An Examination of Background Tree Mortality and Mountain Pine Beetle Disturbance in Subalpine Forests of the Front Range of Colorado, USA

摘要

Subalpine forests in the Front Range of Colorado have experienced increased tree mortality over recent years with the onset of warmer temperatures and extensive bark beetle outbreak. To examine the role of climate on background mortality we measured growth and mortality for trees in an existing network of 40 permanent forest plots located in an area of subalpine forest near the CU Mountain Research Station in the Roosevelt National Forest. The permanent plots contain the following tree species; subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmanii), limber pine (Pinus flexilis), and quaking aspen (Populus tremuloides). Background tree mortality nearly doubled in these stands since the early 1980s, coincident in time with increasing annual moisture deficits and earlier onset of warmer temperatures in spring/summer calculated from temperature and precipitation recorded at the nearby C-1 climate station. A statistical evaluation of temporal associations between drought and tree mortality for one moisture sensitive plot reveals a consistent pattern through time where tree mortality follows major drought by a period of up to 3 years over the years 1872 to 2007 C.E. The occurrence of droughts and major pulses of tree death correlate with the positive phases of the Atlantic Multidecadal Oscillation over that same period.The late 20th century increase in warm and dry conditions has contributed to extensive tree mortality by mountain pine beetle (MPB; Dendroctonus ponderosae), primarily in lodgepole pine stands across its range in western North America. Since information on past outbreaks in Colorado is lacking, we evaluated dendroecological methods for reconstructing previous MPB disturbance. MPB-killed trees were identified based on field evidence of characteristic J-shaped galleries and blue-stain sapwood in extracted cores. Crossdated death years of MPB-killed canopy trees were accurately dated to the late 1970s and early 1980s and relate to subsequent growth releases in both subcanopy (host and non-host) and canopy (host) trees which survived the outbreak in the early 1980s. Furthermore, we analyzed spatiotemporal patterns of recent MPB activity on the western and eastern sides of the Continental Divide from 1997 to 2006 C.E. using overlay analysis in a GIS. The observed MPB damage for each year and on each side of the divide was compared to the proportion of available pine hosts within 8 elevation bands (from c. 1600 to 4400 m) and 8 aspect classes (45 deg each, N = 0 deg). The upslope spread of MPB activity on the western slope is punctuated by a sharp change to MPB activity on the eastern slope in survey year 2004 when the observed spatial extent of MPB activity in the two highest elevation zones east of the divide increases by c. 50%. This is complemented by an order of magnitude increase in estimates of the number of trees killed by MPB on the eastern slope which takes place almost exclusively in lodgepole pine, suggesting that MPB moved from west to east over the Continental Divide ca. 2003.Finally, we examined growth trends in limber pine which were killed by bark beetles or that were apparently killed by drought. Normalization by relative basal area increment shows a steep decline in terminal growth (i.e. last 25 years) for trees affected by drought, while beetle-killed trees show less variable, but slow terminal growth. Moreover, we evaluated the relationship between the initial growth rates and ultimate longevity for all dead limber pine sampled and found that faster growing trees typically have shorter life spans when compared to trees which grew slowly during their initial 50 years. The results of this dissertation contribute to our understanding of how current warming trends are affecting tree mortality in Colorado’s subalpine forest zone and will inform discussions of societal adaptation to these climate impacts.