LAUSR

Abstract Acute Oak Decline (AOD) is devastating thousands of native oaks, Quercus robur and Q. petraea, in the UK and continental Europe. The syndrome is principally caused by multiple, interacting bacterial species that degrade inner bark tissues. The level of host predisposition required prior to AOD infection is unknown. The two spotted oak buprestid Agrilus biguttatus is strongly associated with AOD, although its role remains unclear. To investigate the nature of predisposition in AOD, to explore the role of the beetle in the syndrome, and to examine growth trends after AOD onset, the stem growth of 243 trees with a range of severity of AOD symptoms was analysed at five sites in England. Novel mixed effects dendrochronological modelling methods were developed. The presence/absence of A. biguttatus exit holes in the trees was not specifically linked with reduced stem growth, nor was there evidence of further reductions in growth after the onset of AOD symptoms. Instead, trees with long-term AOD symptoms show significantly reduced growth compared to asymptomatic trees from as far back as the 1930s, following a period of widespread decline in English Oaks. These results suggest that a cohort of oak trees across Britain was permanently damaged in the 1930s and predisposed to develop AOD symptoms decades later. Additionally, correlations with climatic variables suggest that diseased trees are less able to take advantage of good growing conditions in the spring and autumn. This study sheds light on how historical episodes of stress may impact the future resilience of oaks to disturbance, and supports the use of dendrochronological modelling as a technique to study the underlying health status of oak tree populations, and to better understand tree decline episodes.