Management Strategies for Conservation of Tanoak in California Forests Threatened by Sudden Oak Death: A Disease-Community Feedback Modelling Approach

Abstract

We use a new modelling approach to predict the cumulative impact of Phytophthora ramorum on the dynamic distribution of tanoak (Notholithocarpus densiflorus) and other tree species in coastal-Californian forest-communities. We explore the effectiveness of disease-management strategies for the conservation of tanoak at stand level. Forest resources are increasingly threatened by emerging pathogens such as P. ramorum, a generalist that kills hosts and has altered ecosystems in the USA and Europe. In coastal California, P. ramorum has the greatest impact on tanoak through leaf sporulation and lethal bole infections, but also sporulates on the common overstory-tree bay laurel (Umbellularia californica) without significant health impact. Such epidemiological differences impede host-species coexistence and challenge pathogen management. For most disease-impacted natural systems, however, empirical evidence is still insufficient to identify effective and affordable pathogen-control measures for retaining at-risk host populations. Yet, landscape-scale tree mortality requires swift actions to mitigate ecological impacts and loss of biodiversity. We apply a mathematical model of the feedback between disease and forest-community dynamics to assess the impacts of P. ramorum invasion on tanoak under stand-scale disease-management strategies by landowners aiming to retain tanoak and slow disease progression: (1) removal of inoculum through reduction of bay laurel abundance; (2) prevention of tanoak infection through chemical protection (acting epidemiologically like a vaccine); and (3) a combination strategy. The model results indicate that: (1) both bay laurel removal and tanoak protection are required to help maintain tanoak populations; (2) treatment effectiveness depends on forest composition and on threshold criteria; (3) sustainable tanoak conservation would require long-term follow-up of preventive treatments; (4) arresting basal sprouting upon tree removal may help to reduce inoculum. These findings suggest potential treatments for specific forest conditions that could be tested and implemented to reduce P. ramorum inoculum and disease and to conserve tanoak at stand level.

Innate Resistance and Phosphite Treatment Affect Both the Pathogen’s and Host’s Transcriptomes in the Tanoak-Phytophthora ramorum Pathosystem

“…Tanoaks are among the most susceptible wild North American hosts to the pathogen, and experience both the deadly but rarely infectious bole form of the disease this pathogen causes (the widely known “Sudden Oak Death”), and the foliar and infectious form of the disease known as “Ramorum Blight” [ 14 ]. Tanoak has experienced the highest mortality rates among wild hosts [ 15 , 16 , 17 ], to the point that it is widely speculated that extinction of local populations may soon occur in heavily infested areas [ 18 , 19 ]. Agri-fos (Agrichem, potassium phosphite salts) is a registered fungicide in California and is now used for the protection of trees from Sudden Oak Death (causal organism the oomycete Phytophthora ramorum ) in wildlands and particularly for the protection of oaks ( Quercus spp.)…”