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MICHIGAN FORESTS FOREVER
TEACHERS GUIDE
| TREE PHYSIOLOGY |  |
Quite a bit of time is spent on tree physiology, which is key to understanding many of our forest management practices, especially the concepts of shade tolerance and vegetation succession. Additionally, the topics of forest health, hydrologic cycle, and nutrient cycles are discussed.
This is a fairly long section for several reasons. One, it provides the basis for much of what is addressed later in terms of forest management. Two, it has many connections to the Michigan curricula, particularly in science. Three, there are a lot of neat activities and observations associated with the topic.
| Concept List: The Necessities of Life |
Little Known or Interesting Factoids About Tree Physiology
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Some Important Terms
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The following is brief discussion of six key requirements for trees. More detail of some topics is found later in this guide.
1. Sugars supplied by photosynthesis. Air and water are chemically recombined to form glucose, which stores energy captured from the sun. Oxygen is a byproduct.
2. Water is required for most metabolic activities and serves as a vehicle to carry materials through a tree. A large tree may move as much as 50-100 gallons of water on a hot summer day.
3. Nutrients. It’s not how much of a particular nutrient exists in the environment, it’s a matter of how available the nutrient is to the tree. For example, the atmosphere is largely composed on nitrogen, but trees can only use nitrogen in forms that have been altered by soil bacteria and other organisms. The major chemical elements used by plants are: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, and magnesium. You might be able to remember this by a jingle formed using the abbreviations for these elements: C H O P K N S Ca Fe Mg . . . "see hopkins café, might good."
4. Hormones and enzymes. These chemicals are critical in the controlling the timing and activity of physiological processes. They are usually produced in the roots or leaves. We don’t often think of plants having "hormone" deficiencies, but hormones are critical to the survival of any organism, including trees.
5. Mycorrhizae. Pronounced "my-core-HI-zee", this is a group of beneficial fungi associated with most tree roots. It represents an ecologically symbiotic relationship where the fungi receive food from the tree and the trees receive greatly enhanced nutrient and water absorption. Mycorrhizae will also protect tree roots from other invading fungi. There tends to be very specific species relationships between fungus and tree.
6. Environmental factors. A tree needs an appropriate mix of precipitation, temperature, sunlight, and soils in order to thrive. These factors need to occur at the right time. Each tree species has a different set of environmental requirements. Changing climate will lead to changing environmental factors, which can lead to changes in forest ecosystems.
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Activity Suggestions PLT Every Tree For Itself PLT Soil Stories Matching Trees To Soils PLT Dynamic Duos |
| Definition of a tree: A
woody perennial plant, typically large and with a well-defined stem or stems carrying a
more or less definite crown - note, sometimes defined as
attaining a minimum diameter of 5 inches and minimum height of 15 feet at maturity with no
branches within 3 feet of the ground. -Society of American Foresters, 1998 |
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The parts and structure of a tree have obvious components and some not so obvious components.
What makes a tree a tree?
First, a tree has all the characteristics of green plants. Beyond that, a tree is a tall plant with woody tissue. It has the capability to "push" its crown (the primary location for photosynthesis) above other vegetation competing for light. Also, most people don't readily connect trees with having flowers but they do, although our conifers (pines, spruces, firs, etc.) don't have true flowers with petals. The reproductive structures of each species are unique and are used more than any other structure to categorize trees. This categorization is called taxonomy. The tree identification section talks more about taxonomy.
A tree has a dilemma in terms of gathering its resources. It has a distinct light-gathering advantage of having its leaves high above other plants, but there is the problem of getting water and soil nutrients to the upper tissues. The microenvironment in the upper canopy is also rather hostile to sensitive tissues. At the other end of the tree, the roots are dependent upon materials produced way up in the crown. This problem, of course, is solved by the structure of the tree trunk, or bole, a most distinctive feature of trees.
