Doctoral Dissertation Research: Effects of Climate Variation on Tree Mortality Across Spatially Heterogeneous Subalpine Forest Landscapes in the Front Range of Colorado

气候是森林生态系统动态的主要驱动力，并在几个空间和时间尺度上影响森林群落中的树木死亡率。 长期和/或严重干旱等气候事件可引发大规模死亡。 气候对树木死亡率的影响更加复杂，因为树木死亡率可以促进气候敏感的干扰，如树皮甲虫爆发。 这项博士研究的目标是研究气候变化对树木死亡率的影响，在亚高山森林的科罗拉多的前山脉整合死亡率记录永久地块和树木年轮重建的树木死亡日期与空间分析的死亡率模式相关的干旱和树皮甲虫爆发。 10个永久性亚高山森林地块大于400棵树和30个较小的地块，平均50棵树最初安装在20世纪80年代初，重新测量径向生长和树木死亡率。 树种包括英格尔曼云杉（Picea engelmannii）、亚高山冷杉（Abies lasiocarpa）、黑松（Pinus contorta）和软松（Pinus flexilis）。 观测到的树木死亡年数和树木年轮重建的树木死亡年数将与当地和区域气候数据进行统计比较，以辨别与气候事件（如极端干旱）的时间关联。 还将研究地块内死亡率的空间格局，考虑到林冠空隙的相对位置，树木的大小和生长速度，以及不同大小和生长速度的树木的接近程度。 在更广泛的空间尺度上，软松的死亡率将检查在整个亚高山分布在北方前山脉的物理环境的空间变异性。 30个站点的密集采样将提供深入了解的补丁大小和程度的死亡率直接驱动的气候，以及死亡率冥想山松甲虫（Dendroctonus ponderosae）。 交叉日期的树木死亡年将进行同步分析，并与当地和区域气候数据进行定量比较，以测试与气候事件的时间关联。 一个空间逻辑回归模型，预测软松死亡率在区域范围内将开发从地形信息，土壤层，照片解释的物种边界，和历史的害虫信息。 这项研究利用了对永久性地块的长期观察，鉴于森林形成的时间跨度很长，这对了解森林人口至关重要。 此外，分析气候变化和气候相关的昆虫爆发作为树木死亡率的代理人，在当前全球变暖趋势的背景下进行研究。 在科罗拉多，很少有山松甲虫影响高海拔软松的文献记录。 这提出了一种可能性，即目前高水平的甲虫引起的死亡率在软松是前所未有的，在过去的100年左右，可能是一个预兆的模式与目前的变暖趋势在科罗拉多落基山脉。 软松是一种特别重要的野生动物物种，其种群目前正受到干旱、甲虫袭击和疱锈病感染的威胁。 因此，土地管理者需要更好地了解该物种过去和未来可能的死亡率模式，以便为缓解决策提供信息。这项研究的数据将纳入美国地质勘探局的一项合作倡议，旨在调查全球变化对美国西部山区的影响。

Climate is a major driver of forest ecosystem dynamics and operates on several spatial and temporal scales to affect tree mortality in forest communities. Large pulses of mortality can be triggered by climatic events such as prolonged and/or intense drought. Climatic influences on tree mortality are further complicated because tree mortality can be facilitated by climatically-sensitive disturbances such as bark beetle outbreaks. The goal of this doctoral research is to examine effects of climate variation on tree mortality in subalpine forests of Colorado's Front Range by integrating mortality records from permanent plots and tree-ring reconstructions of tree death dates with spatial analysis of mortality patterns related to drought and bark beetle outbreak. Ten permanent subalpine forest plots with greater than 400 trees each and thirty smaller plots with an average of 50 trees initially installed in the early 1980s were re-measured for radial growth and tree mortality. Tree species include Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta) and limber pine (Pinus flexilis). Both observed and tree-ring reconstructed tree death years from trees within the plots will be statistically compared to local and regional climate data to discern temporal associations with climate events (e.g. extreme drought). Spatial patterns of mortality within the plots will also be examined with regard to the relative location of canopy gaps, tree size and growth rate, and proximity of trees of varying sizes and growth rates. At a broader spatial scale, mortality of limber pine will be examined in relation to spatial variability of the physical environment throughout its subalpine distribution in the northern Front Range. Intensive sampling of 30 sites will provide insight into the patch size and extent of mortality directly driven by climate as well as mortality meditated by mountain pine beetle (Dendroctonus ponderosae). Cross dated tree death years will be analyzed for synchrony and quantitatively compared with local and regional climate data to test for temporal associations with climate events. A spatial logistic regression model that predicts limber pine mortality at a regional scale will be developed from topographic information, soil layers, photo-interpreted species boundaries, and historic pest information. This study capitalizes on long-term observations from permanent plots which are crucial to an understanding of forest demography given the long timescales over which forests are shaped. In addition, analysis of climate variation and climate-related insect outbreak as agents of tree mortality situates the study within the context of current global warming trends. There are few documentary records of mountain pine beetle affecting high elevation limber pine in Colorado. This raises the possibility that the current high-level of beetle-induced mortality in limber pine is unprecedented over approximately the past 100 years and may be a harbinger of patterns associated with the current warming trend in the Colorado Rockies. Limber pine is a particularly important species for wildlife, and its populations are currently threatened by drought, beetle attack, and infection by blister rust. Thus, land managers need a better understanding of past and probable future patterns of mortality in this species to inform mitigation decisions. Data from the study will be included in a collaborative USGS initiative aimed at investigating the effects of global change on mountain regions in the western United States.