NOAA/WDS Paleoclimatology - Sniderhan - Trail Valley Creek - PCMA - ITRDB CANA614

Northwestern Canada's boreal forest has experienced rapid warming, drying, and changes to permafrost, yet the growth responses and mechanisms driving productivity have been understudied at broad scales. Forest responses are largely driven by black spruce - the region's most widespread and dominant tree. We collected tree-ring samples from four black spruce-dominated sites across 15 degrees of latitude, spanning gradients in climate and permafrost. We investigated (1) differences in growth patterns, (2) variations in climatic drivers of growth, and (3) trends in water use efficiency (WUE) through 13C isotope analysis from 1945 - 2006. We found positive growth trends at all sites except mid-latitude, where rapid permafrost thaw drove declines. Annual growth was lowest at the tree limit site and highest at the treeline. Climatic drivers of these growth patterns varied; positive growth responses at the northerly sites were associated with warmer winters, whereas d13C trends and climate-growth responses at mid-latitude sites indicated growth was limited by moisture availability. d13C signatures indicated increased WUE at the southernmost site, with no significant trends at northern sites. These results suggest that warming will increase growth of trees at black spruce's northern extent, but southerly areas may face drought stress if precipitation does not balance evapotranspiration.

加拿大西北部的北方针叶林（boreal forest）正经历快速增温、干旱化与永久冻土（permafrost）的改变，但当前针对大尺度下林木生产力的驱动机制与生长响应的研究仍较为不足。该区域的森林生态系统响应主要由黑云杉（black spruce）驱动——它是本地区分布最广、占绝对优势的树种。我们在横跨15个纬度梯度、涵盖气候与永久冻土变化梯度的4个以黑云杉为主的林分中采集了树轮样本，并开展了三项研究：其一为生长模式差异分析，其二为林木生长的气候驱动因子变异分析，其三为基于1945-2006年δ13C同位素分析得到的水分利用效率（water use efficiency, WUE）变化趋势分析。研究发现，除中纬度样地外，所有样地均呈现正向生长趋势，中纬度样地因永久冻土快速消融出现生长量下降；树限（tree limit）样地的年生长量最低，而林线（treeline）样地的年生长量最高。上述生长模式的气候驱动因子存在显著差异：北部样地的正向生长响应与冬季升温密切相关，而中纬度样地的δ13C变化趋势与气候-生长关系表明，其生长受到水分可获得性的限制；δ13C同位素特征显示，最南部样地的水分利用效率显著提升，而北部样地未出现显著变化趋势。本研究结果表明，升温将促进黑云杉分布北界的林木生长，但如果降水无法补偿蒸散（evapotranspiration）量，南部区域的林木可能面临干旱胁迫。