BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Tsuga canadensis | Eastern Hemlock 



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
Applalachian 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 : 
Obligate Climax Species

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 desgination 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].

Related categories for Species: Tsuga canadensis | Eastern Hemlock