The potential of tree resistance to Phytophthora ramorum as an alternative to scorched earth eradication measures on tribal lands
To control the spread of an invasive pathogen in oak-dominated forest of northern California and southern Oregon, host and pathogen eradication through the use of forest sanitation treatments including clear-cutting, burning, and herbicides have been used to mixed success in forest ecosystems invaded by Phytophthora ramorum, the pathogen which causes sudden oak death. Tanoak (Notholithocarpus densiflorus), the native tree most severely impacted by P. ramorum is a vital cultural resource in California tribal communities, so for tribal people current eradication measures are anathema. In collaboration with tribal deparments and forest management agencies, we surveyed tanoak resistance to P. ramorumusing a laboratory inoculation of detached leaves which has previously been shown to be positively associated with survivorship in the field. Resistance surveys were conducted in a range of stand conditions which span cultural uses, management priorities and intensities, and variation in tanoak size, health, and growing conditions. These data were analyzed for patterns of resistance across host characteristics, stand treatments, and spatial association within stands. We linked these data with a dynamic epidemiological model to test the implications of patterns of tolerance vs resistance on potential tribal disease abatement management strategies.
Consistent with previous results, variation in resistance was greater within rather than between stands. Individual tree characteristics such as canopy position, overall health, and diameter at breast height did not show a consistent association with levels of resistance. Between stands, modest increases in resistance were found in stands with lower tree density and clear spacing between tree crowns. Within stands, levels of resistance tended to be random but were also found to be clustered in some locations and uniformly distributed in one stand. These spatial patterns are consistent with previous work showing that tanoak is clonal and suggests the presence of large genets with relatively higher resistance may be useful areas to focus conservation efforts. Our epidemiological model demonstrates that determining whether our data reflect tolerance (survivorship of infected trees with continued pathogen spread) and resistance (higher survivorship, lower transmission) is critical to implementing insights from these measurements in disease management strategies.