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Understory vegetation responses to simulated mountain pine beetle attack and salvage logging in grey attack stage lodgepole pine stands.

Steinke, J., A. C. S. McIntosh, L. Schroeder and S. E. Lori Macdonald, . 2020.

Abstract

Disturbances are an integral part of forest ecosystems and drive successional change. The boreal forest is adapted to stand-replacing fires, which have different ecological impacts than less severe disturbances, such as insect attacks. In recent years, mountain pine beetle (MPB), a bark beetle native to western North America, has undergone an unprecedented range expansion into lodgepole pine forests in west-central Alberta, Canada. MPB influences understory microclimatic conditions and above- and below-ground resources, but does not have the direct impacts on ground vegetation and the forest floor that normally accompany wildfire. The objective of this study was to quantify vegetation responses to MPB in forests for which this is a novel disturbance. Our replicated field experiment included an untreated control, medium and high tree mortality (simulated MPB-attack through stem injection of herbicide), and harvesting to simulate salvage logging. We used univariate and multivariate analyses to examine responses of understory vegetation to these treatments from pre- to seven years post-treatment. The severity of disturbance determined the magnitude of vegetation changes. Only the simulated salvage logging resulted in immediate changes post-treatment. Seven years post-treatment, vascular plant richness increased along the gradient of treatment severity. Total vegetation cover declined in the short term after salvage logging but then returned to pre-treatment levels. Notably, graminoid cover increased with the high-mortality MPB and salvage logging treatment, and remained higher in these two treatments, seven years post-treatment. Changes in species composition were most dramatic for salvage logging followed by the high mortality and then the medium mortality treatments; by seven years post-treatment there were indications that communities were beginning to recover to their pre-treatment composition. Feathermosses were indicative of the untreated control, shade-tolerant species increased their relative abundance in the medium mortality treatment, both shade-tolerant and intolerant species increased their relative abundance in the high mortality treatment, and disturbance-adapted, shade-intolerant species (including lodgepole pine) were indicators of salvage logging. We conclude that stands experiencing medium levels (~50%) of MPB-caused mortality will demonstrate resistance (change less or more slowly) and/or resilience (recover to the pre-disturbance condition more quickly) as compared to those experiencing high levels (~100%) of mortality, which show changes in understory composition that might influence stand successional trajectories. Effects of salvage logging will be much more dramatic than those of the MPB attack. Our results can help inform decisions on management or rehabilitation of stands post-MPB attack.

Key Words

Mountain pine beetle Forest vegetation Forest disturbance Salvage logging Lodgepole pine