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

Abstract

Accelerated forest dieback has been documented at many locations around the world that have experienced severe drought, warming, and wildfires associated with global climate change. Exotic forest diseases pose a comparably large threat to wild forests. Synthesizing surveillance plot data to parameterize spatial epidemiological models, Cobb et al. (2020, https://doi.org/10.1029/2020EF001500) estimate that the introduced invasive water mold, Phytophthora ramorum (Sudden Oak Death), has already killed 43 million trees in coastal evergreen forests of northern California and southern Oregon. Their results highlight the value of long-term surveillance networks for monitoring and modeling the spread of invasive forest pathogens and underscore the need for stronger public policy to reduce the global spread of these extremely harmful organisms. Plain Language Summary In many of the world's forests, trees are being killed by invasive bacteria, fungi, and other forest pathogens that cause disease. A number of these organisms have been inadvertently introduced through the international horticulture trade in live plants. Estimating where and how many trees are dying from introduced diseases is challenging but important because of the long-lasting effects of forest dieback on nature and society and the need to channel management efforts where they will be most effective at limiting future disease spread and tree death. As recently reported in Earth's Future, Richard Cobb and coauthors devised an approach for estimating the number of trees killed in northern coastal California by Phytophthora ramorum ("Sudden oak death"), a water mold introduced into the Bay Area of California roughly 30 years ago. Their findings are a sobering reminder of the damage being caused by introduced forest diseases: At least 39 million tanoak stems and 7 million coast live oaks, along with several million trees of other species, have already been killed, and the disease is still spreading. These numbers are comparable to recent estimates of trees killed in California by extreme drought and related insect outbreaks. Once established, introduced forest pathogens are hard to slow and practically impossible to stop. The best solution is to prevent their introduction in the first place through stronger regulation of trade in live plants and better surveillance to detect and eradicate new introductions before they can establish and spread. Cobb et al. (2020, https://doi.org/10.1029/2020EF001500) provide a compelling case study showing both the need for stronger public policy and the value of long-term surveillance networks.