Hemlock Tree (Gymnosperm)

external image easternhemlocktree.jpg

Classification/ Diagnostic characteristics
Scientific Names:
Tsuga canadensis (Eastern hemlock)
Tsuga heterophylla (Western hemlock)
Tsuga caroliniana (Carolina hemlock)

Kingdom: Plantae - Plants
Division: Coniferophyta - Conifers
Class: Pinopsida - Conifers
Family: Pinaceae - Pine family
Genus: Tsuga Carr. - hemlock
Life Cycle: Perennial

There are many types of hemlock trees that grow around the world. Some inhabit northern parts of Europe and Asia (23), while the eastern, western, and carolina hemlocks live in North America. The hemlocks in North America are very similar except for slight differences life span, needle size, and location. The eastern hemlock’s needles are slightly smaller, but are known to live longer. All hemlocks have small cones about 1/2”-1”, prefer acidic soil and do better than most other conifers in shaded areas.

Relationship to Humans
Although we slaughter thousands of hemlocks every winter for our Christmas trees, we make up for it by helping them get rid of their worst parasitic invasive species, Hemlock Woody Adelgid (HWA). HWA feeds by sucking sap from hemlock trees. This destructive pest was accidentally introduced from Asia and is now targeting eastern hemlocks in 11 states. The decrease of hemlock trees, which is the dominant tree in eastern forests, is quickly changing in the carbon cycle in forests. Since hemlocks do not have any immune system to HWA, humans are interfering to help save the trees. Hemlocks are not in demand in the lumber industry because the wood is brittle and difficult to work with when dry.

Researchers from the Great Smoky Mountains National Park have been working on ways to save Eastern Hemlock trees from the invasive species Hemlock Woody Adelgid (mentioned above), which is a small insect from Japan. The infestation of the Eastern Hemlock trees began in the 1980's and has spread to almost all of the trees. They use chemical insecticides in the soil or directly into the tree as well as introducing new predatory species. (13) (LK)

Native Americans used hemlock trees in a multitude of ways. It was key ingredient in Native American medicine. Its leaves were often used in medicinal teas, and the leaves are actually rich in Vitamin C. The sap was used as an ointment for wounds and burns, and was sometimes mixed with fat to create a salve that protected against sunscreen. The bark is a strong astringent due to its high tannin content, so it is used to treat hemorrhages as well as skin sores. Native Americans also ate the starchy inner bark of the hemlock tree either on its own or made into cakes with berries and fish grease. (JM 19)

Habitat and niche
Hemlocks are perennial, meaning they live all year round. They have no problem being shaded by taller trees, mainly because they prefer to be moist and cool.

Hemlock trees are conical conifers with long and slender branches, they grow to about 60-70 ft. tall, and are extremely shade tolerant, meaning they prefer shaded areas. The needles on the hemlocks are flat and flexible, and are pendant cones, which have a distinctive look. It has thick, deeply furrowed bark, and mostly grows in regions with acidic soils; moist, cool valleys, and rock outcrops. They also often grow in pure stands. These areas of hemlock growth ranges from Novia Scotia to Quebec and Ontario, then to Delaware along the mountains to Georgia and Alabama, then west to Michigan and Minnesota. Although clearly hemlock trees have a habitat preference and do well in their niche, they clearly can live in a multitude of environments (9). (AG)

Below are pictures of where the two hemlocks live.

Distribution Map
Distribution Map
Distribution Map
Distribution Map

Predator avoidance
The white tailed deer and the beaver are two animals that remove bark from the trunk of the hemlock. This can ultimately stop the transport of water and sugars, stopping the process of photosynthesis, eventually killing the tree. Two other deadly creatures that attack hemlocks in their youth are fungi and 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 parasite that attacks the needles of the eastern hemlock is the Melampsora farlowi. This creature causes rust and disease, which will eventually attack the hemlock’s cones, resulting in absorption and will continue to injure the tree.
Diseases in young hemlock seedlings, like molds, can momentarily or permanently effects germination.
To help defend itself, there is a layer of bark over the tree trunk, but the hemlock does not think anything is wrong when an invasive species comes, because it has no immune response.

