Tsuga canadensis (L.) Carr.
Basis for Listing
Tsuga canadensis (eastern hemlock) is one of Minnesota's rarest and most imperiled trees. It was rare even before the era of unrestrained logging and slash fires changed the composition of Minnesota's forests, and it has fared poorly since. The largest reported stand of T. canadensis in Minnesota was situated on 113 ha (280 ac.) in St. Louis County, near the town of Paupore. At one time, this stand had nearly 5,000 T. canadensis trees of all sizes (Lawson 1942). In 1912, about 8,000 railroad ties were cut from this stand and, in an effort to protect the remaining trees, a State Park was proposed for the site. Tragically, the Moose Lake-Cloquet fire of 1918 intervened, destroying all but a few individual trees that all eventually died. Currently, the number of known sites containing T. canadensis is around ten (Laurentian Mixed Forest Province), with a total of perhaps 50 mature trees. The largest population consists of just 12 mature trees. Other sites may have only a single tree, rarely more than four or five, with little if any reproduction.
Tsuga canadensis was listed as special concern in 1984, a status that carries no legal protection in Minnesota. Legal protection is considered essential to the survival of this species in the state, so its status was elevated to endangered in 2013.
Tsuga canadensis is a large forest tree, reaching a maximum height of about 30 m (100 ft.) and a diameter at breast height (dbh) of about 60 cm (2.0 ft.). It has horizontal or somewhat drooping branches and soft evergreen leaves. The shape of the tree is roughly conical, similar to other conifers, except the leader often has a peculiar leaning or drooping aspect, and it may look curiously bare. The bark is gray or brownish, scaly when young, then developing ridges and furrows with age. The leaves are linear or tapered and persist for about 3 years (evergreen). They are 7-13 mm (0.3-0.5 in.) long, 1.5-2.0 mm (0.06-0.08 in.) wide, and borne on a persistent peg-like base, like spruce leaves. The seed cones are 1-2 cm (0.4-0.8 in.) long, pendent, ellipsoid before opening, and ovoid after opening (Smith 2008).
In Minnesota, T. canadensis is usually scattered in mixed hardwood-conifer forests (fire-dependent forest and mesic hardwood forest), with Betula alleghaniensis (yellow birch), Thuja occidentalis (northern white cedar), Pinus strobus (white pine), or Picea glauca (white spruce), typically on moist and well-drained soils in cool and sheltered valleys and ravines.
Biology / Life History
Tsuga canadensis is a long-lived forest tree characteristic of late-successional plant communities. It is wind-pollinated, and the seeds are dispersed by wind and gravity. While the species is very tolerant of shade, it is vulnerable to drought, windthrow, and especially White-tailed Deer (Odocoileus virginianus) predation (Mladendoff and Stearns 1993). The continental climate of Minnesota, especially the cold and dry autumns, may be a limiting factor (Calcote 1986).
Conservation / Management
While T. canadensis has always been rare in Minnesota, which represents the northwestern edge of its range, it has suffered a population decline during the last century and a half. Inadvertent cutting, poor reproduction and recruitment, and significant deer browse have all contributed to the decline. The first conservation requirement for T. canadensis is to protect all surviving stands from cutting and land clearing operations. Tsuga canadensis is so rare in Minnesota that it has no significant value to the timber industry; any damage to individual trees would likely happen incidental to the cutting of more valuable trees. Therefore, if T. canadensis were found in a timber stand that was planned for harvest, it would be relatively easy to exclude it from the timber sale. The boundaries of any T. canadensis populations need to be determined early in the process; and a reserve of at least two tree lengths should be established around the perimeter. Additionally, a buffer should be established around the reserve. The size of the buffer would vary, depending on the position of the population within the landscape.
Once a population of T. canadensis has been protected from direct physical damage, it is imperative to address the long-term viability of the population. This requires a demographic analysis of the population structure, particularly as it pertains to reproduction and recruitment. The large deer population is a significant problem, since T. canadensis is known to be a favorite winter food of White-tailed Deer. Without extraordinary management efforts to protect T. canadensis seedlings from deer, it is unlikely that another generation of trees will be able to establish themselves. Porcupines (Erethizon dorsatum) and sapsuckers (Sphyrapicus spp.) are also known to cause damage to individual trees, though they probably do not pose a threat at a population level. Another potential threat to eastern hemlock in Minnesota is a species of non-native insect called Hemlock Wooly Adelgid (Adelges tsugae).
Best Time to Search
Tsuga canadensis is a coniferous evergreen tree that can be identified any time of the year. Flowers and fruit (cones) are not necessary for positive identification.
Conservation Efforts in Minnesota
Several of the known T. canadensis sites are in forests occurring within public parks and are given tacit protection from logging. The largest known population occurs in a state Scientific and Natural Area, where it receives the highest level of protection available on state-owned land.
Welby Smith (MNDNR), 2008 and 2018
(Note: all content ©MNDNR)
References and Additional Information
Anderson, R. C. and O. L. Loucks. 1979. White-tail deer (Odocoileus virginianus) influence on structure and composition of Tsuga canadensis forests. Journal of Applied Ecology 16: 855-61.
Calcote, R. R. 1986. Hemlock in Minnesota: 1200 years as a rare species. M.S. Thesis, University of Minnesota, Minneapolis, Minnesota.
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Lawson, E. L. 1942. What happened to the hemlock? Minnesota Conservation Volunteer. January: 64-66.
Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.
Mladendoff, D. J. and F. Stearns. 1993. Eastern hemlock regeneration and deer browsing in the northern Great Lakes region: A re-examination and model simulation. Conservation Biology 7:889-900.
Rogers, R. S. 1978. Forests dominated by hemlock (Tsuga canadensis): Distribution as related to site and postsettlement history. Canadian Journal of Botany 56:843-854.
Rooney, T. P., and D. M. Waller. 1998. Local and regional variation in hemlock seedling establishment in forests of the upper Great Lakes region, USA. Forest Ecology and Management 111:211-224.
Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.
Taylor, R. J. 1993. Tsuga. Pages 362-365 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.
Tyrrell, L. E., and T. R. Crow. 1994. Structural characteristics of old-growth hemlock-hardwood forests in relation to age. Ecology 75:370-386.
Winchell, N. H. 1899. The geology of Minnesota / The geological and natural history survey of Minnesota. Volume 4. Johnson, Smith & Harrison, Minneapolis, Minnesota.