California bay removal may be used across a range of spatial scales, from large blocks to small localized patches. Even in areas where extensive removal to protect oaks may not be practical, it may be helpful to remove highly susceptible California bays and those that tend to retain many infected leaves for extended periods. Across a property or landscape, one or more of these strategies may be applied in different areas, depending on the size and density of California bays and oaks present as well as management goals and priorities.
Methods discussed here are based primarily on relationships that have been observed between SOD risk and California bay in non-manipulated coast live oak stands. Long-term controlled studies involving California bay removal are under way, but experimental data are not yet available to show to what degree SOD can be prevented by using California bay removal.
Area-wide removal involves eliminating all California bay from a relatively large contiguous area containing SOD-susceptible oaks. This method provides the greatest SOD risk reduction and is preferable where it is practical and consistent with other management objectives. Removing all California bays within an oak stand can be more efficient than localized clearing around individual trees because larger California bays often increase the SOD risk for several nearby oaks. Candidate sites for area-wide California bay removal may encompass an entire stand or only portions of a stand. If the treated area is smaller than about 0.25 ha (0.5 acre), refer to the discussion of localized California bay removal below.
Area-wide removal of California bay is most likely to be feasible and cost-effective in the following types of stands:
Area-wide removal will be difficult, impractical, or undesirable in stands in which California bay is the dominant canopy species or where much of the California bay canopy consists of large trees.
Remove all California bay size classes, from seedlings to mature trees, from within the treated area if possible. Over the short term, small California bay seedlings (less than 0.5 m tall) that are more than 3 m from a susceptible oak trunk can be left in place. Remove these seedlings before they grow taller than about 1 m. Periodic inspection of the cleared area will be needed to remove California bays that were left in place, have resprouted from cut stems, or have grown from seed. Removing all seed-producing California bays in the treated area will help reduce seedling populations and the need for frequent follow-up inspections and treatments.
In areas where California bay constitutes a high percentage of the stand or where large California bay trees are distributed throughout the stand, it may be prohibitively expensive and/or inconsistent with other management objectives to remove California bay from the entire stand. Removal becomes more difficult and expensive as the size of the tree increases. In addition, it may be difficult to justify removing large healthy California bays in an effort to protect oaks whose disease status is uncertain. In such stands, it may be possible to reduce SOD risk for individual oaks or groups of oaks by increasing oak-California bay clearance in localized areas (fig.3-1). Increased clearance may be attained through a combination of selective California bay removal and canopy pruning.
Figure 3-1—Site before (left) and after removal (right - note stump on left side of image) of a small California bay. Removal of the California bay and several California bay seedlings eliminated all California bay within the immediate vicinity of the two coast live oaks shown, greatly reducing their potential exposure to P. ramorum spores.
In some situations, clearance can be increased by pruning branches or stems from a California bay tree rather than removing the entire tree. As noted above (1.3.1. Environmental Conditions), low, shaded California bay foliage in the inner canopy typically has higher levels of P. ramorum infection than the uppermost, sun-exposed leaves at the top. Removing low branches (via canopy raising and inner canopy thinning) can reduce the amount of inoculum that will impact a nearby oak even if the California bay-oak clearance to upper canopy foliage is less than optimal.
Compared with area-wide removal, localized removal zones may require more frequent inspection and follow-up treatment to maintain the target oak-California bay clearances. Because fewer California bay trees are removed, localized removal may be less costly and time consuming than area-wide removal, but fewer oaks within the stand will be protected.
For high value oaks, chemical treatment may be needed in combination with localized California bay removal to reduce the risk of SOD to an acceptable level.
Individual California bay trees vary in their susceptibility to foliar P. ramorum infections (Meshriy and others 2006, Anaker and others 2008). California bay trees also vary with respect to how many infected leaves they retain over the dry season (Swiecki and Bernhardt 2007,2008a). Because P. ramorum does not appear to affect the survival of infected California bays, these more disease-prone trees are not eliminated from the population by natural selection. If California bays that consistently show high levels of infection or retain high numbers of infected leaves throughout the dry season are identified, they can be targeted for removal. Reducing the density of these California bays in the landscape should help slow the buildup of P. ramorum spores on leaves during the wet season, reducing SOD risk to oaks in the area.
In low rainfall years that are unfavorable for P. ramorum sporulation, few California bays will develop severe foliar symptoms. California bays that have significant numbers of infected leaves by late summer in such years would be candidates for removal. Some of these California bays may have high persistent infection levels due to microclimate effects (e.g., California bays in shaded drainages) rather than high genetic susceptibility. Removing such trees would not help select for more resistant California bay genotypes, but should still help slow the seasonal buildup of P. ramorum in the stand. In years that are favorable for disease, most California bays may be heavily infected and highly susceptible genotypes cannot be identified. It may be possible to identify California bays that are somewhat resistant to P. ramorum infection in favorable disease years. Resistant trees should show especially low infection levels when surrounded by California bays with a high level of foliar disease.
Selective elimination of highly susceptible California bays is more likely to be effective in areas where conditions are less favorable for P. ramorum sporulation, such as drier, more inland areas near the edge of the current P. ramorum range. This tactic is also more likely to be useful in areas with relatively low California bay density.