The recruitment niche of mountain birch (Betula pubescens ssp. tortuosa) and implications for woodland restoration

The extensive restoration of fragmented woodlands calls for practices appropriate to large-scale efforts. These simultaneously require an understanding of ecosystem-level processes and the plant-scale environment. The recruitment niche of the target species is crucial, that is conditions required for seed germination to seedling establishment. Our study contributes to underpinning the science behind successfully promoting the utilization of natural regeneration in woodland restoration in a subarctic environment. We identified the recruitment niche of the only native forest-forming species in Iceland. From 2018 to 2020, we quantified mountain birch seed accumulation, germination, and early seedling survival in relation to substrate types within 500-m-long transects at two study sites on Skeiðarársandur outwash plain, southeast Iceland. At the time of the study, the founding population in this early successional environment had recently reached reproductive maturity. Mountain birch seeds were most likely to accumulate on vegetated surfaces and to germinate in low-growing vegetation, with unimpeded sunlight. Survival was not significantly influenced by substrate types, but was surprisingly high (generally >50%) for the first 1–2 years, although most seedlings were still very small. Overall, recruitment was consistently greater than expected in thin moss (~1 cm), which may be considered a key substrate type for mountain birch recruitment success. Due to high cover of suitable substrate types in the study area, the spatial pattern of the first locally recruited generation of mountain birch was determined at the earliest life history stage, by dispersal limitation. Our study highlights the importance of the recruitment niche for successful restoration and of securing seed input when dispersal may be limited. This allows for scaling up the restoration of severely fragmented woodlands, for which the pending restoration of Icelandic woodlands serves as a case study. The rapid mountain birch establishment on Skeiðarársandur shows that woodland restoration may not need major interventions; however, they must be based on profound knowledge of colonizing processes. Thus, restoration with minimal human assistance can be a practical, low-cost option.