Hemlock woolly adelgid, a soft bodied insect, is an invasive species that feeds on hemlock trees across New England. The surviving hemlock trees have been treated with insecticides or different chemical agents (16). When these insects are newly hatched, they insert their mouth parts into the leaf tissue and ultimately empty the contents of cells in the xylem, which have a mixture of stored carbohydrates, water, and other nutrients. This makes the hemlock tree more susceptible to other agents of stress like drought or other pests. Hemlock trees experiencing a stress factors like drought are more succumb to an attack quicker, so it is important that humans try to maintain good growing conditions. These insects may prevent the hemlocks tree's defenses be absorbing the trees protective chemical (17). Pesticides like Imidacloprid can be absorbed by the hemlock tree's xylem through the roots of the tree and diffused into areas within twigs to target the invasive insects mouth part feeding area. However certain dosages can have a negative affect on the hemlock tree (18). (SM)

Nutrient acquisition
Plants are autotrophs that obtain carbon dioxide through the process of photosynthesis. They obtain nutrients mainly from soil. Soil solution is when mineral nutrients dissolve in the water in the soil as ions. Inside the root, protons swap places with cations in a process called ion exchange. The mineral nutrient cations go into the soil solution where they can be taken up by the hemlock’s roots. If the plant is not getting enough nutrients, the hemlock needles could turn colors or fall off, which is what happens to all the Christmas trees after they are chopped down and brought indoors without soil.

Hemlocks are vascular plants and water and nutrients flow through vessels within them called Xylem (unidirectional vessels) and Phloem (bidirectional vessels). This allows the nutrients to spread throughout the body of the hemlock.


Reproduction and life cycle
Hemlock trees are conifers, meaning they produce cones. Both types of hemlock start producing both male and female cones when they are anywhere from 15-40 years old, and are one of the most frequent cone producers among conifers. Falling off the tree in late summer, conditions are not satisfactory to start growing, so the seed stays in suspended animation, which is when the embryo stops developing. During this time of stopped development, the seed is said to be dormant. In order for the pollen in the male cone to reach the female cone, there must be water. If the seed is germinated, the radicle (embryonic root) will come out of the seed coat, making the seed a seedling. Under ideal conditions, the seedling will grow very slowly. Because of the favorable moisture near the surface of the soil, first-year seedlings may grow only 1 to 1.5 inches in height and the roots extend less than 0.5 inches into the soil. The seedlings grow very slowly and live hundreds of years. According to the Western North Carolina Nature Center, the oldest hemlock tree was an eastern hemlock aged about 988 years old.
Below is a diagram showing the reproduction process of hemlock trees.

Hemlock Life Cycle (12)
Hemlock Life Cycle (12)

Integument is a protective surface structure. In gymnosperms, integument is a layer of tissue around the ovule, which will become the seed coat.
Hemlock trees are surrounded by a gray, tough bark that looks scaly. On mature trees, this bark can be almost an inch thick and can account for 20% of the trees overall volume. This is crucial in animal defense since bugs have to crawl through a lot of wood to get inside. The top of the tree is narrow and pliant to allow for bending without breakage. hemlock trees are conifers meaning they are covered in pine needles and create cones. These cones act as protectants for the seeds. They are made of overlapping scales of bark that protect each scales two seeds. These scales allow water to come in and out and protect the seeds from predators (10). (RG)

Growth and Development
Hemlock trees grow very slowly, there is no rush because they live for hundreds of years.
One of the main hormones in plant growth and development is auxin. In conifers, auxin helps maintain apical dominance, which is when the plant has a single main stem with minimal branching. There is less auxin at the bottom of the tree, so there are more branches, more auxin at the top, so less branches. This explains how the hemlock tree gets its pyramid shape.
Auxin also regulates a response to light in which the plant bends toward a light source called phototropism. This is not as obvious in hemlock trees because of their preference of shade, but more needles will grow on parts of the tree exposed to sun the most.

This image shows how the distribution of auxin in the plant is due to exposure to sunlight and affects the growth of the plant. When the sunlight is equally able to hit the plant on all sides, the plant grows regularly and does not grow towards one side or another because the distribution of auxin is equal in all areas. However, when the sun is moved to one side, as can be seen in (2), the auxin will move towards the side that receives less sunlight. This results in the plant growing towards the sun, which is seen in (3) and (4) to maximize the amount of light it can capture and use during photosynthesis. (21) (CM)

(22) (CM)

Trees grow in so many different locations because the seeds travel before they become seedlings. The wind can carry seeds to another state, or the seed could fall from its tree and get germinated a few inches away.

