Index of Species Information

SPECIES:  Tsuga canadensis

Introductory

SPECIES: Tsuga canadensis
AUTHORSHIP AND CITATION : Carey, Jennifer H. 1993. Tsuga canadensis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].
ABBREVIATION : TSUCAN SYNONYMS : NO-ENTRY SCS PLANT CODE : TSCA COMMON NAMES : eastern hemlock Canada hemlock hemlock spruce TAXONOMY : The currently accepted scientific name for eastern hemlock is Tsuga canadensis (L.) Carr. [35]. Fernald [15] recognizes a dwarf form, T. canadensis forma parvula Vict. and Rousseau, that grows in mats up to 3 feet (1 m) high in Quebec and New England. LIFE FORM : Tree FEDERAL LEGAL STATUS : No special status OTHER STATUS : NO-ENTRY


DISTRIBUTION AND OCCURRENCE

SPECIES: Tsuga canadensis
GENERAL DISTRIBUTION : In the United States, eastern hemlock occurs throughout New England, the mid-Atlantic states, and the Lake States, and extends south in the Appalachian Mountains to northern Georgia and Alabama and west from the mountains into Indiana, western Ohio, and western Kentucky.  At its northern limit, eastern hemlock ranges along the southern border of Canada from southern Ontario to Cape Breton Island, Nova Scotia [20,35]. ECOSYSTEMS :    FRES10  White - red - jack pine    FRES11  Spruce - fir    FRES15  Oak - hickory    FRES18  Maple - beech - birch    FRES19  Aspen - birch STATES :      AL  CT  DE  GA  IN  KY  ME  MD  MA  MI      MN  NH  NJ  NY  NC  OH  PA  RI  SC  TN      VT  VA  WV  WI  NB  NS  ON  PE  PQ BLM PHYSIOGRAPHIC REGIONS : NO-ENTRY KUCHLER PLANT ASSOCIATIONS :    K093  Great Lakes spruce - fir forest    K094  Conifer bog    K095  Great Lakes pine forest    K096  Northeastern spruce - fir forest    K097  Southeastern spruce - fir forest    K103  Mixed mesophytic forest    K104  Appalachian oak forest    K106  Northern hardwoods    K107  Northern hardwoods - fir forest    K108  Northern hardwoods - spruce forest SAF COVER TYPES :      5  Balsam fir     17  Pin cherry     18  Paper birch     19  Gray birch - red maple     20  White pine - northern red oak - red maple     21  Eastern white pine     22  White pine - hemlock     23  Eastern hemlock     24  Hemlock - yellow birch     25  Sugar maple - beech - yellow birch     26  Sugar maple - basswood     27  Sugar maple     28  Black cherry - maple     30  Red spruce - yellow birch     31  Red spruce - sugar maple - beech     32  Red spruce     33  Red spruce - balsam fir     34  Red spruce - Fraser fir     35  Paper birch - red spruce - balsam fir     37  Northern white-cedar     39  Black ash - American elm - red maple     44  Chestnut oak     52  White oak - black oak - northern red oak     53  White oak     57  Yellow-poplar     58  Yellow-poplar - eastern hemlock     59  Yellow-poplar - white oak - northern red oak     60  Beech - sugar maple     97  Atlantic white-cedar    108  Red maple SRM (RANGELAND) COVER TYPES : NO-ENTRY HABITAT TYPES AND PLANT COMMUNITIES : Eastern hemlock occurs as a dominant or codominant in coniferous and mixed-hardwood forests.  It is often the only conifer present in mixed mesophytic forests of the eastern United States [40]. Publications listing eastern hemlock as codominant or dominant are as follows: The natural forests of Maryland: an explanation of the vegetation map    of Maryland [7] A multivariate analysis of forest communities in the western Great Smoky    Mountains National Park [9] The vegetation of Wisconsin [10] The principal plant associations of the Saint Lawrence Valley [11] Field guide: forest habitat types of northern Wisconsin [32] A classification of the deciduous forest of eastern North America [42] The natural communities of South Carolina [45] Forest associations in the Harvard Forest [55] Vegetation of the Great Smoky Mountains [65]

