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Eastern hemlock

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Eastern hemlock
Eastern hemlock Canadian hemlock.jpg
Scientific Classification
Subtaxa / Binomial Name

Tsuga canadensis

[1]

Reproduction Eastern Hemlock.jpg
These are the reproductive cones of the Eastern hemlock.

The Eastern hemlock, also known as the Canadian hemlock or the hemlock spruce is a conifer tree native to eastern North America.[2] This majestic tree is in fact the state tree of Pennsylvania, which is in the region were this hemlock is most prominent. [3] The slow growth rate and tolerance of shade are two of the major characteristics of the Tsuga canadensis that sets them apart from other species in the Division Coniferophyta. These powerful and eloquent trees have endured troublesome times at the hands of man, plants, and animals. [2] Although these hemlocks can live to be over 900 years old, they frequently don’t achieve this milestone due to destruction or disease. [4]

Contents

Anatomy

The dark brown coarse bark of the Eastern Hemlock.

The Tsuga canadensis’s characteristics are similar to the species also in the Pinefamily (Pinaceae). The Eastern hemlock stands erect on average at 30 m tall, but is capable of further growth. The main stem of a mature hemlock tree is a rough brown bark [4] formed over time by the accumulation of cork that is formed by the cork cambium. The texture of this epidermal layer is crusty and cracked, but effectively protects and waterproofs the plant. [5] The branches off the main stem are yellow brown in color. As these branches grow, they begin to droop at the ends, [4] giving the hemlock an excurrent branch pattern. [6] On these branches grow flat, evergreen, needle-like leaves that are about 0.25 - 0.75 in. long and 0.1 in. wide. These fine textured leaves have microscopic serrated edges along the sides of their leaf blades. The Eastern hemlock possesses dark green leaves [7] that are bipinnately compound, meaning the venation of the leaves consist of a pinnate leaf with secondary veins on the leaflets. [8] This hemlock is not a deciduous tree, so its leaf foliage is dense and green all year around. [1]

The Eastern Hemlock uses its needle-like leaves to conduct photosynthesis, which supports the life processes of the plant. In the leaves there is chlorophyll, which receives the sun’s light and energy. Then, by the use of electrons and the electron transport chain, energy or ATP (adenosine triphosphate) is formed. That electron carrier molecule then stores the ATP for further use. [9] Until this energy is needed it is transported by the phloem in the vascular tissue of the hemlock’s stem down into the roots for storage. [10] To attain water and nutrients from the soil, the Eastern hemlock possesses roots and root hairs. This root system is slow growing, like the rest of the plant, but after a period of seedling growth the roots begin to develop at a more rapid pace. [2] The roots take in nutrients and water and disperse them throughout the plant by mean of the xylem in the vascular tissue. [11] If it were not for these complex processes, the Eastern hemlock would die.

Reproduction

An Eastern hemlock becomes capable of producing cones at the age of 15. Beginning around mid-spring, [2] the hemlock begins to create a massive stock of pollen cones and seed cones, [5] which are the smallest cones of the genus, being only 0.5 to 0.75 in. long (13 - 19 mm). The pollen cone, made out of bud scales, contains pollen [2] that holds the male reproductive gametes. [5] The seed cones, formed out of the previous year’s branchlets, contain ovules, which will unite with the pollen to germinate the seed within the cone. Two weeks after the opening of the seed cones the pollen is released. With the assistance of the wind, the pollen travels to another branch on the same, or another hemlock, where it unites with seed cones and germinates the ovules. Fertilization in the Eastern hemlock is completed after approximately 6 weeks. In that period of time, as the embryonic plant within the seed develops it is very susceptible to drying, which can lead to seed failure. As the hemlock seed matures, so does its protective cone. In August or early September, the cones of this hemlock reach their maximum size. Traveling into mid-October, the mature seed cones open, releasing the seeds. Despite the immense number of cones and seeds produced by the tree, the genetic variability in the Tsuga canadensis is extremely low.

