Arboriculture & Urban Forestry 35(1): January 2009 Arboriculture & Urban Forestry 2009. 35(1):5–9. 5 Potential of Tsuga spp. from Western North America and Asia as Replacements for Eastern Hemlock ( Tsuga canadensis ) Paul A. Weston and Richard W. Harper Abstract. Hemlock woolly adelgid ( Adelges tsugae ) has had a devastating impact on Tsuga canadensis and T. caroliniana in forests and managed landscapes in the eastern United States. Species of Tsuga from the western United States and Asia are reported to be tolerant or resistant to A. tsugae . We established plots containing T. canadensis , T. caroliniana , T. chinensis , T. diversifolia , T. het- erophylla , T. mertensiana , and T. sieboldii in Katonah, New York, U.S., an area with high populations of A. tsugae , and monitored tree growth and infestation by adelgids over a 4-year period. Growth and survival of the hemlock species varied widely, the most vigorous species being T. canadensis and T. chinensis . Susceptibility to the adelgid also varied widely among species; some species (particularly T. canadensis ) became readily infested, whereas others (e.g., T. chinensis ) were apparently entirely resistant. Given the ability of T. chinensis to thrive in the climate of southeastern New York State and its apparent resistance to hemlock woolly adelgid, this species might be an appropriate replacement species, especially in managed landscapes. Key Words. Elongate hemlock scale; hemlock; hemlock woolly adelgid; host resistance; Tsuga. Hemlock woolly adelgid (HWA), Adelges tsugae , is a tiny sap-feeding insect endemic to Asia and introduced to the east- ern United States in the 1950s (Gouger 1971 ). Despite its small size, the insect is capable of killing mature hemlocks within several years after an infestation has begun (McClure 1991). It has caused widespread destruction of native hemlocks ( Tsuga canadensis and T. caroliniana ) in forests and landscapes in the eastern United States from Georgia to Maine, principally in the Appalachian Mountains eastward to the Atlantic coast (Ward et al. 2004 ). Species of hemlock from the western United States (e.g., T. heterophylla and T. mertensiana ) are tolerant of hem- lock woolly adelgid because they are often heavily infested but show no signs of injury (McClure 1992). T. chinensis , native to China, is resistant (resistance mechanism unspecified) (Bentz et al. 2002; Del Tredici and Kitajima 2004; Havill and Montgomery 2008). Species of Tsuga from Japan ( T. diversifolia and T. siedboldii ) show varying degrees of susceptibility to HWA; T. diversifolia is reportedly a poor host for HWA, supporting only limited populations (McClure 1992; Bentz et al. 2002), whereas T. sieboldii has been found to vary in infestation from unin- fested to severely infested (Bentz et al. 2002). Thus, species of Tsuga from the western United States and Asia are worthy of investigation as potential replacements for T. canadensis and T. caroliniana . A major consideration determining the suitability of a spe- cies as a replacement for susceptible trees is the ability of the alien species to tolerate the climatic conditions in the new loca- tion. Bentz et al. (2002) suggest that species of Tsuga from the western United States are poorly adapted to the climate in the East (but provide no data). Del Tredici and Kitajima (2004) report that T. chinensis , in addition to being resistant to HWA, performed very well in the Boston, Massachusetts, area, and was hardy to USDA Hardiness Zone 6. No data have been pub- lished on the ability of hemlocks from Japan ( T. diversifolia and T. sieboldii ) to tolerate the climatic conditions of the northeastern United States. An interesting observation on HWA biology was reported by McClure (1991), who found that nitrogen-based fertilizers had highly stimulatory effects on HWA infesting T. canadensis . HWA nymphal survival was twice as high on fertilized trees versus unfertilized controls, and resulting adults produced twice as many eggs each as compared with adults from the control trees. In addi- tion, fertilized trees had five times more adelgids and produced 25% less new growth than unfertilized trees. It is not known if nitrogen fertilization has any impact on the susceptibility of other species of Tsuga to HWA. The main objectives of this study were threefold: 1) to deter- mine the susceptibility to HWA of Tsuga spp. from the west- ern United States and Asia in the eastern United States (Hudson Valley Region of New York); 2) to document the ability of these species to tolerate the climate in the Hudson Valley; and 3) to determine the impact, if any, of nitrogen fertilization on suscep- tibility of these hemlock species to HWA. In addition, we sought to catalog the species of insects (both pests and natural enemies) on the test trees. MATERIAL AND METHODS Research Site The experimental plots were established within the fenced por- tion of Lasdon Park and Arboretum in Katonah, New York, U.S. The test site is within USDA Hardiness Zone 6b; typical monthly temperature extremes and precipitation are shown in Table 1 . The fencing was essential for protecting the young trees from herbivory by white-tailed deer, which are abundant in the area. Three plots were established within the confines of the arboretum, spatially separated and situated in sites varying in topography ©2009 International Society of Arboriculture
January 2009
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