Arboriculture & Urban Forestry 42(4): July 2016 statistically different than the herbicide treatments that averaged 98%. The low percent kill for the cut- only treatment in the second year of Study 2 is due to very low percent kill in only one plot—20% kill in the youngest, northern most treatment plot (Lake George site). The other two cut-only plots in Study 2 had 90% and 100% mortality. It is not clear why the one plot had such low mortality. It may be that the smaller, younger vines in the northern plot, averag- ing 2.9 cm diameter and 15 years old, were not as readily killed with cutting alone, as compared to the older and larger vines treated in the southern plots, where the vines averaged 5.2 and 6.9 cm diameter, and 26 and 36 years old. In addition to vine size considerations to explain high sprout survival in the one plot, it could be that the cut-only plot in the northern site had a more open canopy, allowing for greater survival of stump sprouts due to higher plot- level light levels (discussion follows). Unfortunately, light quantity was not measured in the current study. It was fortuitous that a second year of study was added to observe treatment effects, as the conclu- sions based on the second year are very different than there would have been had the analysis been based on the first year alone. Three other studies of cut-stump treatments conducted for two years affirm the importance of a second year of study. Bal- lard and Nowak (2006) observed that trees treated with cut-stump herbicides oſten were alive aſter the first year, but dead the second year. Petrice and Haack (2011) reported higher mortality for ash tree species treated with cutting only and cut stump with herbicide in the second year compared to the first. Zedaker et al. (1987) observed the opposite: trees that were thought to be dead aſter one year were alive the second year, which still affirms the impor- tance of two years of study. In the current study, at the end of the first year aſter treatment, many treated vines were not dead (yet), and had many large basal sprouts. Nearly all of these sprouts were dead by the end of the second year. Of the 326 vines measured in Year 2 post treatment, only 17 were still alive, and eight of these were in one cut-only plot. It is not clear why nearly all sprouts from the first year died by the end of the second year. One factor could have been herbivory as oriental bit- tersweet is browsed by deer and other mammals (Ashton and Lerdau 2008; Lynch 2009). Impact of deer and other herbivores on current study 261 sprouts was observed to be low, as many sprouts in the second year were observed to be standing dead without evidence of browse, and all sites had large, live sprouts at the end of the second year (albeit a greatly reduced number compared to the first year). Additionally, high second-year mortal- ity was generally consistent across the three broadly separated study sites across the Hudson Valley, with likely differences in browsing pressure that did not cause high site-level variation in mortality (see non-significant block effect for Study 2). A more likely reason for the near complete mor- tality of stump sprouts through two years is that sprouts may have exhausted carbohydrate reserves in the roots aſter the first year. Carbohydrate storage and release are long recognized as key to the vegeta- tive regrowth of woody plants (Kayes and Canham 1991; Kozlowski et al. 1991; Kozlowski 1992), includ- ing vines (Mooney and Gartner 1991). Woody vines have a wide variety of carbohydrate storage patterns, with some species having storage in stems, and others in roots (Mooney and Gartner 1991). Existing infor- mation of vines and their carbohydrate compounds and carbon storage capacity comes from agronomic literature. Vitis species (grapes), because of their economic value, have had detailed plant physiol- ogy study. The bittersweet family (Celastraceae) has similar anatomical vine structure as the grape family (Vitaceae) (Carlquist 1991), so research on grapes could apply to bittersweet. Winkler and Wil- liams (1945) (cited in Mooney and Gartner 1991) found that bark in older grapevine roots had greater than 50% total non-structural carbohydrates (this is the source of carbon available to developing sprouts, in addition to defenses) that was nearly depleted by the end of the growing season—stored carbon in roots was enough to only support one year’s worth of grape development, which classed them as hav- ing a “deep depletion cycle” of total non-structural carbohydrates. It may be that oriental bittersweet has similar carbon dynamics, and can support sprouts for only one year, aſter which each sprout must produce enough photosynthates to carry out its life needs. Apparently, this may not have been the case in the current study where all of the sprouts were growing in full shade of a forest canopy. While oriental bittersweet is considered to be shade tolerant as a seedling with a capacity to shiſt ecophysiology to be more like a shade-intolerant ©2016 International Society of Arboriculture
July 2016
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