Most of a tree trunk is dead woody tissue and serves only to support the weight of the crown. The very outside layers of the tree consists of bark. Underneath the bark is a cork cambium layer that generates new bark. Under the cork cambium lies a thin band of phloem, which is living tissue that transports materials from the crown to the roots. Under the phloem is another vascular cambium zone that produces both new phloem cells and new xylem cells. The wider band of xylem, or sapwood, transports water to the crown, but is not necessarily living. The innermost portion of the trunk is non-living heartwood, which is a repository for many waste products of the tree's living tissue. Only a thin band around the trunk, roughly a centimeter wide in a large tree, is living tissue.
Each year, a tree grows a pair of annual rings (TWO rings each year!). In the spring, the usually wider and thinner-walled layer grows. It is called "springwood". In the summer, through about mid-July, a usually darker and thicker-walled layer is produced. It is called "summerwood". Annual rings are typical in temperate forest trees and tropical forest trees that have regular, annual dry seasons. In tropical humid rainforests, trees grow continually and do not have rings. The oldest portion of a tree is at the bottom and on the inside.
Parts List
Without going into a lot detail, important parts of a tree are:
| Leaves | Broad-leaf or needles, the primary site of photosynthesis and the production of hormones and other chemicals |
| Twigs & Branches | Support structures for leaves, flowers, and fruits. Arrangement varies from species to species by growth strategy. Can sometimes have photosynthetic tissues. Two kinds of growth tissue, at the twig tips and cambium under the bark. |
| Crown | The upper region of the tree made up of leaves, twigs, branches, flowers, and fruits. Crowns of many trees are collectively called the "canopy". |
| Flowers | May have both female & male parts, or only one or the other. Some trees are either all female or all male (e.g. aspen). Flowers may have a full complement of flower parts, or may be missing certain elements. Conifers do not have petals and associated structures. |
| Fruits & Seeds | All trees have seeds. Most trees have seeds inside fruits. Most fruits are NOT edible, but many are, such as apples, cherries, nuts, etc. |
| Trunk or Bole | Most definitions of trees include a "single bole" concept, but many of our tree species sometimes occur with multiple stems. The main functions of a trunk are transport and support. The trunk has growth tissue called cambium. |
| Bark | A highly variable tree part. The main function is to protect the sensitive living tissues from weather and predation (by animals, insects, fungi, etc.) |
| Roots | Roots serve two main functions; collection of nutrients and water, and anchoring the tree. Roots also have growth tissue, bark, and wood. Like twigs and branches, roots have two kinds of growth tissue, at the root tips and cambium under the bark. Fine root hairs are where absorption occurs. |
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Activity Suggestion PLT Tree Cookies |
Photosynthesis and Respiration
All trees (most plants) both photosynthesize and respire. Photosynthesis is a process unique to green plants and produces sugars, which are "tree food." Animals only respire and cannot produce their own food. That’s why plants are called "producers" and animals are called "consumers." These terms have nothing to do with oxygen. I
Photosynthesis can be visualized in a couple ways.
The basic chemical formula for photosynthesis is:
Inputs: 6 carbons, 24 oxygens, 24 hydrogens
Outputs: 6 carbons, 24 oxygens, 24 hydrogens
Note: Inputs and outputs must balance in a chemical equation. In other words, what goes in, must come out!
| Energy IN  |
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Summary equation for photosynthesis |
Energy is stored in the bonds of sugar molecules such as "glucose" and "fructose." Oxygen is a by-product of photosynthesis. The oxygen molecules produced by photosynthesis are not necessarily the same oxygen molecules the plants use for respiration.
These sugars are later broken apart and the released energy drives a variety of metabolic actions. The process of breaking down these sugars is called "respiration." It is the same process that animals (and people) use when they respire (not to be confused with "breathing"). So, either the plant uses its own stored sugars, or some animal (or decomposer) consumes the plant, and uses the stored sugars. In either case, the sugars are valued chemicals because they contain energy, as well as important elements (carbon, hydrogen, and oxygen).