MANAGEMENT CONSIDERATIONS

SPECIES: Tsuga canadensis
WOOD PRODUCTS VALUE : Eastern hemlock wood is of low value because of brittleness and abundant knots [26].  It is used for pulp, light framing, sheathing, roofing, subflooring, and boxes and crates [20]. IMPORTANCE TO LIVESTOCK AND WILDLIFE : Dense stands of eastern hemlock provide excellent wildlife habitat [20]. Cove forests in the southern Appalachian Mountains provide nesting habitat for many species of birds.  The black-throated blue warbler, black-throated green warbler, and blackburnian warbler are especially abundant in virgin eastern hemlock cove forests [25]. Large eastern hemlocks can be climbed by small black bear cubs.  In northeastern Minnesota, black bear mothers and cubs spent more than 95 percent of the time in April and May within 600 feet (183 m) of either an eastern hemlock or an eastern white pine (Pinus strobus) larger than 20 inches (51 cm) in d.b.h. [50]. Eastern hemlock has high cavity value for wildlife [12].  Large hollow trees are commonly used as dens by black bears [49]. The seeds are eaten by birds and mammals [13], and in the winter the foliage is browsed by white-tailed deer, moose, and snowshoe hares [2,59]. PALATABILITY : In the winter, eastern hemlock browse is moderately preferred by moose and highly preferred by white-tailed deer [2,10].  In the summer, white-tailed deer prefer hardwood sprouts and seedlings to eastern hemlock [44].  The seeds of eastern hemlock are not as preferred by white-footed mice, red-backed voles, and meadow voles as red pine (Pinus resinosa) and white pine seeds [1]. NUTRITIONAL VALUE : NO-ENTRY COVER VALUE : Eastern hemlock provides cover to ruffed grouse, wild turkey, fishers, and other wildlife [4,20].  It provides excellent thermal protection and snowfall interception for moose and white-tailed deer in the winter [2,17]. VALUE FOR REHABILITATION OF DISTURBED SITES : NO-ENTRY OTHER USES AND VALUES : From 1880 to 1930, eastern hemlock was extensively harvested for its bark which is a source of tannin [64].  Eastern hemlock is planted as an ornamental [20]. OTHER MANAGEMENT CONSIDERATIONS : Multiple removal cuttings are the best method for regenerating eastern hemlock.  Suddenly released seedlings often die, and a series of removals releases hemlock more slowly [28].  On moist sites, a two-cut shelterwood system leaving about 50 percent cover may be adequate.  On drier sites, a three-cut system is appropriate, initially leaving 70 to 80 percent crown cover and 50 percent after the second cut [62].  If too few residual trees are left, they may die when exposed, and they are subject to windthrow [28].  Scarification of seedbeds and removal of competing hardwoods may be necessary [20].  Eastern hemlock regeneration must be at least sapling size when released if it is to compete successfully with uncontrolled hardwoods [29].  Single tree selection is also an effective method to harvest and regenerate eastern hemlock [62]. Effective reproduction may be absent in areas with high deer populations [3,10].  Regeneration in the Porcupine Mountains in Michigan has declined over the last several decades because of white-tailed deer browsing in the winter [17].  In the Allegheny National Forest in Pennsylvania, the eastern hemlock-northern hardwoods forest type covered 83.4 percent of the land in 1800 and only 15.8 percent in 1986. Extensive harvesting, fire, and overbrowsing are responsible for the decline [64]. Numerous insects attack eastern hemlock, but only a few are of economic importance cause sporadic or local mortality [62].  Mortality usually occurs following complete defoliation by insects [43,62]. Eastern hemlock seedlings are sensitive to damping-off fungi, root rots, and stem and needle rusts [20]. Eastern hemlock appears to be resistant to ozone [21].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Tsuga canadensis
GENERAL BOTANICAL CHARACTERISTICS : Eastern hemlock is a native, evergreen conifer with heavily foliaged and upsweeping branches.  At maturity, it is commonly 60 to 70 feet (18-21 m) tall and 24 to 48 inches (61-122 cm) in d.b.h.  One of the largest eastern hemlock recorded was 175 feet (53 m) tall and 76 inches (193 cm) in d.b.h.  It reaches ages in excess of 800 years.  Eastern hemlock roots are shallow and widespreading [20,26]. RAUNKIAER LIFE FORM : Phanerophyte REGENERATION PROCESSES : Trees begin producing seed when they are 20 to 30 years old.  Eastern hemlocks older than 450 years still produce large seed crops.  This species bears cones every year, and large crops are frequent, usually every 3 to 4 years.  The small winged seeds are dispersed by gravity and wind; most fall within one-tree-height distance from the source [20,54]. The seeds are partially dormant and germinate best when stratified for about 10 weeks at or slightly above freezing.  Germination occurs at a range of temperatures; seeds from the northern portion of its range germinate at lower temperatures than seeds from the southern portion [20,54].  Seeds do not remain viable if they do not germinate the first spring after seedfall [38]. Seeds germinate best on moist substrates, such as rotten wood, mineral soil, mineral soil mixed with humus, well-decomposed litter, and moss mats [14,62].  The number of seedlings established on rotten logs and stumps increases as the wood decays and the moss cover increases. Seedlings commonly establish on "tip-up mounds" formed by fallen trees [10].  Seedlings grow slowly and cannot tolerate full sunlight until fully established, usually when they are 3 to 5 feet (0.9-1.5 m) tall [20]. Eastern hemlock regeneration appears to be periodic and is influenced by fire, windthrow, drought, and stand conditions.  A young dense stand may exclude regeneration for many years because of severe root competition in the upper soil layers, dense low shade, and dry acidic litter [27,56].  