Natural colonisation rates in the UK uplands under different conservation management approaches following the removal of sheep.

Natural colonisation could provide a cost-effective means to increase woodland coverage across upland areas of the UK. However, there is a shortage of evidence as to which factors affect the success of natural colonisation, including grazing management strategies. The species of grazer, grazing intensity and the stage of regeneration may all alter the impact of grazing on the ability of trees to colonise an area. In this study, we quantify the effect of removing sheep from an area in the Yorkshire Dales National Park, England and explore whether cattle or ungrazed management would be better for colonisation after that point. We also test other environmental factors such as distance from woodland, elevation and soil geology to understand the other constraints on natural colonisation and be able to make predictions as to where it may be possible in the future. Naturally colonising trees were recorded within 61 plots at Ingleborough National Nature Reserve and surrounding Yorkshire Wildlife Trust reserves. A series of generalized linear models, with a negative binomial distribution were created to understand the effects that years since removal of sheep, current management, elevation, distance from woodland and soil geology had on the number of naturally colonising trees. A final model for interpretation was selected using AICc. The number of naturally colonising trees per hectare was 16% greater each year after the removal of sheep, with both cattle and ungrazed management being equally effective in promoting natural regeneration. Natural regeneration decreased by 25% for every additional 100 m from the nearest woodland and naturally colonising trees were much more frequent on limestone soil. Predictive maps are also made to indicate where natural colonisation would be expected at Ingleborough National Nature Reserve once sheep are removed. This study provides a demonstration that sheep grazing is a key limiting factor of natural colonisation in the UK uplands, however once this is stopped natural colonisation can be successful using both no grazing and cattle. Tree planting will still be needed to restore missing species and to help increase woodland cover in areas away from any seed source.