood–latewood separation difficult and particular time consuming. The resulting samples may also be too small for efficient cellulose purification or multiple isotopic determinations. As photosynthates from the current year are predominantly used in conifer ring formation at marginal sites with short growing seasons, latewood separation may not be especially advantageous in determining a useful climate signal and therefore unnecessary where resources are limited. To test this hypothesis, Scots pine from Northern Norway and European larch from the Swiss Alps are used. Both sites are tree-line locations where growth is predominantly temperature limited. Tree rings were cut and extracted to cellulose for both the earlywood and latewood of each annual growth ring and stable carbon isotope ratios were measured. Our results demonstrate a very high common carbon isotope signal between earlywood and latewood in both species (r_larch = 0.68 and r_pine = 0.79), which also show high correlations with summer temperature over the investigated period (AD 1980–2004 for larch and AD 1929–1978 for pine). High turnover rates and small reserve pools at these tree-line locations may account for these high common signals. These results suggest that for European tree-line conifers, the separation of earlywood from latewood is unnecessary to resolve an annual isotopic signal and make a reliable climate calibration. Using the whole ring may provide additional analytical advantages and consequently even improve climate calibrations.     

Reconstructed summer temperature in the northern Rocky Mountains wilderness, USA

Ring widths from whitebark pine (Pinus albicaulis Englem.) and subalpine larch (Larix lyallii Parl.) collected at three high-elevation sites were used to develop tree-growth chronologies to reconstruct summer temperature anomalies. A step-wise multiple regression procedure was used to screen potential predictor variables to generate a transfer function capable of skillfully reconstructing summer temperature. The resulting regression model explained approximately 38% of the adjusted variance in the instrumental temperature record. The fidelity of the reconstruction was verified using product mean and sign tests, both of which suggested significant predictive power in the reconstructions (p < 0.05). Reduction of error (RE) and coefficient of efficiency (CE) measures were both positive, indicating the reconstruction contained useful climate information. Cool periods often coincided with reduced solar activity and/or periods of increased volcanic activity. Differences between this reconstruction and others encompassing a broader geographic scale highlight the importance of developing local reconstructions of climate variability, particularly when used in conjunction with ecological data sets that describe the occurrence of fires or insect epidemics. Mixed and divergent climate-response relationships were evident in the whitebark pine chronologies and suggest subalpine larch may be a more useful species than whitebark pine to target for the development of temperature reconstructions in this region.     

Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada

Modelling of tree-ring δ¹³C and δ¹⁸O data from the Columbia Icefield area in the eastern Rocky Mountains of western Canada provides fuller understanding of climatic and hydrologic variability over the past 1000 yr in this region, based on reconstruction of changes in growth season atmospheric relative humidity (RH_grs), winter temperature (T_win) and the precipitation δ¹⁸O-T_win relation. The Little Ice Age (~ AD 1530s-1890s) is marked by low RH_grs and T_win and a δ¹⁸O-T_win relation offset from that of the present, reflecting enhanced meridional circulation and persistent influence of Arctic air masses. Independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies. In contrast, the early millennium was marked by higher RH_grs and T_win and a δ¹⁸O-T_win relation like that of the 20th century, consistent with pervasive influence of Pacific air masses because of strong zonal circulation. Especially mild conditions prevailed during the “Medieval Climate Anomaly” ~ AD 1100-1250, corresponding with evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighbouring western USA.