Understanding and Managing Sudden Oak Death in California

3.2.2.4. Oak-California bay clearance distance—

California bay removal reduces SOD risk by increasing the distance between the spore source (California bay leaves) and the target (a susceptible oak trunk). The number of P. ramorum spores that land on an oak depends on:

• The California bay foliage-oak trunk clearance, and
• The overall amount of California bay canopy (overstory and understory) that is close enough to serve as a source of inoculum (see 1.2.2.1. California bay–oak distance and disease risk and 1.3.1 Environmental Conditions).

Increased California bay-oak clearance should be developed only by removal or pruning of California bay. Pruning off oak branches or stems to increase California bay-oak clearance is not recommended. No single prescription has been verified as effective for localized California bay removal. Consider the following general relationships and recommendations when implementing localized California bay removal:

• SOD risk decreases as the oak-California bay clearance increases and removal of California bay closest to the oak is associated with the largest reduction in SOD risk. The minimum horizontal clearance from California bay foliage to the oak trunk (fig. 1-6) that is likely to have an effect is about 2.5 m; clearances of 5 m or more are preferable.
• Larger California bay-oak distances are needed as the overall amount of California bay canopy in the oak neighborhood increases. If possible, provide additional clearance in the direction of prevailing storm winds, especially if tall California bay canopy is present in that direction.
• In climates that are more favorable for P. ramorum spore production, larger oak-California bay clearance may be needed to achieve adequate risk reduction.
• Removing low inner canopy California bay branches close to oak trunks may help reduce risk even if foliage in the upper portion of the California bay canopy cannot be cleared to the 2.5-5 m minimum target distance.
• California bay seedlings less than 1 m tall should be removed if located within about 3 m from an oak trunk.

When using localized California bay removal, it is often necessary to strike a balance between the maximum risk reduction (greatest clearance possible) and minimizing cost and environmental impacts (smallest clearance that will provide a substantial risk reduction). Each doubling of the desired clearance distance translates to a quadrupling of the area that needs to be cleared, with a corresponding increase in effort (fig. 3-2). In addition, California bay canopy within the area to be cleared may arise from a trunk located outside the clearance distance. Hence, California bays rooted outside of the target clearance distance may also need to be removed or pruned.

Figure 3-2—Bay removal area (blue circles) for four bay foliage-oak trunk clearance distances (orange arrows). Green outline represents a large oak canopy (about 22 m canopy diameter) with trunk shown as the central dot. The smallest clearance likely to provide a significant reduction in SOD risk is about 2.5 m. Although increasing clearances provide greater risk reduction, the area of bay removal quadruples with each doubling of the clearance distance.