The hemlock produces both pollen cones and seed cones on the same tree. Pollen cones occur near the tips of leafy twigs, while seed cones occur at the tips of such twigs. Although the tree does not move, the seeds from the tree will disperse with the help of outside forces such as gravity, wind, animals, and water. The seeds will disperse to new locations so that the new seeds will not accumulate in one area. (SM) [6]

Sensing the environment
It is known that plants will not grow unless the environmental conditions are satisfactory. For example, seeds will not germinate (sprout) unless the soil has plenty of water. To sense the environmental conditions, hemlock trees have something called photoreceptors. These pigments associated with proteins regulate developmental processes by responding to the amount of light the plant gets.

Gas exchange
Tiny pores in the hemlock’s leaves, stomata, regulate gas exchange. A pair of specialized cells, guard cells, controls the opening and closing of each stomata. When the stomata are open, carbon dioxide can enter the leaf by diffusion, and water vapor diffuses out of the leaf simultaneously. Closed stomata keep carbon dioxide out, and water in. At night stomata are closed because no photosynthesis needs to be done (no carbon dioxide is needed) and water is conserved.

Waste removal
The hemlock tree uses its xylem, to remove waste, CO2, from its cells. Xylem transports the CO2 and other minerals through vessels.
Like all plants, the hemlock tree produces oxygen as a waste product of photosynthesis.
This oxygen is excreted through the stomata, openings on the underside of the leaves. Excess water is excreted through transpiration, or evaporation of water from the surface of the leaves and stems. Other waste materials like resins, saps, and latexes are excreted forcefully through hydrostatic pressure. (8) (YR)

Environmental physiology
Hemlocks cannot tolerate high temperatures, especially if they are in direct sunlight because the soil gets too dry. Because they are love cool, moist areas, shade is not a problem but a preference. They love the filtered sunlight, and commonly live in mountainous regions.

Hemlocks also prosper along the banks of streams and creeks, where the soil is moist. Generally, hemlocks grow in small groups or just a single tree in a area. In addition, hemlocks grow more on the north face of mountains rather than the south face of mountains. The south face of mountains generally receives more direct sunlight in the northern hemisphere, which creates relatively dryer, hotter air. The north face of mountains generally receives less sunlight, which results in less evaporation of snow. This more moist environment is ideal for gymnosperms like the hemlock. (11) (RS)

Although both the western hemlock (Tsuga heterophylla) , with its thin bark, flammable foliage, shallow roots, and low-hanging limbs, is very vulnerable to forest fire, the actual prevalence of fire-related damage isn't very high. Not only are most hemlocks covered in lichens, which are moist and offer protection against fires, but they are also often situated in cooler, moderately moist environments. (14). The situation of the the eastern hemlock (Tsuga canadensis) is much the same, though the fact that it often grows in the vicinity of fire-resistant hardwood trees is more responsible for the low frequency of fires. (15) (SS)

Internal circulation
Hemlocks have a vascular system, so the tissue called xylem transports water and minerals. Water moves through the xylem by the transpiration-cohesion-tension mechanism. Transcription is the evaporation of water from cells within the leaves, cohesion of water molecules in the xylem sap occurs due to hydrogen bonding, and tension on the xylem sap result from transpiration. Transpiration is when water vapor diffuses from the leaf, through the stomata, to the outside air.

Chemical control
Hemlock trees are controlled by hormones and receptors, some of which are mentioned above. They respond to environmental cues and perform the plant's functions.

Hemlock trees are controlled by hormones, one of which is known as Auxins. Auxins are chemicals produced at the meristems of young leaves and seeds and primarily play a role in the elongation of cells and maintaining apical dominance, although it plays a role in leaf formation, root initiation, and the development of the embryo.(SF)(20)


1. What is the function of the xylem and phloem in the hemlock tree? What role do they play in nutrient acquisition? Internal circulation? Waste removal? (CM)
2. What happens to carbon dioxide and water when stomata are open? What happens when they’re closed? What time of day do stomata typically close? Why are the stomata closed during this time of day? (AC)
3. Describe how the presence of auxin in hemlock trees explain the plant's shade tolerance. (NU)

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