Hemlock regeneration is present in the understory of stands with a parent overstory density of up to 140 square feet per acre (32 sq m/ha) but is most abundant when eastern hemlock comprises 80 to 100 square feet per acre (18-23 sq m/ha) of the overstory [31]. Eastern hemlock does not sprout and layers only rarely [20]. SITE CHARACTERISTICS : At its western and southern limits, eastern hemlock is confined to moist cool valleys, moist flats, northern and eastern slopes, coves, benches, and ravines.  In the northern part of its range, it tolerates drier and warmer sites.  Eastern hemlock also occurs at swamp borders provided peat and muck soils are shallow [14,20,40,65]. Favorable eastern hemlock sites are moist to very moist with good drainage.  Eastern hemlock grows in a wide variety of acidic soils; textures include sandy loams, loamy sands, and silty loams with gravel of glacial origin in the upper profile [14,20]. While generally considered a moisture-demanding species, eastern hemlock grows on dry sites protected from fire, such as rocky ledges [22].  Two types of eastern hemlock have been described:  one grows in mesophytic habitats and one on subxeric slopes [30].  The types cannot be termed ecotypes, however, because of incomplete habitat differentiation. Eastern hemlock growing on "subxeric" slopes may actually be receiving moisture from seeps [51]. In the northeastern United States, eastern hemlock grows at elevations ranging from sea level to 2,400 feet (730 m).  In the southern Appalachian Mountains it grows from 2,000 to 5,000 feet (610-1,520 m). In the Allegheny Plateau region of New York, Pennsylvania, and Ohio, it grows from 1,000 to 3,000 feet (300-910 m) [13,20,34]. Understory associates are scarce because of acidic infertile humus, low light, and cool conditions [14,34].  Shrub and small tree associates that occur in canopy gaps include sweet birch (Betula lenta), striped maple (Acer pensylvanicum), mountain maple (A. spicatum), hobblebush (Viburnum alnifolium), mapleleaf viburnum (V.  acerifolium), mountain winterberry (Ilex montana), rhododendron (Rhododendron spp.), mountain-laurel (Kalmia latifolia), and witch hazel (Hamamelis virginiana).  Herbs can include Canada mayflower (Maianthemum canadense), star flower (Trientalis borealis), common woodsorrel (Oxalis montana), and goldthread (Coptis groenlandica).  Other associated species include clubmosses (Lycopodium spp.), bracken (Pteridium aquilinum), woodfern (Dryopteris spp.), and sedges (Carex spp.).  Common mosses include Dicranium spp. and Polytrichum spp. [14,20,32,45,65]. SUCCESSIONAL STATUS : Eastern hemlock is very shade tolerant [5].  Seedlings survive in as little as 5 percent of full light [14].  Individuals are able to survive several hundred years of suppression, and many show numerous growth releases and suppressions [6].  Saplings less than 2 inches (5 cm) in d.b.h. may be more than 100 years old [10]. Seedlings are able to establish under the canopy of mature individuals. Eastern hemlock establishes under dense sugar maple canopies and can replace that species [39].  Eastern hemlock uniquely modifies semipermanent soil properties, such as acidity, which favors its reproduction.  Opportunities to establish in a mature forest increase over time as nurse logs and tip-up mounds accumulate [51]. The general designation of eastern hemlock as a climax species has been questioned [22,41].  In some old-growth eastern hemlock stands, the smaller size classes of hemlock are being replaced by American beech (Fagus grandifolia) and sugar maple [41].  Because of this lack of regeneration, Hemond and others [22] suggest that eastern hemlock requires disturbance to perpetuate itself. In contrast, other authors suggest that disturbance is responsible for the lack of regeneration in mature hemlock forests [3,6,51]. White-tailed deer populations have increased since presettlement times because logging of virgin forests opened up habitat, predators declined, and the deer were protected.  Deer often consume all eastern hemlock seedlings and saplings in the winter.  Where deer populations are low, eastern hemlock appears to be able to reproduce in its own shade and become a component of a self-perpetuating homogenous climax forest [3]. Eastern hemlock requires partial shade for establishment and is a late colonizer of disturbed sites [24].  In the Pisgah Forest in southwestern New Hampshire, 80 percent of old-growth eastern hemlock established within 37 years of disturbance.  Hardwoods grew rapidly into the canopy while eastern hemlock grew slowly as shade-tolerant saplings.  Eastern hemlock extended into the canopy following subsequent disturbance [23]. The understory population of eastern hemlock readily takes advantage of canopy gaps.  Eastern hemlock increased in importance as American chestnut (Castanea dentata) declined from chestnut blight [8].  It is currently replacing American beech where that species is succumbing to beech bark disease [53].  Eastern hemlock is not successful in regenerating in canopy gaps in areas such as the New York Botanical Forest, where the occasional light arson fire, trampling, and other urban stresses kill seedlings.  In addition, the removal of fallen logs in the forest decreases the amount of adequate substrate for germination [52]. The slow invasion of oak-dominated sites by eastern hemlock appears to be related to heavy leaf litter and the absence of favorable seedbed conditions [22]. SEASONAL DEVELOPMENT : Eastern hemlock male strobili open and pollen is dispersed in late April to early June, depending on locality.  This is usually 2 weeks after the leaf buds open.  Fertilization is complete in about 6 weeks, and cones reach full size in late August or early September.  The cones open in mid-October, but seed dispersal may extend into the winter [20].  Cones close in wet weather and open again in subsequent dry weather, prolonging seed dispersal.  Germination occurs in the spring [10].

