Understanding and Managing Sudden Oak Death in California

3.5.1. Species Selection

The term "restoration" implies a return to a previous condition.  Because forest composition changes over time, various forest types may have occupied a site in the past.  Forest stands in most of California have been substantially altered by human activity over the past two centuries.  Due to tree cutting, changes in fire frequency, grazing, and other cultural practices, current stand composition and structure may differfrom the conditions prior to  European/American settlement.  For example, tanoak-dominated stands in some areas developed after clearcutting of redwood/Douglas-fir forests in the mid- 20th century. 

Previous forest cover indicates what may have grown in the past.  Also consider how site conditions may have changed since that forest type was established. In addition to the presence of P. ramorum, sites may be changed due to:

Such changes may inhibit the establishment of species that once occupied the site.  Some of these factors can be addressed by management actions.  For instance, surface litter can be removed or protection provided from herbivores, although these interventions may increase costs.  Other constraints that cannot be eliminated need to be dealt with through appropriate species selection.  For example, warmer, drier conditions associated with climate change may render sites unsuitable for species that were only marginally suited to the area in the past.

Use of SOD canker hosts for restoration—

Under some conditions, it may be feasible to replace SOD-killed oaks with the same species.  If the stand will be managed to eliminate California bay and any other significant sources of P. ramorum spores, SOD-killed oaks may be replaced with susceptible species such as coast live oak and California black oak.  It may be possible to recruit existing natural oak regeneration in SOD-induced canopy gaps. 

Although tanoaks vary in their susceptibility to P. ramorum, highly SOD-resistant tanoaks have not yet been identified. Until tanoaks with useful levels of SOD resistance can be identified, regenerated tanoak stands will continue to be at risk. Replace tanoak canopy lost to SOD with species other than tanoak.

Choosing replacement species—

In SOD-affected tanoak and mixed-hardwood stands with high amounts of California bay, species that are not susceptible to SOD should be favored. To achieve a stand that will be stable over the long term, the species that will dominate the replacement stand must be both well-adapted to the site and stocked at an overall density that is sustainable over time.

Canopy cover and tree species characteristics both need to be considered in designing restoration projects that will fulfill specific restoration goals (table 3-8).  Visual considerations may be especially important when choosing trees to plant around homes.  For example, replacing lower-growing oaks with tall conifers may obstruc views or sunlight.  Conversely, where visual screening is desired, evergreen species will be more effective than deciduous trees.  Replacing SOD-killed coast live oaks with deciduous blue or valley oaks will retain many of the same ecosystem services, but will not provide the same level of visual screening during the winter.

Table 3-8—Canopy cover and tree characteristics that will achieve various restoration goals

Restoration goal Canopy cover Tree characteristics

Soil stabilization


Fast establishment, wide fibrous root system, evergreen

Moderation of stormwater flows


Evergreen or deciduous with high leaf surface area during rainy season

Maintenance or improvement of wildlife habitat / biodiversity

Varies, diversity of cover levels may be important

Provides food (e.g., acorns, insects), cover, nesting sites

Maintenance or improvement of native plant habitat / biodiversity

Varies, diversity of cover heights may be important

Open canopies may allow for better development of understory plants

Suppression of undesirable invasive species


Dense canopy and/or allelopathic (suppressive) properties

Visual screening


Evergreen trees with high canopy density at desired height range

Aesthetics / property value enhancement



Shading for energy conservation

High in areas near structures

Deciduous trees with relatively early leaf drop and late leaf emergence

Minimize fire hazard in defensible space

Low in firebreak area; low to moderate in reduced fuel zone

Relatively low flammability, high canopy with clear lower trunk, low litter production

Although varying combinations of canopy cover and species composition can meet multiple goals, no single forest structure is likely to meet all goals.


When evaluating a site's capabilities, remember that conditions can change over relatively short distances due to factors such as soil type and depth, topographic position, slope, and aspect.  Species that perform well in deep soil at the base of a slope near a seasonal creek may not be adapted to growing on a west-facing slope with shallow soils a short distance away.  Some SOD canker hosts, such as coast live oak, can tolerate a broad range of site conditions.  Unless the replacement species are equally adaptable, multiple species may be needed to fill the niches previously occupied by SOD-killed species.

Integrating restoration with other land management goals and practices—

Organizations that manage large areas of forest and other natural habitats (e.g., park and open space districts, land trusts, water districts) typically have overall land management goals that apply across their holdings.  When setting goals for restoration of SOD-affected sites, resource managers need to consider how the restored sites will function in the context of broader land management goals.

Individual private landowners commonly manage smaller parcels. These parcels occur within the context of a wider landscape which may impact restoration projects. For example, it may be difficult to suppress invasive exotic plants encroaching from adjoining lands. For wildlife species that range across multiple land ownerships, consider how the restoration site fits in the overall landscape. Where possible, coordinate management efforts with adjacent landowners.

For safety, and to comply with state and local regulations and guidelines related to fire hazard near structures, consider fuels management. Minimizing fire hazard in the reduced fuels portion of areas designated as defensible space depends on proper vegetation selection, placement, and management.

Most small scale restoration projects do not involve activities that are addressed by various federal, state, and local environmental and land use regulations. However, larger scale projects that involve intensive inputs such as land clearing, grading, or burning may require agency permits or approvals. Landowners should check with appropriate local and state agencies before undertaking these activities.