Predisposition of forests to biotic disturbance: Predicting the distribution of Acute Oak Decline using environmental factors

In the UK, Acute Oak Decline (AOD) has caused much concern, due to its distinctive symptoms and its potential to impact oak species that form the largest component of native broadleaf woodland. Decline complexes involve multiple biotic and abiotic factors, which combine to reduce host vigor. In order to investigate forest decline, it is necessary to take a systems approach by considering biotic agents and their interactions with environmental factors, as unfavorable conditions may predispose host trees to pests and diseases. AOD affected trees have lesions in the phloem caused by necrogenic bacteria associated with galleries of the two-spotted oak buprestid (Agrilus biguttatus). Here, we test the extent to which AOD is influenced by environmental predisposition factors traditionally associated with oak decline. These are often factors that reduce water availability. During 2013 and 2014 extensive surveys were undertaken, which systematically visited oak woodlands across England and Wales. These locations were used in conjunction with reports from landowners, which have been collected from 2006 onwards. In total 544 locations have been used to assess relationships with soil type, climatic factors and pollutant deposition, notably atmospheric nitrogen, using logistic regression models. The resulting model has been used to produce a detailed risk map for England and Wales and predictions have been validated using the locations of an independent dataset of A. biguttatus sightings collected by entomologists. This spatial study re-emphasises the importance of predisposition factors in decline syndromes and suggests avenues for future management and mitigation.