Introduction

Oak Forests of Wisconsin

Oaks (Quercus spp.) are a dominant component of the extensive oak-hickory forests of the Central U.S.A. (Leopold and others 1998). In Wisconsin, timberland typed as oak-hickory forest was estimated at 2.9 million acres in 1996 (Schmidt 1997). Growing stock volume for red oak (Lobatae) was estimated at 2.4 billion cubic feet, whereas white oak (Quercus) volume was estimated to be 927 million cubic feet (Schmidt 1997).

Oak Wilt – Primary Disease of Concern

Oak wilt, the oak disease of greatest concern in Wisconsin, occurs in 51 of the State’s 70 counties (http://www.na.fs.fed.us/fhp/ow/maps/ow_dist_fs.shtm). Thousands of oaks in woodland and urban settings succumb to the disease every year. The causal fungus, Ceratocystis fagacearum, is spread from diseased to healthy oaks belowground through functional root grafts or aboveground by insect vectors (Tainter and Baker 1996). Species of the sap beetle family (Coleoptera: Nitidulidae) are considered the primary vectors in Wisconsin. New disease centers are established when C. fagacearum-contaminated beetles visit fresh xylem-penetrating wounds, (e.g., axe blazes, logging wounds, branch-pruning wounds) on healthy oaks and successfully inoculate them with propagules of the fungus (Gibbs and French 1980, Juzwik and others 2004). Stump surfaces created by tree felling and wounds to branches, stems, and roots by heavy equipment or adjacent falling trees are avenues for infection during timber stand improvement or harvesting activities. In a timber sale unit near Waube Lake, WI, many new infection centers occurred over a large area following a May 2001 timber harvest (Personal communication. December 18, 2006. M. Mielke, Plant Pathologist, Northeastern Area State and Private Forestry, USDA Forest Service). Felling of diseased oaks adjacent to healthy oaks can lead to intensification of the disease within stands if root connections exist. Slow movement of the pathogen through grafted roots of healthy trees felled within 50 feet of a diseased tree explained the sporadic appearance of oak wilt in subsequent years at the edge of clear-felled areas (Yount 1955).

Need for Statewide Guidelines

The Wisconsin Department of Natural Resources (DNR) identified the need to develop consistent, statewide guidelines for timing harvest activities in oak timberland in order to minimize potential for oak wilt introduction or spread or both in existing and future stands where oak regeneration is the management objective. A committee of government, industrial, and consulting foresters was formed to develop such guidelines. Both scientific and experience-based knowledge of the oak wilt host – pathogen system were the basis of the guidelines. The approach used to: (1) analyze the risk and the potential for introduction and spread of oak wilt in stands targeted for harvest, and (2) develop guidelines for timing harvest are described in this paper.