Patterns and Drivers of Pest and Disease Occurrence in UK Treescapes

Tree pests and diseases are a key threat to woodland biodiversity and commercial forestry worldwide. In the UK, the ongoing spread of pests and diseases is severely affecting a range of nationally important tree species, resulting in substantial ecological and economic impacts. As the risk posed by pests and diseases varies across the UK's treescapes, understanding the patterns of risk and the factors underlying these patterns is crucial for designing and implementing effective mitigation strategies. To address this challenge, we modelled the distribution of pests and diseases across mainland Great Britain, focusing on the total pest and disease burdens for nine host tree species of particular ecological, economic and cultural importance. Using integrated species distribution models, we combined two datasets-totalling 18,871 pest and disease records across 22 years-to model the spatial patterns of risk. To examine the factors underlying these distributions, we used graph-based causal inference approaches to inform our model design and to explore the robustness of our conclusions to variations in our modelling assumptions. We found that pest and disease burdens for broadleaved host trees exhibited hotspots in England, while burdens for conifer hosts tended to be high in Scotland. We identified urban area, human population density and local recreation as important drivers for several species, mainly native broadleaves. By contrast, woodland connectivity, afforestation and the level of conifer coverage were the most important drivers of pest and disease burdens for conifer hosts. Deforestation was also an important driver, with effects on pest and disease burdens for both conifers and broadleaves. Our findings have implications for the management of the UK's treescapes in the face of continuing threats from tree pests and diseases, including supporting targeted surveillance and the prioritisation of tree species for future planting.