Understanding and Managing Sudden Oak Death in California

3.3. Monitoring Oaks With SOD to Assess Survival and Failure Potential

3.3.1 Overview

Three factors influence a tree’s failure hazard:

The likelihood that the tree or a part of it will fail in a given time period;
The ability of the failing part to cause damage (due to its size and the distance that it may fall);
The presence of a valuable target within the failure's impact zone.

The USDA Forest Service and other organizations have developed standardized numerical tree hazard rating systems based on evaluation of these factors (see below Sources of Information on Tree Hazard Assessment Methods—).

Although many trees on a given property may have an elevated likelihood of failure, only those trees that have the potential to cause personal injury, death, and/or property damage need to be included in most tree hazard surveys. If a target zone is rarely occupied (for instance, a low-use trail), it is not likely to be occupied at the moment of tree failure. Hence, the risk of damage can be low even if the tree is likely to fail. In contrast, if the target area is continuously occupied, any failure is likely to cause injury or loss. In such sites, even trees with a moderate failure potential need to be considered for possible hazard reduction.

As discussed in Part 2 (see 2.2.3.1. Survival and failure potential of SOD-affected trees) trees with late stage SOD symptoms have an elevated likelihood of failure overall compared to similar trees lacking these symptoms. Specific factors related to failure risk in SOD-affected trees are shown in tables 3-2 and 3-3. These risk factors should be incorporated into the tree hazard evaluation process in areas where SOD is present and should be considered in combination with other tree failure risk factors.

Monitoring. As SOD symptoms develop over time, the likelihood of failure also changes. Monitor SOD-infected trees so they can be removed in a timely manner to minimize hazards.

A one-year reinspection interval is a useful starting point in many situations. Of all the factors that contribute to failure risk (tables 3-2, 3-3), factors related to P. ramorum infection tend to change the most rapidly over time. Collect detailed observations on individual trees to serve as an initial baseline survey so that changes in condition are readily apparent when follow-up inspections are made (table 3-4).

Sources of information on tree hazard assessment methods—

Johnson, D. W. Revised 1981. Tree Hazards: Recognition and Reduction in Recreation Sites. USDA Forest Service State and Private Forestry Rocky Mountain Region, Technical Report R2-1. Available: http://www.na.fs.fed.us/spfo/pubs/hazardtrees/treehazards/thazards.pdf

Pokorney, J.D. 2003. Urban Tree Risk Management: A Community Guide to Program Design and Implementation. USDA Forest Service, Northeastern Area, State and Private Forestry Publication NA-TP-03-03. Available: http://www.na.fs.fed.us/spfo/pubs/uf/utrmm/

US National Park Service Hazard Tree Guidelines. Available: http://na.fs.fed.us/fhp/hazard_tree/pubs/misc/nps.htm

Other publications on tree risk assessment are available for purchase from the International Association of Arboriculture http://secure.isa-arbor.com/webstore.

Table 3-2—Guidelines for assessing mortality risk and hazard potential in wildland coast live oaks based on P. ramorum canker symptoms (see also table 3-3)

Risk factor	Factor level	Effect on mortality risk	Contribution to failure potential within 1 year	Additional considerations and interactions
Early P. ramorum canker symptoms (bleeding cankers only)		Low risk of mortality within the next year	None	Symptom remission is most likely in trees with small cankers (<20% of circumference girdled). Trees with extensive girdling (>80% girdled) are likely to die within 2-4 years, but cankers normally become invaded by secondary organisms before mortality or failure. Mortality risk may be underestimated if cankers are larger than indicated by bleeding. Trees be monitored at least annually for symptom progression.
Late P. ramorum canker symptoms (cankers plus beetle boring and/or Annulohypoxylon thouarsianum sporulation)	Varies with amount of trunk affected	Moderate to high	Mortality risk and failure potential increase as the degree of colonization by secondary organisms increases. Failure potential is also influenced by the presence of other types of decay and defects present (table 3-3).
Fruiting of A. thouarsianum or Phellinus gilvus	2.5% to 50% of trunk circumference with sporulation	Moderate risk of mortality within 1-2 years	Moderate	Mortality and failure risk in live trees increases as percent of the trunk circumference affected increases. Failure risk interacts in an additive fashion with other decay columns present in tree. Decay occurs before fruiting bodies develop, so sapwood decay may be extensive even if fruiting bodies are not seen. Older fruiting bodies may have fallen off or degraded.
Fruiting of A. thouarsianum or Phellinus gilvus	50% or more of trunk circumference with sporulation	High risk of mortality within 1-2 years	High
Beetle boring	2.5% to 50% of trunk circumference with exit holes	Moderate risk of mortality within 1-2 years	Moderate	Mortality and failure risk in live trees increases as percent of circumference affected increases. Beetle boring is usually associated with decay. Beetle boring seldom penetrates more than 10 cm into the wood, so beetles damage alone is of less importance in large diameter trees. Inactive galleries may be difficult to detect if boring dust has washed off.
Beetle boring	50% or more of trunk circumference with exit holes	High risk of mortality within 1-2 years	High

