398 glyphosate versus those that did not, we found that glysophate-treated plots had significantly lower native flowering plant species richness than those treated with herbicide mixtures that did not contain glypho- sate (t = −2.44, df = 1, p = 0.04; Table 1). Regardless of type of herbicide used, mechanically treated plots had significantly higher numbers of tree species than chemically treated plots (t = 3.10, df = 1, p = 0.009; Table 1). DISCUSSION Our research supports past results from this study area (e.g., Yahner and Hutnik 2005) and from other ROW (De Blois et al. 2004; Yahner et al. 2008; Wagner et al. 2014) that demonstrate selective herbicide use as part of an IVM plan is an effective approach to lim- iting tree species on ROW while maintaining native plant species richness (Clarke et al. 2006; Wagner et al. 2014). Although our study took place under trans- mission lines within an eastern forest landscape, studies from a variety of forest areas indicate that selective use of herbicides does not significantly reduce native plant diversity and may cause less disturbance to native ecosystems than mechanical removal approaches. Menges and Gordon (2010) found that mechanical-only vegetation management treatments increased soil compaction and disturbance, while treatments that used mechanical vegetation removal in conjunction with targeted herbicide application were best at reducing hardwood abundance and maintaining native species in Florida upland habitats. Furthermore, Fortier and Messier (2006) found that manual brush cutting (sim- ilar to hand cutting) was the least effective vegetation management approach at reducing competing decid- uous trees and shrubs in forests in Canada. However, they also found that non-selective (broadcast) appli- cation of herbicides can decrease plant species rich- ness and abundance over time. In boreal forests, native plant species abundance was reduced following severe mechanical site preparation but was maintained with targeted herbicide application to competing non-native plants (Swift and Bell 2011). Our research indicates that the type of herbicide mixture applied (as well as application method) may affect species richness of native flowering plants—a topic that is receiving more attention. For example, Isbister (2016) found that herbicide mixtures that con- tained imazapyr caused more damage to non-target plant species than triclopyr in boreal forests. However, ©2020 International Society of Arboriculture Mahan et al: Native Plants on Rights-of-Way Lowe et al. (2007) found that herbicide mixtures that contained both imazapyr and triclopyr effectively controlled non-native invasive plants and permitted restoration of native plant communities in central Indiana. Glyphosate has been an important tool in the removal of invasive, non-native vegetation in forest communities, and a systemic review of the use of gly- phosate in forest environments indicates no signifi- cant risk to wildlife and plant communities (Rolando et al. 2017). Furthermore, recent research in central Pennsylvania demonstrated the resilience and recov- ery of native plant communities when invasive, non-native plants were controlled through selective and careful application of glyphosate (Maynard-Bean and Kaye 2019). However, the potential negative effects of glyphosate on forest soil microbial and earthworm communities indicate caution for long-term application, especially in northern ecosystems (see Helander et al. 2012 for review; Gaupp-Berghausen et al. 2015; Aristilde et al. 2017). Therefore, we urge further study into the effects of specific herbicide mixtures on native and non-native plant and soil com- munities, especially in forest (versus agricultural) set- tings (Kettenring and Adams 2011). Our research is unique due to the long-term nature of IVM management at the study site. However, our research is hampered by this feature as well. Over the 50+ years of treatment, herbicide mixtures, amounts, and application approaches have changed, but the basic research objective of understanding IVM as compared to mechanical approaches to ROW manage- ment have remained consistent. Our research focuses on measuring response of wildlife and plants to the treatments and, in general, our study supports the findings of other researchers that non-selective mechan- ical treatments (e.g., mowing, hand cutting) facilitated the invasion and abundance of trees in transmission line ROW (Mercier et al. 2001; De Blois et al. 2004). Integrated vegetation management on ROW, which includes selective herbicide treatment, provides oppor- tunities for maintaining native plant species richness while limiting the invasion of tree species. We note that our vegetation sampling in mid-summer may miss the effects of IVM on plants with short emer- gence (e.g., spring ephemerals), but we do note the presence of native plant communities dominated by Ericaceae and Asteraceae that dominate ROW in other northeastern studies (Wagner et al. 2019). Other com- mon native plant species found along the ROW corri- dor in our study, such as Rubus and Solidago, have
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