Abstract
Forests are expected to become more vulnerable to drought-induced tree mortality owing to rising
temperatures and changing precipitation patterns that amplify drought lethality. There is a crucial
knowledge gap regarding drought–pathogen interactions and their effects on tree mortality. The
objectives of this research were to examine whether stand dynamics and ‘background’ mortality rates
were affected by a severe drought in 2012; and to evaluate the importance of drought–pathogen
interactions within the context of a mortality event that killed 10.0%and 26.5% of white (Quercus
alba L.) and black (Q. velutina Lam.) oak stems, respectively, in a single year.We synthesized (i)
forest inventory data (24 years), (ii) 11 years of ecosystem flux data with supporting biological data
including predawn leaf water potential and annual forest inventories, (iii) tree-ring analyses of
individual white oaks that were alive and ones that died in 2013, and (iv) documentation of a
pathogen infection. This forest displayed stand dynamics consistent with expected patterns of
decreasing tree density and increasing basal area. Continued basal area growth outpaced mortality
implying a net accumulation of live biomass, which was supported by eddy covariance ecosystem
carbon flux observations. Individual white and black oaks that died in 2013 displayed historically
lower growth with the majority of dead trees exhibiting Biscogniauxia cankers. Our observations
point to the importance of event-based oak mortality and that drought–Biscogniauxia interactions are
important in shaping oak stand dynamics in this region. Although forest function has not been
significantly impaired, these drought–pathogen interactions could amplify mortality under future
climate conditions and thus warrant further investigation.
