Sheep removal could allow woodlands to self-plant in UK uplands

UK uplands could naturally establish new woodlands in areas where sheep are removed. Research from the Wild Ingleborough project, in the Yorkshire Dales, highlights the potential for ‘natural colonisation’ as an alternative technique to traditional tree planting. 

In the UK, uplands are often open landscapes with few trees or wooded areas. However, pollen records show that these landscapes contained many more trees in the past. Restored tree cover in these upland areas is urgently needed to benefit wildlife, store carbon, and reduce downstream flooding in the current biodiversity and climate crises. 

Two key options for restoring tree cover are planting and ‘natural colonisation’ – the process of trees seeding themselves through natural processes to grow outside of currently wooded areas. Natural colonisation has the potential to maintain local genetic diversity and create woodlands that are more structurally diverse than is typically achieved by tree planting however, despite these benefits, land managers often choose to plant trees themselves. This may be in response to a low diversity of nearby parent trees or because successful natural colonisation is overwhelmingly restricted by grazing pressures from sheep or deer. 

Another big factor limiting the use of natural colonisation to increase tree cover is the unpredictability of its results, particularly within the context of grant funding. To provide evidence addressing this issue, a research team led by George Porton – a scientist from the University of Leeds – collected data to help understand factors limiting natural colonisation at Ingleborough, in the Yorkshire Dales. 

Ingleborough National Nature Reserve, now part of the Wild Ingleborough project, provides a great opportunity to study patterns of natural colonisation after sheep removal. This is because different parcels of land on the reserve have had sheep removed and then replaced with cattle or left ungrazed over the past 45 years. However, the area has a woodland cover of less than 3%. The team surveyed trees in 62 circular plots, each 30 metres wide, allowing them to predict suitable areas where natural colonisation is likely to be effective in the future. 

The study’s results highlight that natural colonisation at Ingleborough is being severely limited by sheep grazing. The selective grazing style of sheep means they target any new tree seedlings before grazing other less nutritious plants nearby. Natural colonisation was, however, successful on both cattle grazed and ungrazed sites. There was no significant difference in tree density between cattle grazed and ungrazed sites, however, future data may highlight a more subtle effect. 

The distribution of natural colonisation in the study was clustered around existing woodland, which acted as the main seed source within the landscape. A more detailed analysis of this suggested that 25% fewer trees were able to naturally establish for every 100 metres further they were from an existing woodland. 

Naturally colonised trees were also found at much higher densities on limestone soil. The authors of the study suggest that this may be a result of limestone pavement in the area protecting seedlings from cattle and providing more areas of bare soil in the gaps between the limestone, where tree seedlings face less competition from other vegetation. 

The results overall show that natural colonisation is a viable method for restoring trees to upland areas of the UK, like Ingleborough, once sheep are removed. The new models from this research predict 16% of Ingleborough National Nature Reserve could have at least 1000 stems per hectare 30 years after sheep are removed. 

George Porton, the lead author of the study, summarised that “We were able to predict where, when and at what density natural colonisation would be expected. This helps to provide evidence to inform decisions on where to plant trees and where natural colonisation can be used”. 

He emphasises too that “Tree planting should still play a role to restore tree cover in areas away from seed sources and to plant rare or poorly dispersing species.” 

The research is published with open access to read online in Ecological Solutions and Evidence.