The Magnitude of Regional‐Scale Tree Mortality Caused by the Invasive Pathogen Phytophthora ramorum

Abstract

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. Here we harness the rare data set 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 [confidence interval]) 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 suggest infection in 95.2 million (±8.6) California bay laurel (Umbellularia californica), which does not suffer mortality from infection and represents a critical source of continued spread. Prevalent infection as of 2012 suggests the cumulative number of disease‐killed stems likely increased from 20.8 to 42.8 million between 2012 and 2019 for all species. While these impacts are substantial, most host populations occur in a yet to be invaded region of northern California indicating that the disease will intensify in the coming decades.

Phytophthora Introductions in Restoration Areas: Responding to Protect California Native Flora from Human-Assisted Pathogen Spread

“…The goal of restoration is to improve ecological function and health, but it is also a type of disturbance with many activities that pose risks for invasive species introductions, (e.g., planting, soil movement, use of heavy construction equipment, and frequent worker ingress and egress). Nursery stock outplanted into wildland settings presents an opportunity for pathogen infections acquired in nurseries to be moved into new locations along with the plants, which may inadvertently result in lasting environmental damage to surrounding natural communities and habitats [18]. In California, native plant nursery stock outplanted for restoration has served as a pathway of introduction for a diverse array of Phytophthora species into natural areas and critical habitats [12].…”

“…Many introductions in restoration plantings occurred in the same region damaged by P. ramorum Werres, de Cock and Man in't Veld. Nearly 50 million native trees in California and Oregon have been killed by P. ramorum since its introduction on nursery stock, estimated to have occurred in the 1980s to early 1990s [18].…”

More Trees Are Dying Due to Drought and Wildfire but Do Not Lose Sight of Forest Pathogens

“…Their overall framework yields statistical models to test hypotheses about factors controlling infection and disease spread, estimates of cumulative impacts on tree hosts, and species-specific maps of current disease hotspots as well as areas with projected high probability of infection. Cobb et al (2020) conservatively estimate that by 2012 the disease had already infected 95 million California bay laurel (Umbellularia californica)-an important tree host implicated in the disease spread-and killed roughly 18 million tanoak stems while infecting another 71 million stems. Some 9 million coast live oaks had also been killed or infected by that time along with 1.7 million California black oaks.…”

“…The surveillance data analyzed by Cobb et al (2020) extend only to 2012, but P. ramorum has continued spreading to new host populations, albeit at different rates in different regions. The authors estimate that by 2019 perhaps 39 million tanoak stems and 7 million coast live oaks, roughly 2-2.5% of the regional population of each species, had been killed by this pathogen.…”

“…Climate change has also increased wildfire risk and annual area burned in forested regions of the state (Goss et al, 2020). Cobb et al (2020) provide a sobering reminder of the equally potent role of exotic forest pathogens as agents of large-scale tree mortality and forest change. The culprit in this case is the water mold Phytophtora ramorum (Sudden Oak Death), which was introduced into the greater San Francisco area of California some 30 years ago via the horticultural trade.…”