Risk Assessment

Risk refers to the chance of injury or loss defined as a measure of the probability and severity of an adverse effect to health, property, the environment, or other things of value (North American Forest Commission 2004). Our risk analysis includes: (1) the assessment of risk posed by the oak wilt pathogen to oak timberland scheduled for harvest and regeneration to oak, and (2) recommendations for minimizing frequency of pathogen introduction to and spread within such stands. A rule-based, expert-driven model, such as that used for pest risk assessment in the Exotic Forest Pests (ExFor) system (North American Forest Commission 2004), was adapted for this analysis. This approach falls under the umbrella term of multi-criteria decision analysis, which seeks to take multiple criteria into account when groups explore decisions that matter, e.g., natural resource management decisions (Mendoza and Martin 2006). Two criteria were evaluated within the risk assessment process.

Criterion 1: Risk of Ceratocystis fagacearum introduction to the stand [between-stand spread] or for initiation of new centers within the stand [within-stand spread] by insect vectors

Statements were developed for this criterion that considered two factors: (1) time of year during which harvest activities would occur (resulting in fresh wounds suitable for infection), and (2) proximity of existing oak wilt centers in other locations to the stand in which harvest activities would occur. A risk rating, ranging from very low to very high, was then assigned to each of the possible combinations of time and proximity. The risk values were determined through a group consensus process after review of pertinent scientific literature and of each individual’s experience working with the disease.

Criterion 2: Risk of C. fagacearum belowground spread within an oak stand following pathogen establishment

Statements for this criterion included three factors, (i.e., stand conditions): (1) density of oaks, (2) general topographic relief, and (3) general soil type in the stand to be harvested. Each of these factors is known, either through scientific studies or experiential knowledge or both, to influence the frequency and the distance over which intraspecific root grafting occurs. Two or more levels were selected for each factor. Basal area (square feet per acre) levels for describing red oak species composition and density were: less than 15, between 15 and 35, and greater than 35. The general levels for topographic relief were: (a) flat to rolling terrain, and (b) steep hills with deep valleys terrain. Soil type was divided into light textured (sandy, loamy sand, and sandy loam) and heavier textured (all other types depicted in classic soil texture triangle). A risk rating, ranging from very low to very high was then assigned to each of all possible combinations of statements by factor. The risk values were determined by a group consensus process.

Overall risk: combined risk rating for the two criteria

The ratings for each criterion were then used to generate the overall risk of oak wilt’s threat to the stand of interest following a timber harvesting event. The overall rating, ranging from very low to very high, was assigned to each stand scenario based on the combination of introduction and root graft spread factors. As before, the risk values were determined through a group consensus process.