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ENERGY OUT |
Summary equation for respiration |
| Apply the Concept: The "10% rule
of thumb." Plants are able to "fix" about 10% of the solar energy that reaches plant surfaces (usually less, however). "Fixing" means converting solar energy into chemical energy (sugars). Organisms that consume plants, are able to extract about 10% of the energy stored in the plant. Organisms that consume other consumers can extract only about 10% of the energy stored in their prey. These levels of energy consumption are called "trophic levels." Energy flow through an ecosystem (large or small) is a key life process. Threads of energy transfer are called "food chains." Food chains also include the transfer of chemicals other than sugar. Many nutrients, amino acids, and other compounds are digested and recombined by consumers along any particular food chain. |
| Apply the Concept: Crown size and
photosynthesis All the leaves and branches of a tree are collectively called the "crown." All the crowns of a forest are collectively called the "canopy." As forests age and trees grow, crowns begin to touch each other and the forest canopy closes. Most of the trees will be unable to grow as rapidly as if they had free space to occupy. The photosynthetic capacity will be spread among a greater number of trees. That means less photosynthesis per tree, which translates into slower growth. Slow growth can be a contributor to tree stress, which can lead to tree health problems. Foresters understand how different forests grow in different ways. They can recognize a forest that is too crowded and prescribe a thinning, where some trees are removed so that others may grow better. In addition to channeling more growth onto a fewer number of trees, thinning the canopy can have a very positive impact on the understory. More light to the forest floor will stimulate the regeneration of trees and promote more vegetation in the understory layers of a forest. More vegetation in the understory creates more vertical structure, which often leads to greater species diversity in the forest. |
What does a tree use its photosynthate for (glucose and fructose) in addition to energy storage and subsequent release?
A note about energy allocation within trees. Energy is not a limitless resource for trees. A tree will typically move energy according to these following priorities. As energy in the form of glucose becomes limited, a tree will begin to reduce resources spent beginning with the lowest priority. As you can see, a tree with a diminishing crown will become more vulnerable to insects and diseases rather quickly. That’s one reason why foresters are so keen to maintain a vigorous growing environment.
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Activity Suggestions PLT Air Plants PLT Sunlight & Shades of Green |
Chlorophyll is the chemical compound where solar energy (light) is captured and photosynthesis happens. Chlorophyll is continuously produced and broken down during the growing season. The heart of the chlorophyll compound is a magnesium molecule. The magnesium molecule is bonded to many molecules of hydrogen, carbon, oxygen, and nitrogen.
Chlorophyll "a", one of
several forms of chlorophyll
| Experiment Suggestion: Grow plants (beans, peas, fast-growing plants) in containers of different light. Transparent plastic wrap covering containers will filter light spectra. Compare growth rates of plants. |
There are different kinds of chlorophyll that absorb different colors in the light spectrum. The only color that is pretty much useless to plants is green, which is why plant tissues containing chlorophyll appear green . It’s the color that is reflected back into the environment. The process of photosynthesis is very complicated and driven by a series of enzymes. Enzymes function within fairly narrow temperature windows. Within these temperature windows, heat accelerates photosynthesis to a certain point and cold slows it down. Outside the temperature window, photosynthetic activity drops off, most quickly with hot temperatures.
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Activity Suggestion PLT Sunlight & Shades of Green |
So, photosynthesis produces all this glucose . . . what then? Essentially, the energy in glucose is used by trees (and most other living things) to drive metabolic processes that produce tissues and maintain life functions. Keep in mind that this whole thing called life is a big solar powered system!
A tree will draw nutrients and minerals from the soil, break them down and put them back together to form compounds and chemicals that we recognize as a tree. The most common material made by a tree is "cellulose." Cellulose is a complex sugar that is the main component of wood and many other plant tissues. It’s also an extremely useful material for lots of human uses, such as food products, paper, strengthener in plastics and concrete, clothing, and other things.
Wood is the answer to the tree challenge of pushing a crown as high as possible to obtain the best light-capturing position as possible, while maintaining a connection with water and nutrient supplies in the soil.
