Risk Analysis Results
The combined risk ratings for Criterion 1 (see Risk Assessment) are shown in (Table: Combined risk ratings for Criterion 1). The risk of overland pathogen transmission by sap beetles was considered to increase as proximity to an existing oak wilt center decreased. The existing centers would be the source from which inoculum-laden beetles would originate, assuming oak wilt mats were formed on recently wilted red oaks in that originating center. Menges and Loucks (1984) and Shelstad and others (1991) found higher efficiencies of vector spread over short distances, (e.g., ≤ 300 m); longer distance spread occurs very infrequently and on a random basis. Although the number of new centers occurring at greater distances is small, over time they can have a significant influence on distribution of oak wilt within the total forest area (Shelstad and others 1991). Timber harvest activities would result in wounding of residual oaks in shelter wood cut situations or create stump surfaces of removed healthy oaks or both. Such xylem-exposing cuts are attractive to dispersing sap beetles. The risk of pathogen transmission to such wounds by certain sap beetle species is high during the spring months, low from midsummer to early fall, and none during the late fall and winter (Ambourn and others 2005, French and Juzwik 1999, Juzwik and others 2006).
The combined risk ratings for Criterion 2 (see Risk Assessment) are shown in (Table: Combined risk ratings for Criterion 2). Frequencies of root graft spread increase with increasingly lighter textured soils, e.g., from silt loam to sands (Menges 1978). Furthermore, frequency of root graft transmission is highest for stands with > 60 percent red oak density (Menges and Loucks 1984). Lastly, oak wilt is very common in areas of low topographic relief in portions of Iowa, Michigan, Minnesota, and Wisconsin, (e.g., Albers 2001, Menges and Loucks 1984). In areas with obvious topographic relief, oak wilt is most common on upper slopes and ridge tops (Anderson and Anderson 1963, Bowen and Merrill 1982, Cones and True 1967).
Each of the 108 stand condition scenarios described by combinations of the five factors was assessed for overall risk of oak wilt occurrence based on the individual criterion ratings. Overall risk ratings were high to very high for eight stand-condition scenarios where oak wilt was not known to be present in the stand (Table: Overall high or very high risk). Overall risk rating of very low, however, was often determined by a late fall – winter timing for the harvest.
- Timber Harvest Guidelines/Risk Analysis and Guidelines : Three different formats of the risk analysis results and the harvest guidelines were developed for end users.
Encyclopedia ID: p3473
