Regeneration response and seedling bank dynamics on a Dendroctonus rufipennis-killed Picea engelmannii landscape

Question: How does regeneration response to a host-specific, high-severity, infrequent Dendroctonus rufipennis outbreak differ from our conceptualization of high-severity, infrequent/low-severity, frequent disturbance regimes in Picea engelmannii-Abies lasiocarpa communities? Location: Southern Utah, USA. Methods: One hundred and seven plots across a high-elevation P. engelmannii forest were sampled to reconstruct pre-outbreak overstory and seedling bank densities, and calculate their associated metrics of diversity. Decade of establishment by seedling bank trees indicated "chronic" and "pulse" regenerators. Results: The post-outbreak overstory and seedling bank were dominated by A. lasiocarpa. Although Pinus flexilis, Pinus ponderosa, Picea pungens, and Psuedotsuga menziesii were present in the overstory, they were virtually absent in the seedling bank. Seedling bank recruitment of A. lasiocarpa and P. engelmannii has been occurring chronically for at least the last ∼205 and ∼152 years, respectively. A pulse response of seedling bank Populus tremuloides was apparent; however, results were complicated by intense ungulate browsing. Conclusions: Despite some similarities to the high-severity, infrequent/low-severity, frequent conceptualization of regeneration response to disturbance, the high-severity D. rufipennis outbreak is best described by explicitly considering host specificity and severity. Although, the outbreak simultaneously promoted both a pulse of P. tremuloides and a release of chronically regenerated A. lasiocarpa, the P. tremuloides response was generally masked by ungulate browsing, and the regeneration response came overwhelmingly from the A. lasiocarpa seedling bank. In this landscape, once dominated by P. engelmannii, the chronically regenerating seedling bank, typically thought to take advantage of canopy gaps associated with low-severity disturbances, is poised to dominate forest reorganization in response to the host-specific outbreak.