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，在几个空间和时间尺度上影响森林社区的树木死亡率。长期和/或严重干旱等气候事件可能会引发大量死亡脉冲。气候对树木死亡的影响更加复杂，因为树皮甲虫暴发等气候敏感干扰可能会促进树木死亡。这项博士研究的目标是通过将永久地块的死亡记录和树木死亡日期的年轮重建与与干旱和树皮甲虫暴发有关的死亡模式的空间分析相结合，来检验气候变化对科罗拉多州前线亚高山森林树木死亡的影响。对20世纪80年代初建立的10个永久性亚高山林地和30个较小的样地进行了径向生长和树木死亡的重新测量。树种包括恩格尔曼云杉(Picea Engelmannii)、亚高山冷杉(Abies Lasiucpa)、樟子松(Pinus Trorta)和软木松树(Pinus Flexilis)。从地块内的树木观察到的和树木年轮重建的树木死亡年份都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)的时间联系。还将根据林冠空隙的相对位置、树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来研究样地内死亡的空间模式。在更广泛的空间尺度上，软木松材线虫的死亡率将与其在北部前锋山脉的整个亚高山分布的物理环境的空间变异性有关。对30个地点的密集采样将提供对气候直接驱动的斑块大小和死亡程度的洞察，以及山松甲虫(Dendroctonus Ponderosae)思考的死亡率。将分析交叉日期的树木死亡年份的同步性，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。根据地形信息、土壤层、照片解译的物种边界和历史虫害信息，将建立一个空间Logistic回归模型，预测区域尺度上的松材线虫死亡。这项研究利用了对永久地块的长期观察，鉴于森林形成的长期时间尺度，这些观察对于理解森林人口统计学至关重要。此外，对气候变化和与气候有关的昆虫暴发作为树木死亡因素的分析，将这项研究置于当前全球变暖趋势的背景下。在科罗拉多州，很少有山松甲虫影响高海拔松树的文献记录。这增加了这样一种可能性，即目前由甲虫引起的松树高死亡率是近100年来前所未有的，可能是与科罗拉多州落基山脉目前变暖趋势有关的模式的先兆。松树是一种特别重要的野生动物物种，其种群目前受到干旱、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模式，以便做出缓解决定。这项研究的数据将被包括在美国地质调查局的一个合作倡议中，该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.