Neighbours matter and the weak succumb: chalara ash dieback infection is more severe in trees with fewer conspecific neighbours and lower prior growth rate

The epidemiology and severity of ash dieback (ADB), the disease caused by
the ascomycete fungus, Hymenoscyphus fraxineus, has been linked to a
variety of site conditions, however, there has been a lack of analysis at
an individual-tree scale.  Symptoms of ADB were scored
on ca. 400 trees of Fraxinus excelsior (ash) in permanent sample plots
during two successive years in a UK natural woodland reserve. Using
comprehensive plot records maintained since 1945, and detailed spatial
records updated since 1977, we assembled an array of potential explanatory
variables, including site environment factors, ash tree density, previous
and present tree condition and near neighbourhood summary statistics
(NNSS), such as species mingling and size dominance. Their impact on the
severity of ADB of focal ash trees was tested with generalised linear
mixed effects models (GLMM). The severity of ADB was much greater
in the lower slope parts of the site with moister soils and least in a
managed area subject to tree thinning in the previous 35 years. Severity
of ADB had a negative association with focal ash tree prior relative
growth rate over a period of a decade immediately before the disease was
detected at the site. Greater ADB severity was also significantly
associated with smaller diameter-at-breast-height of ash trees.
Additionally, ADB was significantly positively associated with a greater
proportion of heterospecific trees amongst the six nearest neighbours of
the focal tree.  Synthesis. The relationship of the
severity of ADB disease with site environment, tree condition and
neighbourhood is complex but nevertheless important in the progression of
the disease. The findings suggest some silvicultural interventions, such
as thinning to increase the vigour of retained ash trees, might reduce the
impact of ADB.