Room to regenerate: microhabitats and the recruitment niche as drivers of mountain woodland restoration

Abstract

Introduction Natural regeneration is an efficient, minimal-intervention approach essential for facilitating resilient, self-sustaining ecosystems during woodland restoration. However, recruitment can be slow, uncertain, or highly restricted, particularly in mountain woodland systems where tree planting and large herbivore management are necessary to reverse habitat fragmentation. Objectives We sought to determine the factors limiting natural regeneration from planted montane willow scrub by investigating the recruitment niche of Downy willow (Salix lapponum), an arctic-alpine species requiring restoration across Europe. We studied seed dispersal phenology, germinability, and the effect of microhabitat on seedling establishment to aid seed collection, restoration site selection, and management for self-sustaining populations. Methods Using a long-term landscape-scale restoration project in Scotland featuring a large herbivore exclosure containing abundant planted individuals, we quantified catkin maturation and seed viability across multiple years and integrated field surveys of natural regeneration with a direct seed sowing experiment. Results Seed dispersal occurred from mid-June to early July, with an average seed viability of 60%, but 8% of female plants experienced total catkin development failure. Bryophyte-dominated microhabitats in landslips had greater seedling frequency and survival compared to bare soil or dense vegetation. Conclusions Seed production and viability are not limiting S. lapponum regeneration from large numbers of planted individuals, but may be affected by weather fluctuations and plant-specific genetic factors. In contrast to the common assumption that bare soil is essential for Salix establishment, we show that wet bryophytes are prime regeneration substrates for S. lapponum, revealing a complex recruitment niche influenced by disturbance and microclimate.