Daniel Patrick Bebber , Ilya MacLean, Jonathan Mosedale, Benjamin Youngman

Plant pests and diseases (PPDs) pose a serious threat to trees and forests globally. In the wake of the ash dieback epidemic, the UK Government instigated the UK Plant Health Risk Register (PHRR) to provide semi-quantitative estimates of invasion probability and impact on host plants for PPDs thought to pose a risk to the UK to help prioritize biosecurity activities. The PHRR currently contains 636 PPDs potentially affecting 74 tree species found in the UK. Of these, the genera Prunus, Pinus, Malus, Pyrus and Quercus have the largest number of associated PPDs. Here, 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, such as ash dieback and horse chestnut bleeding canker, 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. Invasion probabilities and losses must be much lower than our plausible values to maintain reasonable levels of tree productivity in future. 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.