For the Eastern hemlock’s seeds to successfully grow they must reside in a limited region and only under certain conditions. The ideal temperature for a hemlock seedling is around 59°F. [2] When it comes to the growth of the young seedling, the hemlock develops best in a shaded region with 70-80% vegetative crown cover. [4] The ideal soil for growth should be warm and most with a decomposing layer of vegetation. As in the development of the seed, a young hemlock seedling is in danger of dying by drying. In an experiment, 60% of the seedlings died as a result of 2 hours of drying. It is vital that a hemlock develop in moist, shaded soil.

When it comes to the hemlock’s rate of growth, the numbers are shocking. The Tsuga canadensis is an extremely slowly developing tree, only growing about 1 - 1.5 in. vertically and the root system traveling out only about 0.5 in. annually. However, after about the second year, the roots develop deep enough into the soil that they are no longer hindered by dry soil on the surface. After this, the hemlock will increase in its growth rate. When reaching about 3 - 5 ft. in height, the hemlock becomes independent of the overhead vegetation and no longer needs its protection from the sun’s rays. [2] Even though the Eastern hemlock rate of growth is time-consuming, it is a perennial tree, meaning it continues to steadily grow year after year. [1]


Due to its slow growth rate, it takes about 250 - 300 years for a Tsuga canadensis to fully mature. Even then, the hemlock only stands 105 ft. tall. The life span for an Eastern hemlock can be over 900 years. [4] As these trees grow they will increase in both their height and diameter. The record measurements for the Eastern hemlock include: age- 988, diameter- 84 in (7 ft), height- 160 ft (49 m).

The methods by which the Eastern hemlock can propagate include cutting and grafting. Although these methods are effective, they are only used in ornamental production. However, there have been reports in Wisconsin of natural root grafts in hemlock trees. Even so, the majority of the hemlocks planted in woodland environments or decorative gardens are grown from the seed. [2]

Ecology

The habitat range of the Tsuga canadensis in North America.

Habitat

The Eastern hemlock resides in the eastern regions of the United States and in a southeast portion of Canada. These trees stretch from Minnesota and Alabama, to Ontario and Nova Scotia where it grows on rocky ridges, hillsides, and gorges at altitudes of 600-1800 m. [4] In environments with cooler and moister conditions, these hemlocks thrive best. The Eastern hemlock grows successfully in course soil that ideally is well drained, because that allows for a deeper root system. [2] When it comes to the weather, these hemlocks prefer non-windy regions without periods of extended heat. [4] However, Eastern hemlocks are very stalwart in the cold, being able to endure temperatures as low as -33° F. When it comes to precipitation, the Tsuga canadensis can survive with a minimum of 32 in. and a maximum 55 in. of rainfall annually. [1] The Eastern hemlock is considered one of the most shade tolerant trees on the planet. This hemlock can reside in heavily shaded regions where it receives only 5% of the sun’s full light. [2]


Relationships

The White-Tailed Deer uses the Eastern Hemlock for a source of food and shelter.

The Tsuga canadensis possesses many relationships with the plants and animals residing in their region, some of these relationships are beneficial to the hemlock, and others are not. The Eastern hemlock shares a beneficial relationship with white-tailed deer, ruffed geese, turkeys, mice, voles, squirrels, and other such rodents. These creatures come to the hemlock for both food and shelter. Some of these animals eat the bark, thus injuring the tree. However, others consume as well as redistributed the plant’s seeds throughout the area. This relationship provides the tree with an expanded region of growth and these animals with food and refuge.

However, the Tsuga canadensis suffers relationships with many dangerous plants and animals that injure the hemlock. The white tailed deer and the beaver both are two animals that remove bark from the trunk of the hemlock, [2] which can result in the girdling of the tree. If the hemlock is girdled it will be unable to transport water and sugars, thus stopping the process of photosynthesis. This will eventually kill the tree. [12]

There are a variety of dangerous organisms that can cause diseases in young hemlock seedlings. Such organism include the molds in the genus Botrytis, which momentarily or permanently hinders germination. Also, two other deadly creatures that attack hemlocks in their infancy are the damping-off fungi and the root rots. These terrible fungi reside in poorly drained soil as well as well-drained regions, so they are common organisms on the Eastern hemlock. The root rots also infest the tree as a seedling and are very commonly seen on hemlocks.

