Forty-four years of change: Capercaillie and Black grouse responses to changes in forestry, climate and vole decline in icentral Sweden

Human land use has greatly affected the natural landscapes in most parts of the world, and Scandinavian industrial forestry has substantially altered the age structure of the boreal forest over the last 75 years. In this study, we analyzed 44 years of line transect counts of Capercaillie and Black grouse collected in an industrial forest landscape in central Sweden between 1980 and 2024. The area was heavily logged in the 1960s and 1970s, followed by large-scale replanting. A hierarchical Gompertz state-space model to assess long-term population dynamics of both species. The adult state model included a brood production submodel, and forest age structure, snow depth, vole abundance, and spring frost were covariates. Our results reveal strong density dependence and temporally variable environmental effects. Capercaillie and Black grouse exhibited contrasting decadal trends: Capercaillie increased markedly during the early study years, whereas Black grouse declined steeply before recovering later in the series. Increasing proportions of forest aged 21-40 years had a clearly positive effect on Capercaillie, whereas snow depth in the previous winter negatively influenced Capercaillie but not Black grouse. Brood production exhibited substantial interannual unexplained variation, although late spring frost reduced brood size in Capercaillie. Toward the end of the study, the two species reached similar levels of latent adult abundance, demonstrating that both can persist at sustainable densities across a broader range of forest structures than suggested in earlier studies.