Optimizing opportunities for oak woodland expansion into upland pastures

Abstract

Woodland expansion is widely advocated for the mitigation of climate change and its impacts. This is supported by ambitious targets for increasing tree cover in the United Kingdom and elsewhere to aid carbon storage, flood mitigation and biodiversity provision. However, it remains unclear whether natural tree establishment can supply demand for expanded treescapes in remote, anthropogenically modified upland landscapes. We assessed natural establishment of NW-European native oak (Quercus robur, Q. petraea) saplings (< 12 years) in UK upland pasture systems adjacent to established ‘Atlantic’ oak woodlands on Dartmoor, SW England. We compared the extent of natural sapling colonization (abundance) into pasture sites on the moorland fringe and assessed their survival and growth throughout early life history using long- and short-term grazing exclusion experiments. Natural oak establishment typically occurred on naturally freely draining pasture slopes and at high densities (up to 1900 saplings per ha–1) within 20 m of the nearest adult congeneric. Beyond 20 m from a likely seed source, establishment was limited with no recorded establishment between 75–100 m. The natural establishment of oak saplings in grazed pastures was specific to ontogeny with livestock exclusion only favouring the density of older recruits (8–12 years). Results suggest an age-dependent relationship between ground cover and sapling performance; that is positive association between bare-ground and height for 4–7 year old trees, but little effect for seedlings and younger (0–3 years) saplings. Our scoping study highlights that with informed livestock management, there is significant opportunity for natural expansion of oak woodland into upland pastures where existing propagule sources are present (woodland edge, isolated trees). We signpost, however, that rapid expansion of oak woodland into UK uplands for climate mitigation is likely to require targeted planting and temporary grazing cessation, and there is need for improved evaluation of the effects of grazer exclosure and ontogeny specific ecology to better facilitate native woodland expansion efforts.