Disturbance ecology and vegetation dynamics at varying spatial and temporal scales in southern Rocky Mountain Engelmann spruce forests.

摘要

High-severity disturbances are the primary drivers of Engelmann spruce-subalpine fir ecosystems in the southern Rocky Mountain. Recently, an unprecedented, landscape-wide (at least 250 km2) spruce beetle outbreak killed virtually all the Engelmann spruce on the Markagunt Plateau in southwestern Utah, USA. Results from dendroecological analyses suggested the combination of antecedent disturbance history and drought-driven stand development was responsible for creating suitable host conditions prior to the recent outbreak. Multiple and consistent lines of evidence suggested mixed- and high-severity fires shaped the development of the Markagunt Plateau. Subsequent stand development, influenced by species-specific differential tree-ring response to drought, resulted in the gradual increase of Engelmann spruce dominance across the landscape.Spatiotemporal outbreak dynamics included the early, independent and spatially synchronous building of beetle populations in moist sites with large Engelmann spruce across the landscape. As the outbreak evolved over time, it is likely temperature anomalies accelerated beetle population growth, leading to more rapid spruce mortality. In the wake of the spruce beetle outbreak, results from simulated potential fire behavior suggested there was a reduction in probability of active crown fire for one or two decades on near-pure Engelmann spruce sites after the outbreak. This counterintuitive result suggested extreme fire behavior is not an inevitable consequence of spruce beetle outbreaks. Regardless of the occurrence of fire, forest response is likely to be dominated by advance regeneration in the seedling bank. Furthermore, because spruce was virtually absent from the understory, forest reorganization is likely to be dominated by subalpine fir.In response to recent outbreaks such as the Markagunt Plateau, silviculturists are questioning what they can do to limit the loss from these likely inevitable spruce beetle outbreaks. Concepts of resistance and resilience can be used in planning vegetation management intended to indirectly control beetle populations by manipulating their habitat (vegetation). Resilient landscapes will ideally have spruce age class diversity and size class diversity in spatially discontinuous patches.