Introduction and Cultivation of Chinese Hemlock (Tsuga chinensis) and its Resistance to Hemlock Woolly Adelgid (Adelges tsugae)

CHINESE HEMLOCK IN CULTIVATION

While T. chinensis has a widespread distribution in mountainous regions of eastern, central, and southwestern China between 1,000 and 3,500 m (3,282 to 11,487 ft) (Wu and Raven 1999), the species is poorly represented in North American botanical gardens. E.H. Wilson is credited with the introduction of Chinese hemlock into cultivation with seed he collected in Xing Shan, Hubei Province in October 1901 (collection # 952), while working for the Veitch Nursery Company of Chelsea, England (Sargent 1913–1917, 2:38; Rehder 1940; Clausen and Hu 1980; Howard 1980). One of the very few specimens of T. chinensis of known provenance is growing at the Arnold Arboretum of Harvard University, under accession #17569. Wilson collected this plant as a seedling in Fang Xian, Hubei Province, China, in September 1910 and sent it to the Arnold Arboretum, where it arrived in February, 1911 (Sargent 1913–1917, 3:446; Howard 1980). As of winter 2004, Wilson’s tree was 15 m (49.2 ft) tall with a diameter at breast height (dbh) of 37 cm (14.4 in.) and a branch spread of 12 m (39.4 ft), and showed no sign of adelgid infestation.

Over the years, Arnold Arboretum staff had propagated both seedlings and cuttings from the Wilson tree and distributed them to various botanical gardens and nurseries in the United States. Between 1915 and 1945, arboretum records show at least 16 separate distributions of T. chinensis #17569, involving a total of 28 plants. Many of the older Chinese hemlocks now growing in botanical collections in the United States are the direct descendants of Wilson’s Hubei seedling.

Apart from Wilson’s collections in 1901 and 1911, wild-collected germplasm of T. chinensis does not appear to have entered North America until 1979 and 1980, when visiting delegations of Chinese botanists presented their hosts with seed of Tsuga chinensis from the Chinese Academy of Forestry. Since then, numerous American and European expeditions to China have collected and distributed seeds of T. chinensis from wild populations growing at altitudes between 1,000 and 2,650 m (3,282 and 8,697 ft) in Sichuan, Hubei, Shaanxi, Fujian, Zhejiang, and Yunnan provinces (Table 2).

SITE CONDITIONS, PLANT MATERIALS, AND METHODOLOGY

The Hemlock Hill portion of the Arnold Arboretum consists of approximately 10 ha (22 ac). The north-facing slope is covered with a nearly pure stand of T. canadensis. Bedrock is close to the surface on much of Hemlock Hill, and the soils that overlay it are well-drained and nutrient poor. Four soil samples collected from the north-facing slope of Hemlock Hill were analyzed in May 2003 and showed pH levels between 4.2 and 4.5, organic matter content between 11.3% and 19.5%, and high levels of ammonium and low levels of nitrate (Del Tredici et al. 2003).

HWA was first discovered on Hemlock Hill in April of 1997, and the pest has spread rapidly throughout the hemlock population, which consisted of some 1,905 individuals with dbh’s greater than 5 cm (2 in.). The entire population was labeled, mapped, and qualitatively assessed for condition and HWA damage during the winter of 1997–1998. Eastern hemlocks along the base of Hemlock Hill have been sprayed annually in the fall with dormant oil since 1997, which has effectively protected them from HWA. Eastern hemlocks growing on the interior portions of Hemlock Hill have been left untreated and are now in a serious state of decline. When the entire population was recensused during the winter of 2002–2003, 263 trees were found to have been removed (all were either dead or near death), and those remaining had lost foliage and were in poor health (Del Tredici et al. 2003). Table 3 shows the dramatic decline in the condition of the hemlocks that occurred between 1998 and 2002 as a result of the HWA infestation.

The senior author undertook a study of the resistance of T. chinensis to HWA using seedlings that were raised from a lot of seed purchased from Sheffield Seed Company in Locke, New York, in February 1994 (lot # 9101, received from the China National Seed Company in March 1992). The senior author determined that the seed, while of unknown provenance, was collected from trees growing in the wild. At the Arnold Arboretum, the Sheffield seed was accessioned under #100-94 and given a 3-month coldstratification period, after which it was sown in a warm greenhouse. In April 1999, 74 five-year-old seedlings were planted out on the arboretum grounds. At the time, they were growing in 12 L (3 gal) containers and were between 60 and 110 cm (23.4 and 42.9 in.) tall. To specifically test the resistance of T. chinensis to HWA, some 42 of the seedlings were planted on the interior portions of the Hemlock Hill section of the arboretum in groups of three to six individuals in scattered light-gaps. The canopy for all the seedlings consisted of eastern hemlocks that were badly infected with HWA.