Table 3-3—Additional factors related to hazard potential in SOD-affected trees (see also table 3-2)¹

Risk factor	Factor level	Contribution to failure potential	Additional considerations and interactions
*Decay and related factors*
Tree dead or partly dead	Present	High	Small-diameter stems tend to fail sooner than larger stems. Levels of decay and other factors influence how quickly failure occurs.
Decay	<25% of the stem cross section affected	Low	Failure potential increases with increasing decay. Decay in structurally critical areas (e.g., branch junctions) can drastically increase failure potential. Decay assessment methods (drilling, etc) may be needed to assess amounts of decay in standing trees. In the absence of direct decay assessments, use other indicators of decay (fruiting bodies, canker rot symptoms, decline symptoms, beetles) to estimate extent.
	25-50% of the stem cross section affected	Moderate
	More than 50% of the stem cross section affected	High
Fruiting of wood decay fungi other than Annulohypoxylon thouarsianum	Inonotus spp., Phellinus spp., Laetiporus gilbertsonii and other primary decay fungi	High	Presence of fruiting bodies indicates decay is present. This decay interacts in an additive fashion with decay caused by A. thouarsianum. Failure potential varies somewhat between fungal species. In absence of positive identification, consider any fruiting body emerging from or through bark to be important.
	Stereum spp, Trametes spp. and other secondary decay fungi	Moderate
Cavities	>50% of stem cross sectional area affected	Moderate to high	Risk increases as the percent of cross sectional area affected increases. Cavities increase failure potential primarily if decay and other risk factors are also present
Canker rot symptoms	Symptoms present, but no fruiting bodies	Moderate	In the absence of fruiting bodies, canker rot symptoms provide an indication that decay columns are present. Decay from canker rot fungi interacts in an additive fashion with decay caused by A. thouarsianum and P. gilvus.
Decline due to other agents	Tree in severe decline, but no fruiting bodies	Moderate	In the absence of fruiting bodies, symptoms of severe decline may indicate that extensive decay columns or root disease are present.
Old failures, large decayed stubs	Present	Low to moderate	Can serve as point of weakness where subsequent failure is likely to occur. May also serve as indicators of internal decay.
*Tree structure factors*
Number of stems from ground	Multiple stems from ground	Low to moderate	Mainly increases failure potential in trees with dead stems, decay, and other substantial risk factors.
Multiple branches from one point	Present (especially if crowded)	Low	Mainly increases branch failure potential in trees with dead stems, decay and other substantial risk factors.
One- sided canopy	Present	Low	Mainly increases bole failure potential in trees with dead stems, decay and other substantial risk factors. Severe trunk lean can have a similar effect.
*Stand factors*
Sky- exposed canopy rating	<50% of canopy exposed to overhead sunlight	Low	Failure potential increases as sky exposure decreases (i.e., as degree of overtopping increases). Trees with dead stems, decay, and other substantial risk factors are most likely to show increased failure risk due to this factor.
Tree neighborhood altered	Other dead or failed trees present within 2-3 canopy widths of tree	Low	This factor may primarily serve as an indicator of a P. ramorum disease cluster. Increased exposure to wind possibly increases failure risk in trees with dead stems, decay and other substantial risk factors.

¹Most of these factors are not strongly related to mortality risk in the near term.

Table 3-4—Methods for assessing factors related to mortality and failure potential in SOD-infected trees

Factor	Assessment methods	Notes
Phytophthora ramorum canker activity/ expansion	Note presence of current-year bleeding Estimate percent of trunk circumference affected Note which sides of trunk are affected using compass directions (N, NE, E, etc.) Mark edges of canker using paint or lumber crayon Document with digital photos Note if callus is developing along edge of older cankers	Comparisons between annual assessments will indicate whether cankers are expanding or inactive.
Trunk decay	Types of fruiting bodies present (photograph if not identified) Estimate percent of trunk circumference with A. thouarsianum or other fruiting bodies Note location and extent of cavities, decay zones	Decay can be extensive before fruiting bodies appear. Intensive decay-detection methods (drilling, imaging) can provide more detail, but are expensive and time-consuming. Decay fungi present can be identified via DNA testing of wood shavings from drilled holes.
Beetle colonization of trunk	Estimate percent of trunk circumference affected Note which sides of trunk are affected using compass directions (N, NE, E, etc.) Note presence of fresh boring dust	Bark and ambrosia beetle boring is most easily rated when fresh boring dust is present. Beetle holes may be difficult to see when dust has washed away, so the extent of colonization may appear to decline over time.

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