The tree ring growth histories of UK native oaks as a tool for investigating Chronic Oak Decline: An example from the Forest of Dean

Declines are a distinctive category of tree disease, complex to define and quantify and challenging to mitigate due to their multiple causes and heterogeneous tree response patterns. In many parts of Europe oak decline syndromes are severely impacting tree health and having a measurable economic impact on forestry. In the UK the impact of periodic oak declines is expanding against a backdrop of multiple environmental pressures, to levels capable of threatening the UK's native oak woodland. Here we explore the growth histories of oak trees at a site symptomatic of Chronic Oak Decline (COD), in the South of England; Speculation Cannop in the Forest of Dean, Gloucestershire. The dendrochronological picture at the site reveals that trees with current external COD symptoms have shown suppressed growth, in relation to the regional average, from early on in their lives. Moreover, there is an amplified reduction in minimum ring width in Symptomatic trees as compared to a healthy subsample of Control trees, likely to be heavily dominated by reduced latewood width in affected trees, as decline sets in. Broadly, the site reveals the initial appearance of decline, roughly 40 years after planting, in 1860. There is considerable variability in the later decline history pattern in Symptomatic trees but there are clusters of decline episodes in the 1920s, 1960s, 1980s and 1990s at this site. The Control trees are not always unaffected but rather show growth releases after each historical decline phase. The trees that currently show external decline symptoms do not have a history of these growth releases. We conclude that investigating the tree ring growth histories at sites impacted by COD could provide an important management tool, and ring width histories of trees at affected sites should be used in the identification of the decline predisposing factors, that a management strategy requires.