Biological legacies and seedling bank dynamics of a spruce beetle-killed Engelmann spruce landscape

Background/Question/Methods

Regeneration response to disturbance is often thought to be a function of the disturbance regime severity. Response to high-severity disturbance is typically described as an event or ‘pulse’ where regeneration is triggered by the type of disturbance, e.g. aspen (Populus tremuloides) sprouting after stand-replacing fire. In contrast, response to low-severity disturbance is often thought to be ‘chronic’ where advance regeneration responds to an open environment, e.g. a single tree windthrow releasing a shade-tolerant individual from the seedling bank. In this paper we describe an alternative model of regeneration response to disturbance expressly considering host-specificity and biological legacies in the wake of a spruce beetle (Dendroctonus rufipennis) outbreak. The host-specific mortality across a mature Engelmann spruce (Picea engelmannii) landscape left two types of biological legacies: the seedling bank and some non-host overstory trees. While the non-host overstory, where present, will influence regeneration, the seedling bank will play the primary role in the future of the forest. Understanding the influence of chronic versus pulse regeneration dynamics in these communities will help elucidate the long-term consequences of the outbreak. Fourteen sites in southern Utah, USA were measured to reconstruct pre-outbreak community composition and quantify its influence on post-outbreak biological legacies. Three measures of diversity (Shannon diversity and evenness, and Berger-Parker dominance) were calculated to compare over- and understory components. Species-specific analysis of understory ages were made to evaluate chronic versus pulse regeneration dynamics.
Results/Conclusions

Comparisons of pre-outbreak overstory composition to understory composition revealed a substantial shift in dominance from Engelmann spruce to, in order of magnitude: subalpine fir (Abies lasiocarpa), aspen, Engelmann spruce, Douglas-fir (Pseudotsuga menziesii), limber pine (Pinus flexilis), blue spruce (Picea pungens), and ponderosa pine (Pinus ponderosa). Results suggested overstory diversity did not ensure understory diversity. Little, if any, regeneration has occurred post-outbreak; however, seedling bank recruitment of subalpine fir and Engelmann spruce has occurred chronically for the last ~ 205 yr and ~ 152 yr, respectively. Limber pine, ponderosa pine and Douglas-fir, while present in the overstory, were virtually absent in the understory. Understory aspen was predicted to respond as a pulse; however, results were complicated by intense ungulate browsing. While the spruce beetle outbreak was clearly a high severity disturbance, regeneration dynamics, overwhelmingly influenced by biological legacies, have much more in common with low severity disturbance regimes.