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Where does a tree grow? In three places.
One region of tissue expansion or tree growth is at the tips of both twigs and roots, called the "meristem." This is unspecialized tissue that can form wood, buds, or flowers. Each year, trees will lengthen twigs and roots, produce flowers and fruit, and grow new buds. The meristem and newly produced tissues are rich with nutrients and are often the target of attack by diseases, insects, and animals. Deer, for example, are Michigan’s most significant browser. In areas of high populations, deer can destroy years of growth on small trees and entirely eliminate regeneration.
Most of a tree trunk, branch, or root is dead wood. The living
part is only a narrow band on the outside edge. This living layer is produced by thin
bands of regenerating tissue called "cambium." Cambium produces new wood
on the inside and new bark on the outside. The cambium grows only from the inside out, not
up or down the length of a trunk, branch, or root. For awhile, the new wood and bark are
living. The wood actively transports many materials up and down the tree and performs
other functions. After the wood dies, it still serves as a transport route for several
years. Eventually, even that function is diminished and the wood serves primarily as
structural support.
Each year the cambium produces TWO distinct rings of tissue. In the spring, a layer of thinner-walled cells are grown. In the summer, a layer of thicker-celled, sometimes larger cells are grown. The layers are called "springwood" and "summerwood," respectively. When counting the age of tree "cookie," either the springwood or summerwood rings can be counted, but don’t count both (unless you divide your sum by two!). Most people count the typically narrower and darker summerwood. Tree such as oaks, ashes, and all the conifers produce fairly distinct rings which are easy to count. Other trees, such as aspens, maples, and birch have less distinct rings. Foresters can count rings without cutting a tree down. A tool called an "increment borer" will extract a thin wood core from the tree, which can be used to age the tree.
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Activity Suggestions PLT Tree Cookies PLT How Plants Grow PLT Every Tree For Itself PLT How Big Is Your Tree? PLT Trees In Trouble |
Why do leaves change colors in the Autumn?
| Project: Have kids collect different colored leaves in the fall. Categorize leaves by species and color. The same species may have many different colors, especially red maple. Also, have kids record the dates when trees at home, at school, or in another selected place begin to change color. Make notes by species and see if any patterns can be observed. It would be interesting to have a "sister" school in a different part of the state to compare color change with. |
The short answer is that chlorophyll production drops-off as night length increases. The green part of the light spectrum is no longer reflected and other compounds, chemicals called "anthocyanins" (reds) and "carotenoids" (yellows), become the dominant pigments in the leaves. The longer answer involves discussions of changing day lengths and weather, and strategies dealing with nutrient loss with the dropping of leaves.
What is the story behind Autumn leaf fall?
The purpose of Autumn leaf fall is to prepare for winter dormancy. The cold temperatures prevent trees and plants from functioning in at least three ways. Water would freeze in the plant tissues, causing cell rupture. Water in the upper soil layers often freezes, making absorption impossible. Lastly, the low temperatures are far outside the operating windows for the enzymes that control a tree’s metabolic processes, such as photosynthesis and respiration. To avoid these environmental limitations, trees prepare for dormancy in the Autumn.
Trees drop leaves because they are too difficult to "winterize" (unlike most conifers that have strategies to maintain their green parts during the winter and needles have a much different structure than broad leaves). Or, in the case of conifers, the needles that have grown old after two to three years, no longer receive as much light, and are shed each Autumn. However, dropping tons of biomass per acre presents the problem of losing significant amounts of valuable nutrients. Much of the sugars and valuable nutrients are resorbed from the leaves, but the annual leaf drop still means the loss of a lot of good "stuff." In our north temperate climates, dropped leaves become part of the "organic layer" on the surface of the soil, to be recycled (in part) by decomposers.
There are two components influencing the Autumn color display, the timing and the intensity. The timing is usually controlled by lengthening nights and the intensity is strongly influenced by weather.
