Forest management legacies affect demographics and population dynamics of spruce grouse in northern Maine

Abstract

Conserving wildlife in managed forests requires knowledge of the interplay between species’ ecology and forestry practices. Spruce grouse (Falcipennis canadensis) are obligate residents of North American conifer forests with a southern range terminus in the northern U.S. We studied survival and reproduction of spruce grouse from 2012 through 2018 in north-central Maine, USA, within a landscape managed extensively for commercial forest products. We used radio telemetry data from 151 unique spruce grouse to monitor nest survival, brood success, and mortality of two age classes (juvenile and adult), and related these factors to a geospatial layer of stand harvest histories that provided a record of the timing and method of silvicultural treatments over the previous 30 years. We evaluated how demographic rates were affected by these forest practices, constructed stage-structured models of spruce grouse population growth, and used these models to quantify potential effects of management legacy on population dynamics. Nests in clearcuts with post-harvest treatments (herbicide and pre-commercial thinning) had lower success than those located in other stand types. Success of broods during summer increased with greater use of residual stands (unharvested for > 31 years), however, juvenile spruce grouse that made greater use of residual stands following independence had reduced survival. Adults that used a greater proportion of clearcuts lacking post-harvest treatments had the greatest annual survival. Deterministic and stochastic population models predicted values of λ < 1.0, suggesting population decline, and we found that λ was most sensitive and elastic to survival of adult females. Combined results from demographic analyses and our population model showed that greater use of untreated clearcuts by adult females had the largest potential to increase population growth, and that a stable to increasing growth rate (λ ≥ 1.0) could be achieved with particular combinations of stand use at different life stages. In this system the demographic mechanisms affecting relationships between forest management and spruce grouse population dynamics were complex. Ongoing changes in forest practices within the region may shift forests away from conifer-dominated composition, with negative consequences for the future status of the species.