Arboriculture & Urban Forestry 42(5): September 2016 Tsuga mertensiana (Bong.) Carrière (moun- tain hemlock)—A significantly smaller tree than its native western counterpart (max. ht. about 30 m), T. mertensiana is rarely found growing in close association with T. heterophylla, as it typi- cally occupies the less temperate, more elevated mountainous sites of the pacific northwest U.S., Canada, and Alaska (Means 1990). Several vari- eties of this tree are available commercially (Anonymous 2014), presumably for landscape purposes. Although it is described as being hardy to USDA Zone 5, and though numerous manu- scripts describe host resistance to HWA (Table 1), only two of the original 18 research specimens have persisted at Lasdon Arboretum (Table 2). Tsuga chinensis (Franch.) E. Pritz (Chinese hemlock)—Described as attaining heights in excess of 30 m in China (Dirr 2011), T. chinensis has been growing successfully in Jamaica Plain, Massachusetts, U.S. (USDA Zone 6b) since a speci- men was established at the Arnold Arboretum in 1910 by E.H. Wilson (Havill and Montgomery 2008). Thirty-eight T. chinensis seedlings that had been planted in 1999 were inventoried 09 March 2004, identifying no post-winter mortality despite exposure to a low temperature of -22.5°C, which occurred on 16 January 2004 (Del Tredici and Kitajima 2004). Dirr (2011) describes T. chinensis hardiness to Zone 5, and 17 of the 18 specimens at Lasdon Arboretum have persisted (Table 2). Tsuga diversifolia (Maximowicz) Masters (northern Japanese hemlock)—Occurring in northern Japan at higher elevations, this tree, records indicate, has been in commercial culti- vation and trade since 1861 (Anonymous 2014). Although only three of the original 18 research specimens (Table still persist 2), Dirr (2011) describes at Lasdon Arboretum survivorship of this tree as far north as the state of Maine, U.S. (Zone 4) and reports observing specimens at heights of over 12 m in Congreve Gardens, Ireland. Although many manuscripts report host resistance to HWA (Table 1), findings among all references are not entirely consistent. Tsuga sieboldii Carrière (Japanese hem- lock)—Found naturally growing in southern Japan at lower elevations, this tree, records indi- cate, has been in cultivation and trade, suc- cessfully, since 1914 (Havill and Montgomery 349 2008). One of the original 18 research specimens remains living at Lasdon Arboretum (Table 2). Tsuga canadensis, T. chinensis, and T. hetero- phylla were sourced from Eby Nursery, Wilsonville, Oregon, U.S., and T. caroliniana, T. diversifolia, T. mertensiana, and T. sieboldii were obtained from Forestfarm Nursery, Williams, Oregon, U.S. Plot Design Trial plots were approximately 12 m × 14 m, each, including six rows of the research trees spaced at approximately 2 m intervals. Species were planted in each row in random order and were intended to include at least one of each of the aforementioned seven Tsuga species as well as a second representative of T. canadensis per row. Because seven specimens of T. heterophylla and one T. caroliniana perished before time of planting (30 September 2003), some rows were lacking a complete complement of trees (the remaining specimens of T. heterophylla were distributed as evenly as possible among plots, Table 2). Daylight penetration on the plots ranged from full to partial sun throughout the day (Front Gate and Magnolia Garden plots), to nearly full shade provided by deciduous trees (Hemlock Hedge plot). Plots had been mulched annually, commencing in autumn 2006, with 5–10 cm depth of whole-tree mulch (i.e., assorted wood chips) from a municipal composting facil- ity for the purposes of weed management, mois- ture retention, and soil temperature regulation. Prior to annual mulching, the area between the trees was mowed as needed during the summer to reduce vegetative competition from grasses and weeds. Soil samples within the plots were submitted to the University of Massachusetts (UMass) Soil Diagnostic Laboratory on 31 May 2013. Soil compaction readings were also taken in each plot at 12 random locations with a soil penetrometer (soil compaction tester, DICKEY- john®, Auburn, Illinois, U.S.) on 29 May 2013. Trees were assessed for performance and pest occurrence on 19 and 24 July 2012. The height of trees was measured to the nearest 0.03 m using a tele- scopic measuring rod (Jameson, LLC, Clover, South Carolina, U.S.), and overall health was assessed visually, where each tree was assigned a rating from 0 (dead) to 5 (lush, dark green needles). The pres- ©2016 International Society of Arboriculture
September 2016
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