Arboriculture & Urban Forestry 36(4): July 2010 within the primary VPL and up to 15 m beyond or as the situ- ation required. An inventory of all the mapped oak stems was taken and recorded electronically. Species, stem diameter(s), and tree condition (e.g., live, no oak wilt; currently wilting, dead due to oak wilt; dead due to other causes), were noted for each tree. Locations and diameters of oak stumps were re- corded when present. Coordinate data, location data for each oak stem in relation to the plot center (i.e., azimuth and dis- tance), and tree data were imported into a GIS (ArcView 8.03, Redlands, CA) and stem maps were created for each site. Other Data Collection Wilting tree data Location, species and size of each wilting oak detected was recorded by park staff during ground disease surveys conduct- ed between July and late-August of each year between 1997 and 2005. GPS coordinates for each wilting tree were used to create modified stem maps for each site with tree condi- tion status for each year between the treatment year and 2005. VPL location data Using a handheld GPS, park staff recorded locations of all plow lines installed in the 25 sites. These line data were imported into the GIS project as a VPL layer for each site. Supplemen- tal location data (GPS obtained coordinates) for witness trees were also imported and used to adjust line placement when each stem map was overlaid with the VPL layer. Hand drawn maps and tree maps created prior to VPL treatment were also used to adjust line placements in the GIS. Ground checking was also con- ducted in summer 2006 for twelve sites where questions about line placement remained. Final VPL placement for each site was then used to create annual site maps containing all oak stems and their conditions between 2001 and 2005. Length of the prima- ry and secondary lines installed on each site were measured in the GIS and compared with estimated line length on the origi- nal field data sheets as recorded by the park forestry technician. Evaluation of VPL Effectiveness The location of wilting trees in relation to installed primary and secondary VPLs on each site map were examined to determine whether the disease had spread beyond the VPLs each year be- tween 2001 and 2005. Knowledge of separate oak wilt infection centers developing in the vicinity of the treated centers was also used to determine whether wilting trees appearing outside the VPLs were likely to be from an encroaching center, due to overland spread of the fungal pathogen by insect vectors, or attributable to failure of the installed VPLs to contain the original infection(s). The resulting determination was recorded for both secondary and primary VPLs in each site. The overall success or failure rate of plow treatment was based on number of sites where primary lines apparently failed to contain the disease per total number of centers treated. The year of apparent failure was also noted. Comparison of Potential Spore Producing Tree Sanitation Options During the years of the study, the park reserve did not employ ei- ther a “cut-to-the-line” or a “monitor and remove” strategy to re- duce potential spore mat producing trees; thus, no red oaks were 173 identified and removed in the 25 sites during the course of this study. However, the number of potential PSPTs that arose each year could be estimated using the stem maps and the GIS. To ob- tain estimates of trees that would have been removed had the “cut- to-the-line” option been implemented by the park, the number of all live red oaks within the primary lines on each site in the year of plow treatment were tallied. The cumulative number of red oaks that wilted on each site during the study period was used to esti- mate hypothetical removal under a “monitor and remove” scenario. Comparison of VPL Placement Models Hypothetical, model-placed VPLs based on the Pemene soils equation (Equation 1) of the statistical model devel- oped by Bruhn et al. (1991) were evaluated. Specifically, two lines representing the 95% and the 80% confidence level were created in a GIS layer of theoretical VPLs for each site. [Equation 1] D = (dtt + dst)/e({ - Z - [0.5659735 * (Soil)]} / 2.481853) where D = inter-tree distance (feet) associated with 95% or 80% probabilities that a tree will not contract oak wilt through root grafts from an inoculum source tree for one year follow- ing its death; dtt = diameter of healthy tree; dst = diameter of inoculum source tree; Z = a variable based on the level of con- fidence desired (= 1.645 for 95% confidence level and 0.842 for 80% confidence level); and Soil = 1 for Pemene soils. Because a 3 m width clearance is required for plow op- eration, these modeled lines were adjusted to account for such practical considerations. Hypothetical red oak remov- als under a “cut-to-the-line” scenario and a hypothetical “monitor and remove” scenario were then estimated and com- pared for the two modeled VPLs (at two different confidence levels) and the installed VPLs for each site using the GIS. RESULTS Characteristics of Installed VPLs For the ten active oak wilt sites treated in 1997, only primary VPLs were established in two, while both primary and second- ary VPLs were placed in the remaining sites (Table 1). Both types of VPLs were established in the nine sites treated with the plow in 1998 and the six sites treated in 1999. Length of barri- ers established varied greatly, with length of primary VPLs ex- ceeding those of secondary ones in all but one of twenty-three sites receiving both barrier types. Primary VPL lengths across all sites averaged 70 m and ranged from 16 to 116 m. Secondary VPLs for 23 sites averaged 41 m and ranged from 14 to 77 m. Efficacy of Vibratory Plow Treatments Between 1998 and 2005, oak wilt was found outside the second- ary but inside the primary VPLs (i.e., secondary line failure) for five of eight sites in which both line types were established in 1997 (Table 2). Oak wilt was found outside the primary VPLs (i.e., primary line failure) in one site. For one of these sites where the wilting tree was 3 m outside a primary line that was 26 m long, failure of the VPL (in 2003, six years after plow- ing) was assumed; no secondary lines were installed on this site. Overland spread of the pathogen (in 2002) was cited in ©2010 International Society of Arboriculture
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