Growing at the edge: Modelling sapling colonization, performance, and effective range of mountain birch (Betula pubescens ssp. tortuosa)

Colonization of trees from existing woodlands into adjacent open lands is a critical process of passive restoration, leading to the formation of secondary woodlands. Important drivers of this process include sheltering from surrounding woodlands, which can affect both the spatial range of colonization and the performance of individual saplings. In this study we analysed the effects of sheltering and canopy height on the patterns of natural colonization and individual sapling performance of mountain birch (Betula pubescens ssp. tortuosa) in different regions of Iceland. We measured sapling densities and individual sapling morphology along transects, and modelled colonization using hyperbolic kernel functions. We subsequently used these colonization kernels to define and calculate the effective spatial range for mountain birch colonization. The results showed a clear effect of regional environments on the performance of mountain birch saplings. The effective spatial range of colonization varied greatly between regions, ranging from 16.6 m in areas with birch of low stature, to 67.4 m in the region with the tallest, straightest stems. We furthermore saw that individual sapling performance was a direct product of sheltering, as one kernel function parameter was significantly correlated with the tree morphology index used in this study. Our results highlighted the importance of sheltering from surrounding woodlands on both colonization patterns and on individual sapling performance, though sheltering effects are generally low in Iceland. This should be considered in research as well as practice by combining natural colonization with efficient localized planting. The effective spatial range can aid in finding an appropriate balance between these two restoration methods.