Arboriculture & Urban Forestry 44(2): March 2018 ences and allow for more accurate identification. However, there are uncommon yet unique species, like Ginkgo biloba, where identification appears to be reliable, possibly given the uniqueness of Ginkgo biloba. Truly unique species without a common look-a-like may require less attention during train- ing. Training methods that involve a test or quiz component—to isolate species or genera likely to be misidentified—could help to focus training on problem species, thereby increasing data quality. Species identification was problematic even for genera identified correct with high frequency. The genus Fraxinus was correctly identified 98% of the time, yet species within the genus were only cor- rectly identified 44% of the time (Table 3). While managers requested identification of trees to spe- cies level in most communities, the use of spe- cies information may not always have practical value. Invasive insect species that currently pose threats to the forests and woodlands of Minnesota include emerald ash borer (Agrilus planipennis) and gypsy moth (Lymantria dispar). Emerald ash borer infests all three native species of the Fraxi- nus genus (i.e., white ash, green ash, and black ash) predominantly found in Minnesota’s urban forests, while gypsy moth will defoliate hundreds of species of plants, albeit oaks and aspen tend to be more common tree hosts (U.S. Forest Service 2017). Asian longhorned beetle (Anoplophora gla- bripennis), not yet present in Minnesota, primarily uses trees in the Acer genus as its preferred host (Minnesota Department of Agriculture 2017). Volunteer ability to accurately identify tree gen- era is encouraging and warrants considerations for resource managers as knowledge of genus-level diversity will likely prove adequate to assess poten- tial canopy losses, or insecticide treatment costs needed to formulate management objectives based on projections from tree survey or inventory data. For the nine communities, measurement of DBH had an agreement frequency of 51% (Table 2); however, when allowing for a margin of error of 2.54 cm, the percentage agreement increased to 85% (Table 2). All communities except three (Hutchinson, Rochester, and Starbuck) were above the 80% agreement threshold proposed by Blon- iarz and Ryan (1996), with volunteers in St Cloud attaining a 98% agreement frequency. For both DBH and DBH ±2.54 cm, volunteers trained under 81 Method Two had statistically greater frequency of agreement (Table 3). The greater agreement for volunteers trained by Method Two may be attrib- utable to several differences and possible intro- duced bias regarding the assessment of increased agreement in DBH and CRW measurements. The first difference was the increased technical assistance in which researchers accompanied vol- unteers for their first few inventory outings, which allowed researchers to identify and correct errors in measurement techniques, such as measuring too low or high along the trunk, crooked or twisted tapes, and errors reading the tape. An additional source of improvement is likely attributable to the use of diameter tapes provided by the university research team to communities trained by Method Two. The use of diameter tapes helped with the consistency of measurement and data recording versus linear tapes (circumference measurements), but the diam- eter tapes used by volunteers trained by Method One, where some of the measurements needed to be converted from circumference to diameter by entering data in the appropriate spreadsheet col- umn, likely increased error. The combination of increased technical assistance and improved mea- surement devices for volunteers trained by Method Two increased the overall agreement by 6% (Table 3), with volunteers approximately 1.6 times more likely to agree with university researchers (Table 5). Additional sources of error in agreement may have been introduced by temporal differences in measurement between volunteers and research- ers. An increment core was taken from a small sample of trees in each community and measured in the field (data not shown) to verify that less than 2.54 cm of growth had occurred between the time of volunteer and researcher measurements. Even though trees were believed to have added less than 2.54 cm in diameter between measure- ments, it is plausible that some error was intro- duced due to tree growth. A source of bias was unintentionally introduced as researchers mea- sured DBH in all nine communities using only the measurement techniques and tools employed by the communities in training Method Two. The higher level of agreement seen in commu- nities trained with Method Two likely reflects, at least partially, the introduced bias. An unbi- ased assessment of agreement should have com- ©2018 International Society of Arboriculture
March 2018
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