The magnitude of large-scale tree mortality caused by the invasive pathogen Phytophthora ramorum

Forest pathogens are important drivers of tree mortality across the globe
but it is exceptionally challenging to gather and build unbiased
quantitative models of their impacts, which has resulted in few estimates
matching the scale of disease. Here we harness the rare dataset matching
the spatial scale of pathogen invasion, host, and disease heterogeneity to
estimate infection and mortality for the four most susceptible host
species of Phytophthora ramorum, an invasive pathogen that drives the most
important biological cause of tree mortality in a broad geographic region
of coastal California and southwest Oregon. As of 2012, the most current
field survey year, we estimate 17.5 (± 4.6 95% CI) million tanoak
(Notholithocarpus densiflorus) stems were pathogen-killed with an
additional 71 (± 21.5) million infected. We estimated 9.0 million (± 2.2)
coast live oak (Quercus agrifolia) and 1.7 million (± 0.5) California
black oak (Quercus kelloggii) stems are disease impacted (mortality and
infection combined). Lastly, our estimates showed 95.23 million (± 8.6)
million infected California bay laurel (Umbellularia californica), which
does not suffer mortality from infection and represents a critical source
of continued spread. Using an extensive dataset on mortality rates of
infected individuals we estimate that cumulative mortality likely
increased from 20.8 to 42.8 million individual stems between 2012 and 2019
for all species. While these impacts are substantial, the majority of host
populations occur in a yet to be invaded region of northern California
indicating that the disease will intensify in the coming decades.