Dissertation Research: Dynamics and History of Treeline in the Southern Sierra Nevada

9411573 Graumlich对微量气体引起的气候变化可能造成的生态后果的担忧，促使人们研究过去的气候变化如何影响群落结构和组成。特别令人感兴趣的是交错带边界的古生态史，那里的物种预计会对气候变化做出敏感和可预测的反应。在过去的几千年里，内华达山脉南部高山树线的海拔位置发生了变化；已有几千年历史的死木和亚化石木材在比现在的树线高出80米的海拔处仍被保存下来。以前的研究已经证明，这种木材可以用树状年代学方法测年，提供了过去三千年来生态区变化的临时准确记录。这项研究将利用这些丰富的古生态信息来分析气候在推动内华达山脉南部高山树线过去移动中的作用。季节性气候波动的长期记录(1000年)将与树线运动的记录进行比较。利用林分重建技术，这项研究还将调查过去一千年来气候变化如何影响亚高山森林的更新和死亡速度。本论文的研究结果将可以量化气候和树线之间的时变关系，并探索这种关系是如何通过气候对亚高山森林种群进程速率的影响来调节的。

9411573 Graumlich Concern of the possible ecological consequences of trace-gas induced climatic change has led to examinations of how past variation in climate has influenced community structure and composition. Of particular interest is the paleoecological history of ecotonal boundaries, where species are expected to respond sensitively and predictably to climate change. The elevational position of alpine treeline in the southern Sierra Nevada has shifted over the past several millennia; dead and sub-fossil wood, up to several thousand years old, remains preserved at elevations of up to 80 m higher than present treeline. Previous research has demonstrated that the wood is dateable using dendrochronological methods, providing a temporarily precise record of ecotonal shifts over the last three millennia. This research will use this wealth of paleoecological information to analyze the role of climate in driving past movements of alpine treeline in the southern Sierra Nevada. Long-term records (1000 yr) of seasonal climate fluctuations will be compared with records of treeline movement. Using the technique of stand reconstructions, this study will also investigate how climate variation over the last millennium has affected rates of recruitment and mortality in subalpine forests. %%% Results from this dissertation research will allow quantification of the time-varying relationship between climate and treeline, and exploration of how this relationship is mediated through climatic effects on rates of population processes in subalpine forests.