Thinking outside the bog: Planting dwarf birch (Betula nana) for mountain woodland restoration beyond habitat refugia constrained by overgrazing

Abstract

The contemporary distribution of threatened species may not be in equilibrium with their full ecological niche. Understanding range contraction due to anthropogenic land-use pressures will reduce bias in management aimed at expanding present-day forests. Many mountain woodlands currently occur as fragments, with uncertainty regarding their optimal distribution and restoration potential, particularly in landscapes shaped by large herbivore management. Our high-altitude planting experiment replicated at three sites in Scotland investigates the influence of browsing on the extent and condition of mountain woodland. Planting of Betula nana (dwarf birch) in three different habitats monitored for up to six consecutive years tested whether this upland tree species may currently be confined to blanket bog because of historical and contemporary overgrazing. At red deer (Cervus elaphus) densities managed at 3-6 per km2, browsing of planted B. nana and mortality linked to browsing were much greater in grass and heath vegetation than in blanket bog. In contrast, survival was similar for all three habitats when large herbivores were excluded, while growth outputs were not more productive in blanket bog at any of the three sites, and at one location interannual growth was significantly greater in the grass and heath. Therefore, blanket bog is not more favourable for B. nana establishment in Scotland compared to potential alternatives based on distributions elsewhere in Northern Europe, and may instead represent a refugium where relict populations escape overgrazing. Our results suggest that active management to reduce deer densities to approximately 3 per km2 or below is currently pivotal for mountain woodland restoration outside of such refugial sites in Scotland. This research illustrates the value of a “thinking outside the box” approach in applied forest ecology whereby restoration management moves beyond simply reproducing the features of constrained present-day fragments and accounts for land-use legacies while addressing the original causes of habitat loss at scale.