Assessing the use of natural colonization to create new forests within temperate agriculturally dominated landscapes

Abstract

There is a global drive to increase forest cover to protect biodiversity and help combat climate change. Tree planting is widely used to increase forest cover, although there is growing interest in using natural processes. However, predicting the outcome of natural colonization is challenging as it is a highly variable process and evidence is sparse, especially in temperate landscapes. Our study aims to evaluate the spatial and temporal scale over which natural colonization may be an effective approach to create new forests within temperate agriculturally dominated landscapes. We examine the spatial patterns in tree density and height across 90 sites in the United Kingdom that attempted to use natural colonization to create new forests between 1994 and 2004. Tree density and tree height were measured using light detecting and ranging point cloud data, an increasingly common technique for surveying forest characteristics. This research shows that natural colonization is a highly variable, relatively slow process in temperate agricultural landscapes, spatially restricted to a narrow fringe around existing forests and trees (105 m; 95% CI: 70–174 m, within approximately 19 years), although exact distances vary with former land use. This suggests that in some circumstances natural colonization will need to be assisted and supplemented with tree planting to increase forest cover, especially in areas away from a seed source. In reality, a blend of approaches will be needed to meet policy and land manager objectives, increase forest cover and tackle the biodiversity and climate crises.

Active management is required to regenerate the Caledonian forest: Alladale as a case study

“…Possible explanations for the reported patterns at Alladale can be either specific, grounded in particular mechanistic biotic interactions, or general, grounded in the dynamics and stability of the ecosystem as a whole (Sandom & Macdonald, 2015). Specific mechanisms may include depleted seed sources (Bauld et al., 2023), low levels of disturbances limiting germination potential (Sandom et al., 2013) and deer herbivory preventing germinated seedlings from escaping the browse layer (Tanentzap et al., 2013). Interestingly, these same mechanisms may also explain the lack of landscape level change in response to climate change, as seen in the arctic (see e.g.…”

“…herbivore culling; Tanentzap et al., 2009), introducing absent taxa (e.g. tree planting; Bauld et al., 2023) or engineering (e.g. re‐wetting; Kreyling et al., 2021).…”