A pest that attacks the needles and twigs of the Eastern hemlock is the Melampsora farlowi. This creature causes rust and disease, which will eventually attack the hemlock’s cones. Such an attack will result in cone absorption and continue to injure the tree.

The Tyromyces borealis is another infamous insect that infests into the heartwood of the Tsuga canadensis. This infestation consequently creates white speckles in the tree’s wood. More importantly, this attack can also cause a puncture to form in the plant’s pith.

The Wooly Adelgid is the worst enemy of the Tsuga canadensis, infesting and killing countless hemlocks on the east coast.

Despite the 24 insects that infest and destroy the Eastern hemlock, none is more deadly or more prominent than the wooly adelgid. [2] The Adelges tsugae (wooly adelgid) has created a disastrous range reduction in the Eastern hemlock population. This pest was first introduced to the America’s in 1924 from Asia. Around the mid-1980s, this insect was considered a minor threat in New Jersey, Pennsylvania, Virginia, and New York, but nothing more. However, in recent years the population of the wooly adelgid has increased and the Eastern hemlock’s population and range has dramatically decrease. In 2007, a list of the 15 largest Eastern hemlock trees in the world was taken, but only a few years later all were dead as a result of the wooly adelgid. Despite efforts to protect famous hemlock trees from the devastation of the Adelges tsugae by pesticides, these pests have eluded these precautions and infested countless trees. [3]

Commercial Uses

Today, the commercial uses of the Eastern hemlock are predominantly in the pulp and paper industries. [2] This tree’s wood mainly manufactures newsprint, wrapping paper, and pulp. The Eastern hemlock is not considered a quality timber tree due to its brittle wood. In comparison to the other hemlocks of North America, the Eastern Hemlock’s lumber is second-rate. [3]

Between 1890 and 1910, the Eastern hemlock reached its peak in production. During this time period, the wood of the Eastern hemlock was primarily used in sheathing, roofing, light framing, boxes, crates, and subflooring. [2] Another main reason for the Eastern hemlock’s production surge in the late nineteenth and early twentieth century was due to demand for tannin. The tannins produced in the bark of the Eastern hemlock were a vital source for the production of leather. This demand drove the destruction of the northeastern forests of North America. Loggers fell the trees and stripped them of their bark, but failed to restock the woodlands of what they had taken. As a result, the commercial use of Eastern hemlock suffered. From 1935 –1955, the timber volume in Michigan decreased by 69% and the volume of the growing stock was depleted by 71%. Due to reckless harvesting in the past as well as the forests not being replenished, Eastern hemlocks have had to endure many tribulations. [2]

Besides the lumber production of the Eastern hemlock, this tree is also commercially used in gardens and landscapes. The Eastern hemlock has been selectively bred into dwarf forms, which allows people to plant these miniature trees within in their yards. It also has been observed that the Eastern hemlock can be pruned into an effective screen or hedge with a dense leafy cover. Besides being bred to a smaller form, this hemlock as also been bred for a diversity of different colors. [2] The various commercial uses of the Eastern hemlock make this tree a valuable member of the vegetation in North America.

Gallery

References

  1. 1.0 1.1 1.2 1.3 Tsuga canadensis (L.) Carrière eastern hemlock Author Unkown, United States Department of Agriculture, 5/03/2011.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 Tsuga canadensis (L.) Carr. Eastern Hemlock Author Unknown, USDA Forest Service Northeastern Area State & Private Forestry, 5/8/2011.
  3. 3.0 3.1 3.2 Tsuga Canadensis Christopher J. Earle, The Gymnosperm Database, 2/25/11.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Tsuga canadensis (L.) Carrière Eastern hemlock Matthew D. Hurteau, et al., Encyclopedia of Life, 5/8/2011.
  5. 5.0 5.1 5.2 Porch, and Batdorf. Biology with Laboratory Exercises. South Carolina: Bob Jones University Press, 1991. (p.358).
  6. Porch, p.355
  7. Tsuga Canadensis Mark H. Brand, UConn Plant Database, 5/3/11.
  8. Porch, p.349
  9. Porch, p.100
  10. Porch, p.345-346
  11. Porch, p.345
  12. Porch, p.358
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