Potential impacts of plant pests and diseases on trees and forests in the United Kingdom

Societal Impact Statement The UK Plant Health Risk Register (PHRR) has so far identified 581 Plant Pests and Diseases (PPDs) that could invade the United Kingdom and affect 74 tree species. The combined effects of multiple invasions on trees are little understood and seldom considered. We estimate future invasion rates and tree losses from the PHRR risk scores using historical data. We project potentially severe losses to ecologically and economically important species like oak, apple, poplar and pine due to combined impacts of multiple PPDs. Our analysis provides a framework to better understand and address these pressing biosecurity challenges. Summary Plant pests and diseases (PPDs) pose a serious threat to trees and forests globally. The UK Plant Health Risk Register (PHRR) provides semiquantitative estimates of invasion probability and impact on host plants for PPDs thought to pose a risk to the United Kingdom to help prioritize biosecurity activities. The PHRR currently contains 636 PPDs potentially affecting 74 tree species found in the United Kingdom. We explore the implications of converting these ordinal likelihood and impact scores to quantitative estimates of invasion probabilities and losses to tree productivity, in terms of increased mortality or reduced growth, in the next 25 years. We use recent invasions to quantify likely losses, and generate plausible invasion probabilities from historical observations. Assuming that the expectation of loss in 25 years is the probability of invasion multiplied by the fractional loss, and that the impacts of multiple PPDs affecting the same host species are independent, we find that many tree species would suffer severe production declines under plausible invasion probability and impact estimates. Despite several important knowledge gaps, our analysis provides a framework for projecting how trees and forests might be impacted in future and helps to highlight the risk posed by PPDs to biodiversity and ecosystem services.