Arboriculture & Urban Forestry 46(1): January 2020 triacanthos) (Neely 1973). But later, Neely (1988) maintained that wound closure rates were directly related to tree vigor, and that slow-growing trees are able to close wounds more quickly than fast-growing trees like American elm. Among the several methods available for detec- tion of internal decay, tomography is one of the least invasive (Johnstone et al. 2010), an important consid- eration for long-term management. Sonic tomogra- phy (SoT) measures heterogeneity in sonic velocity, which is directly proportional to wood density (Arciniegas et al. 2014). Electrical-resistance tomog- raphy (ERT) measures heterogeneity in electrical fields, indicating the presence or absence of moisture and electrolytes in the wood (Bieker and Rust 2010; Benson et al. 2019). When used together, SoT and ERT provide an accurate estimate of the incidence and severity of internal decay in living trees (Brazee et al. 2011; Marra et al. 2018). Due to the accuracy with which SoT estimates the geometric configura- tion of internal decay, it can also be used to estimate the loss of load-bearing capacity in trees with internal decay (Burcham et al. 2019). Because of its accuracy and minimal invasiveness, tomography is well suited for decay assessments while limiting any lasting injury to culturally and historically significant trees such as American elms. The primary goal of this study was to better under- stand the effects of long-term fungicide injection on the health of American elm using SoT and ERT to nondestructively determine the incidence and sever- ity of internal decay. Fungicide injection by itself does not result in decay. However, the process associ- ated with fungicide injection generates numerous wounds on the root flares, which could function as points of entry by wood-decaying fungi to colonize these trees. We hypothesized that trees undergoing regular fungicide injection treatments would have a higher frequency of internal decay compared to trees with irregular or no known injections. Additional objectives of the study were to: (1) provide baseline data on the incidence and severity of internal decay for American elm, data that are lacking for urban and suburban trees; and (2) support existing long-term management efforts for American elms threatened by DED in urban and suburban settings. MATERIALS AND METHODS Tree Selection American elms sampled in this study were located with the assistance of tree wardens, arborists, ecolo- gists, and homeowners. Based on the injection his- tory information obtained from these individuals, elms were partitioned into two injection history groups: (1) regular injection; and (2) irregular injection or no known history of injection. Regular injection refers to trees receiving preventative root flare injections with thiabendazole hypophosphite or propiconazole (Alamo®, Syngenta Crop Protection, Greensboro, North Carolina) at one to three year intervals. Some of these trees also received therapeutic injections when DED was identified. Trees with irregular injec- tion cycles were typically treated one to three times over a 30-year period, while the remaining trees had no known history of injection. The exact method of injection was not recorded. Trees were sampled in six states (Massachusetts, Connecticut, Rhode Island, New Hampshire, New York, and Minnesota), Wash- ington D.C., and Quebec, Canada over a three-year period (2016 through 2018) from May through Octo- ber. The location of sampled elms, the number of trees at each site by injection-history group, and the number of trees with decay can be found in Table S1. Prior to tomographic scanning, symptoms and signs of internal decay of the lower trunk were visually assessed and recorded. Sonic and Electrical-Resistance Tomography To capture sonic and electrical-resistance tomograms, the PiCUS® Sonic Tomograph 3 and TreeTronic 3 (Argus Electronic GMBH, Rostock, Germany) were used in this study. For specific details on the place- ment and collection of tomograms, refer to Marra et al. (2018). Briefly, tomographic cross sections were obtained on the lower trunk at a horizontal plane as close to the soil line as possible in order to account for the dramatic root flaring and subsequent cross sec- tional geometry exhibited by many American elms. At this location, the diameter at sample height (DSH) was recorded. For certain trees, additional sampling planes were established above the lowest plane for further scanning to corroborate the presence of decay. Galvanized roofing nails, 5.1 to 6.4 cm in length, were then inserted to a depth just beneath the outer ©2020 International Society of Arboriculture 3
January 2020
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