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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: 
https://www.fs.usda.gov/database/feis/plants/tree/tsucan/all.html [].


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.

FIRE REGIMES : 
Find fire regime information for the plant communities in which this 
species may occur by entering the species name in the FEIS home page under
"Find Fire Regimes".

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
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