Recovery of a boreal ground-beetle (Coleoptera: Carabidae) fauna 15 years after variable retention harvest.
Retention harvests are preferred over traditional clear-cuts for sustainable forest management because maintenance and re-establishment of native forest biodiversity is a priority. However, few studies have examined long-term responses of biotic assemblages to retention harvest at particular sites. We studied the effects of decreasing initial harvest intensities (clear-cut, 10%, 20%, 50% and 75% dispersed green-tree retention) on carabid beetle assemblages relative to assemblage changes in un-harvested control stands in four successionally ordered cover-types of boreal mixedwood forest. We also studied temporal effects by comparing assemblages over a 16-year pre- and post-harvest period, using data collected through monitoring of the EMEND (Ecosystem Management Emulating Natural Disturbance) experiment in NW Alberta, Canada. Retention harvests affected assemblages differently across cover-types. Assemblages in compartments harvested in the earlier forest successional stages of 'deciduous' or 'deciduous with spruce understorey' converged towards the pre-harvest structure of corresponding controls over time. In contrast, beetle assemblages in 'mixed' or 'conifer' compartments, that represent later successional forest, moved steadily away from their pre-harvest structures during the first post-harvest decade. These latter assemblages became strikingly more similar to those under deciduous canopies by 15-year post-harvest. Synthesis and applications. Variable retention harvests will promote and maintain biodiversity better than clear-cutting. Higher retention levels promote faster recovery, but towards fauna typical of early successional forest in all cover-types. Carabids associated with conifer habitats are less resistant to impact from harvesting than are those from broadleaf deciduous forest. Therefore, conifer-dominated stands present the most significant management challenge and higher retention levels are required to promote rapid and effective faunal recovery in such late successional stands.