FIRE ECOLOGY

SPECIES: Tsuga canadensis
FIRE ECOLOGY OR ADAPTATIONS : Eastern hemlock is very susceptible to fire because of its thin bark, shallow roots, low-branching habit, and heavy litter deposits [20,51]. It is possibly the most fire-sensitive mesophytic tree species in its range [51]. Eastern hemlock usually escapes fire because it occurs in moist habitats and is often associated with hardwoods which do not readily burn.  If a fire starts in a cutover area, a windfall area, or an area with dead standing timber, it may carry into a northern hardwoods forest if there is strong wind [18].  In Michigan, the average return time for severe crown fires in the hemlock-white pine-northern hardwoods type is estimated to be about 1,400 years [63].  In northeastern Maine, the average return interval for fire in spruce-fir forests in which eastern hemlock is a minor component is about 800 years [37]. Vogl [61] considers eastern hemlock a fire-initiated species rather than a fire-independent species because it benefits from fire-prepared seedbeds.  However, suggestions that fire promotes regeneration of eastern hemlock are not well documented.  Given the difficulties in accurate age estimates because of heart rot, Rogers [51] suggests that even-aged eastern hemlock forests that regenerated after fire may actually be uneven-aged. POSTFIRE REGENERATION STRATEGY :    Tree without adventitious-bud root crown    Secondary colonizer - off-site seed

FIRE EFFECTS

SPECIES: Tsuga canadensis
IMMEDIATE FIRE EFFECT ON PLANT : Low-severity fire readily kills seedlings and saplings of eastern hemlock, and may also kill larger trees.  A low-severity ground fire in a northern hardwoods community in south-central New York killed 93 percent of the eastern hemlock saplings.  Sixty percent of the mature eastern hemlock died or were badly injured as a result of the fire [58]. The presence of fire scars indicates that larger trees have thick enough bark to survive low-severity surface fires [18,36]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT : NO-ENTRY PLANT RESPONSE TO FIRE : Eastern hemlock appears to invade burned sites over time.  In the Pisgah Forest in southwestern New Hampshire, 80 percent of old-growth hemlock germinated within the first 37 years after a major fire in 1665 [23]. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : The Research Project Summary Early postfire effects of a prescribed fire in the southern Appalachians of North Carolina provides information on prescribed fire and postfire response of plant community species, including eastern hemlock, that was not available when this species review was originally written. FIRE MANAGEMENT CONSIDERATIONS : NO-ENTRY

REFERENCES

SPECIES: Tsuga canadensis
REFERENCES :  1.  Abbott, Herschel G. 1962. Tree seed preferences of mice and voles in the        Northeast. Journal of Forestry. 60: 97-99.  [20402]  2.  Allen, Arthur W.; Jordan, Peter A.; Terrell, James W. 1987. Habitat        suitability index models: moose, Lake Superior region. Biol. Rep. 82        (10.155). Washington, DC: U.S. Department of the Interior, Fish and        Wildlife Service. 47 p.  [11710]  3.  Anderson, Roger C.; Loucks, Orie L. 1979. White-tail deer (Odocoileus        virginianus) influence on structure and composition of Tsuga canadensis        forests. Journal of Applied Ecology. 16: 855-861.  [20403]  4.  Arthur, Stephen M.; Krohn, William B.; Gilbert, James R. 1989. Habitat        use and diet of fishers. Journal of Wildlife Management. 53(3): 680-688.        [8671]  5.  Baker, Frederick S. 1949. A revised tolerance table. 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