Arboriculture & Urban Forestry 36(4): July 2010 (i.e., a 300% increase overall). Estimates of removal costs under both scenarios were not addressed in this study; however, both options were deemed economically feasible in a general sense. Based on results of this study and other considerations, the park reserve staff has selected the “monitor and remove” option. Alternative VPL Placement Models Spatial analysis using the geo-referenced maps also allowed for theoretical comparison of how VPL placement differs with the lines placed by park staff in this study and how potential red oak loss (either through disease or PSPT removal) could vary by line placement model used. On all sites but one, primary lines placed using the 95% confidence level of the statistical model were lo- cated at a greater distance from the original diseased oaks than were the installed lines. The theoretical placement of lines ac- cording to the 80% confidence level of the same model was simi- lar to that of the installed lines on approximately one-third of the sites while the modeled lines for half of the sites were farther out, distance-wise, than the installed lines. The 95% and 99% confi- dence level versions of the statistical model are recommended for VPL placement by university extension and natural resource agencies in Michigan (Bruhn and Heyd 1992; Adams and Blan- kenheim 2001), and Wisconsin (Carlson and Martin 2005). Oak wilt eradication in the affected stands and one time treat- ment are the goals for many of the sites for which the model is used in Wisconsin (Kyoko Scanlon, pers. comm.). Thus, conser- vative VPL line placement coupled with “cut-to-the-line” red oak removal is a logical approach to achieve such goals. In hypotheti- cal spatial analysis for Murphy-Hanrehan Park, a “cut-to-the-line” option coupled with primary VPLs placed according to the 95% confidence level statistical model represented a 250% increase in numbers of red oaks that, theoretically, would have been removed. In contrast, the line placement model currently used by the park reserve represents a trade-off between trying to preserve as many oaks as possible while trying to minimize oak losses over time due to oak wilt. In essence, the park’s staff have chosen to manage the disease versus eradicate it from their forest lands. Consistent with that approach, the park has now adopted a “monitor and re- move” option for reducing the potential for overland spread of the pathogen within the park reserve and has chosen to continue using their modified version of French and Stienstra’s model (1978). One other point arose during the theoretical comparison of VPL placement models. The confidence levels of the statisti- cal model examined are based on the inter-tree distance associ- ated with selected probabilities (e.g., 95%) that a tree will not contract oak wilt through root grafts from an inoculum source tree for one year following its death for the Pemene soil series (Bruhn et al. 1991). The use of the published statistical mod- els (Bruhn et al. 1991) for controlling oak wilt in stands on soil types other than the Pemene and Grayling series could result in unnecessary loss of trees (Bruhn and Heyd 1992). The rate at which the oak wilt fungus moves belowground varies greatly by soil type and topography, as well as other factors (Juzwik 2009). Therefore, the hypothetical use of the statistical model developed for the Pemene soil series in the study may have placed the VPLs farther out than necessary for the soil type characteristic of Murphy-Hanrehan Park. Indeed, extensions of the statistical model need to be developed for other common soil types where the model is used (Bruhn and Heyd 1992). CONCLUSIONS For the management forester of the Murphy-Hanrehan Park Re- serve, overall oak wilt treatment and program success is measured by 1) the total number of trees saved at the infection center level and across the landscape, 2) the numbers of infection centers con- tained by effective VPLs, and 3) the numbers of new centers initi- ated by insect spread. For the park, the most parsimonious strategy for controlling belowground fungus spread is to place the barrier such that the fewest numbers of oaks are left within the primary VPL, without excessive risk that the pathogen is already in the root systems of trees outside the primary plow line. Based in part on an analysis of the present study, the park’s forestry division plans to continue using their modified version of the French and Stienstra (1978) model for placing VPLs. Re-treatment of sites with the vibratory plow is feasible and considered worth the effort to save additional trees when the disease continues to be quite active sev- eral years after the initial plowing or a when a “breakout” results from unintentional under-placement of the original primary VPLs. Diligent annual removal of recently wilted red oaks inside the primary barriers would reduce incidence of new pathogen intro- ductions (i.e., new infection centers), affording even greater control in the reserve’s oak wilt management program. The park reserve was able to fully implement the “monitor and remove” sanitation option in 2008 in the hopes of increasing program effectiveness. Acknowledgments. Funding for stem map data collection and mapping was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative Commission on Minne- sota Resources and from the Special Technology Development Program of the US Forest Service. Special thanks to R.L. Heyd, K. Scanlon, and A.D. Wilson for review and helpful discussion of an earlier version of the manuscript. The authors also thank G. Watson and two anonymous indi- viduals for their review and helpful suggestions of the submitted manu- script. The technical assistance of Shawn Neuman and Stacey Olszewski is gratefully acknowledged. LITERATURE CITED Adams, G., and Z. Blankenheim. 2001. Oak wilt in Michigan. MSU Ex- tension Bulletin EZ764, Michigan State Univ., E. Lansing, MI. 8 p. Billings, R.E. 2000. State forest health programs: a survey of state forest- ers. Journal of Forestry 98:20–25. Bruhn, J.N., and R.L. Heyd. 1992. Biology and control of oak wilt in Michigan red oak stands. Northern Journal of Applied Forestry 9:47–51. Bruhn, J.N., J.B. Pickens, and D.B. Stanfield. 1991. Probit analysis of oak wilt transmission through root grafts in red oak stands. Forest Science 37:28–44. Carlson, J.C., and A.J. Martin. 2005. Oak wilt management – what are the options? University of Wisconsin Extension Publication. G3590. 6 pp. Cook, S.J. 2001. Current practices and suppression methods for man- aging oak wilt disease. pp. 93–100. In: C. Ash (ed.), Shade Tree Wilt Diseases, APS Press, American Phytopathological Society, St. Paul, MN. French, D.W., and W.C. Stienstra. 1978. Oak wilt. Univ. of Minn., Agri- culture Expt. Sta., Extension Folder 310. 6 pp. Haight, R.G., S.V. Mehta, T. Hovie, D.J. Smith, R.C. Venette, F.R. Homans, and A.J. Walter. 2009. An economic impact assessment for oak wilt in Anoka County, Minnesota. p. 31. In: K. McManus and K.W. Gottschalk (eds.), Proceedings of the 20th U.S. Department of ©2010 International Society of Arboriculture 177
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