Arboriculture & Urban Forestry 35(1): January 2009 Table 3. Percent increase in height of test species during the 4 years of the project. z Percent increase in height Species T. canadensis T. caroliniana T. heterophylla T. mertensiana T. chinensis T. diversifolia T. sieboldii mean. y Unfertilized 165.2 ± 10.0 2.8 ± 38.5 120.3 ± 31.2 –18.4 ± 6.5 162.6 ± 16.1 57.4 ± 57.1 136.3 ± 55.3 Fertilized y 162.0 ± 13.3 41.7 ± 9.7 254.8 ± 90.8* 16.7 ± 73.9* 174.3 ± 18.9* 176.2 ± 76.5 25.0 ± 18.4* z Note the species × fertilizer interaction. Values are means ± SE of treatment Asterisk (*) indicates significant difference between fertilizer treatments for individual species at P = 0.05. adelgid and elongate hemlock scale ( Fiorinia externa ), a more recent pest problem of hemlocks apparently associated with the presence of HWA. The visual rating incorporated needle color, needle loss, and overall appearance as indicators of tree vigor. Data Analysis Tree vigor and pest infestation were analyzed with split-plot anal- ysis of variance (ANOVA) with fertilization as the subplot treat- ment. Because survivorship of the test species was measured on a subplot basis rather than on a per-tree basis, this variable could not be analyzed with split-plot ANOVA. Instead, survivorship was analyzed with factorial ANOVA with tree species, fertilizer treatment, and their interaction as the factors after transforming the data with the arcsine transformation. The association between presence of hemlock woolly adelgid and elongate hemlock scale was tested with c 2 contingency tables. The presence of remaining arthropods on the test trees was tested with randomized complete block ANOVA. RESULTS AND DISCUSSION The test species varied widely in their survivorship, vigor, and growth measured after 4 years ( Table 2 ). Not surprisingly, T. canadensis had the highest survivorship and vigor of the North American species included in the study. Of the Asian species, T. chinensis was most similar to T. canadensis ; its survivorship and vigor were very comparable to that of T. canadensis , indi- cating that it was well suited to the climate in the study area. In general, vigor was higher in spring except for the western North American species ( Figure 2 ). although the overall effect of season was not significant ( F = 2.4; P = 0.068; df = 1, 21). An exception was T. heterophylla , which had higher vigor rat- ings in the fall than in the spring ( F = 3.3; P = 0.018; df = 6, 21). This species apparently sustained cold damage because the nee- dles, especially those on branches more than 50 cm (20 in) above the ground and presumably not protected by snow cover, were noticeably browned in spring. By fall, however, the specimens of T. heterophylla looked as healthy as those of the most vigorous species. Specimens of T. mertensiana looked equally poor in fall and spring, consistently lower than all other test species. Judging differences in growth among the species is compli- cated because the starting heights were quite different, so the percent increase in growth over 4 years is more informative. Figure 2. Vigor ratings for the seven test species of Tsuga dur- ing fall and spring averaged over the 4 years of the study. Heights of the bars indicate average vigor ration on a 0 to 5 scale, higher numbers corresponding to higher vigor rat- ings. can = T. canadensis; car = T. caroliniana ; het = T. het- erophylla ; mer = T. mertensiana ; chi = T. chinensis ; div = T. diversifolia ; sie = T. sieboldii . Percent growth varied significantly among species ( F = 11.2; P < 0.0001; df = 6, 65). Although the overall effect of fertilizer on percent growth was not significant ( F = 7.4; P = 0.113; df = 1, 2), the interaction between species and fertilizer was significant ( F = 3.2; P = 0.0085; df = 6, 65) ( Table 3 ). Growth of T. canadensis was virtually identical with and without fertilizer, but the remain- ing species (except T. sieboldii ) showed varying degrees of growth stimulation in the fertilizer treatment. Oddly, the growth of T. sieboldii was actually lower in the fertilized plots. Infestation by hemlock woolly adelgid varied greatly among species ( F = 41.9; P < 0.0001; df = 6, 65). Like with tree growth, the overall effect of fertilizer was not significant ( F = 3.2; P = 0.21; df = 1, 2), but the interaction between tree species and fertilizer treatment was significant ( F = 2.53; P = 0.029; df = 6, 65). Infestation of T. canadensis was uniformly high in both fertilizer treatments; in unfertilized plots, 87.5% ± 8.5% of trees were infested versus 92.9% ± 7.1% in fertilized plots. T. caroliniana showed a high rate of infestation in the fertilized plots (75% ± 25%) but was unin- fested in unfertilized plots. The small sample size for this species, however, rendered this result less than robust (two trees survived in the unfertilized plots and four in the fertilized plots). The only other species to exhibit signs of adelgid infestation was T. hetero- phylla , for which one of four trees (25% ± 25%) was infested in the unfertilized plots and none in the fertilized plots. The remaining species were free of adelgid infestation as of Spring 2008. Elongate hemlock scale also showed variability in infestation rate among test species ( F = 3.2; P = 0.0087; df = 6, 65), but infestation was not influenced by fertilizer ( F = 0.0; P = 0.99; df = 1, 2) or its interaction with species ( F = 1.6; P = 0.160; df = 6, 65). Overall rates of infestation by elongate hemlock scale were rather high, ranging from a high of 83.3% ± 16.7% on T. caro- liniana to a low of 25% ± 16.4% on T. heterophylla . T. canaden- sis was at the higher end of this range (73.3% ± 8.2%), whereas T. chinensis was at the lower end (35.3% ± 12%). On T. canaden- sis , the presence of elongate hemlock scale was strongly associ- ated with the presence of hemlock woolly adelgid; there were no scales on T. canadensis that were free of adelgid, whereas 81% of the trees that had adelgids were also infested with scale ©2009 International Society of Arboriculture 7
January 2009
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