On June 25 and 26, 2003, the authors evaluated the growth and resistance of 38 of the 42 Chinese hemlocks that had been planted on Hemlock Hill in 1999. Four individuals were left out of the study because they were not able to be located at the time of the evaluation. As a “control” group, the authors tagged some 33 seedlings of T. canadensis that were growing spontaneously on the north-facing slope of Hemlock Hill, adjacent to the planted Chinese hemlocks. The fact that only 33 seedlings could be located on the north-facing slope is testament to the paucity of natural hemlock regeneration and explains why some of the plants used in this study were larger than optimal [up to 380 cm (148.2 in.) tall]. The major differences between the two trial groups were as follows: The T. chinensis seedlings had 4 years of exposure to HWA, while the T. canadensis seedlings had 6; and the majority of T. chinensis seedlings (68%) were growing in light gaps that received some direct sunlight during the day, while the majority of T. canadensis seedlings (94%) were growing in understory positions that did not receive any direct sunlight.

The authors measured the heights of the 38 T. chinensis and 33 T. canadensis seedlings and recorded their canopy position, which was rated as “sun” (growing in a large canopy gap with moderate amounts of direct sun), “gap” (growing in a small canopy gap with minimal amounts of direct sun), or “shade” (growing in complete shade). To assess the level of infestation with HWA, two branches on opposite sides of each tree were selected at random, and the six top-most shoots on each branch, consisting of growth from both 2002 and 2003, were selected for evaluation. For each of the 12 shoots per tree, the authors (1) measured its length to the nearest millimeter, (2) counted the number of HWA egg sacs, (3) noted the presence or absence of new (2003) growth, and (4) noted the presence or absence of spider mite damage. The cottony HWA egg sacs were counted with the naked eye on the undersides of the 2002 shoots, and spider mite damage was assessed by looking for the characteristic leaf stippling on the uppersides of the 2002 shoots (Montgomery 2003).

The senior author recensused the 38 Chinese hemlock seedlings on March 9, 2004, in order to obtain final height measurements for the 2003 growing season and to assess their survival following an extremely cold winter when the temperature at the arboretum reached a low of −22.5°C (−8.5°F) on January 16.

RESULTS AND DISCUSSION

Results of this study are summarized in Table 4. The most dramatic finding was the total absence of HWA egg sacs on any of the 38 T. chinensis seedlings in comparison to a mean of 45.9 egg sacs per 12-shoot sample for T. canadensis. Another indicator of T. chinensis resistance to HWA was the difference in new growth between the two species. One hundred percent of the terminal buds on sampled shoots of T. chinensis produced new growth compared with only 45% for T. canadensis. Lastly, the mean shoot length for T. chinensis was 10.3 cm (4 in.), more than twice the 4.9 cm (1.9 in.) length recorded for T. canadensis. These results clearly indicate that Chinese hemlock possesses a high degree of resistance to HWA under conditions that were optimal for infestation of eastern hemlock. This conclusion supports anecdotal reports of Chinese hemlock’s resistance to HWA (Montgomery et al. 2000; Bentz et al. 2002).

The remeasurement of the 38 Chinese hemlocks on March 9, 2004, showed them to be 17% taller than they were in the summer, when their shoot tips were drooping, with an average height of 197.8 ± 50.7 cm (77.1 ± 19.8 in.). Given that the seedlings averaged between 80 and 90 cm (31.2 and 35.1 in.) tall when they were planted out in April 1999, the average increase in height for T. chinensis over four growing seasons has been over a meter (Figure 1). This was a remarkable figure given the shady conditions on Hemlock Hill, the thin soil, and the minimal after-care the plants received. Finally, it should be noted that the plants showed very little winter damage at the time of the March resurvey, despite the low temperatures recorded in January 2004.

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Figure 1.

One of the Tsuga chinensis seedlings (AA #100-94) growing in a canopy gap on Hemlock Hill at the Arnold Arboretum. Photographed in summer 2003.

The results of this study indicate that T. chinensis is fully hardy in USDA zone 6 and is a suitable replacement for T. canadensis in landscape situations because of its relatively rapid growth rate, tolerance of shade, and resistance to HWA.