The most dependable seasonal environmental factor is the change in daylight, or more accurately, the lengthening dark period. Such things as rainfall or temperature might "fool" a tree into retaining leaves too long. For this reason, the timing of leaf-drop is regulated by the consistent movement of the Earth around the Sun. However, a late spring or extremely dry summer can postpone the response to lengthening nights by a week or two. Just "when" a tree begins to turn color varies from species to species, and geographically from north to south. In our northern forests, black ash is the first to change color. Tamarack (a needle-bearing tree) is the last.
The intensity or brilliance of the color change is influenced by weather conditions during the period of declining chlorophyll production. A series of sunny days and cool nights (above freezing) result in a more colorful display. The warm days increase production of both sugars and anthocyanin pigments. Sugars "stranded" in the leaf and greater concentrations of anthocyanins bring out the scarlets and reds, especially the deep purple of northern red oak. Carotenoids yield the yellow and golden colors but tend to remain at fairly constant concentrations regardless of weather.
So, how might weather affect the fall colors?
What causes the leaves to actually fall off?
Wind, most commonly. As nights lengthen, a layer of cells forms in the leaf stem near the twig, called the "abscission layer." Abscission means cutting or severing. This layer blocks transfer of materials to and from the leaf. The abscission layer also makes a weakened connection. Eventually, wind, rain, snow, or animals will knock the leaf from the twig.
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Activity Suggestion PLT Signs of Fall |
Sunlight and Tolerance of Shade
| Relative Sunlight
Requirements For Representative Tree Species |
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| Paper Birch Tamarack Jack Pine |
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| Quaking Aspen Silver Maple Red Pine |
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| Red Maple Red Oak White Pine |
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| Yellow Birch Balsam Fir White Spruce |
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| Sugar Maple Basswood Cedar |
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It’s commonly known that trees and plants need sunshine to live. However, not all trees need the same amounts of sunlight. Trees that require high amounts of sunlight are sensitive to shade. Foresters call this sensitivity "shade tolerance" or just "tolerance". The shade tolerance of some tree species will vary with age.
Tree species such as aspen, cherry, paper birch, jack pine, and red pine require lots of sun and are not tolerant of shade. That’s part of the reason stands of these species tend to be all about the same age. Seeds of these species that germinate under a canopy of shade do not survive.
Other tree species are more tolerant of shade, such as sugar maple, beech, balsam fir, hemlock, and cedar. They can survive as seedlings or saplings under a fairly heavy canopy of shade for many years. When exposed to light, the small trees (not always young trees!) can sometimes quickly grow to take advantage of the new light regime.
There are a number of tree species that fall into the moderately tolerant category, such as red oak, red maple, yellow birch, white ash, white pine, and white spruce. They may be able to grow under the light canopy of an aspen or paper birch stand, but would not be very successful under the shade of a maple-beech-basswood stand.
Shade tolerance is key component of forest management systems. And, a species tolerance to shade can change with age .
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Activity Suggestion PLT Sunlight & Shades of Green |
Other Environmental Factors
There are many environmental factors, both living and not living, that influence the growth of trees. This guide has already discussed some of them, such as light, nutrients, and temperature. Many of these factors interact with other. That’s part of the reason why forest management can be complex. Tree adaptation to various environmental factors runs along gradients. Some tree species are more sensitive to a particular gradient than others.
Rainfall or Precipitation
Average annual rainfall varies across a wide geographical area. Some tree species can survive with less annual precipitation. As you move north and west, rainfall declines, and so do the number of tree species. More locally, available water may vary with microsites. The south sides of slopes will be drier, so will a sandy plain or areas with bedrock close to the surface. Deep snows during very cold periods help protect latent seeds and dormant root systems. The timing of rainfall matters, too, along with the frequency and size of events. A few large downpours is less desireable than moderate rains at regular intervals. Climate change is altering these patterns .
Soil Variability
Scientists have identified over 475 soil types in Michigan. It stands to reason that different tree species have preferences for certain types of soil. Red pine and jack pine are well-known for their ability to grow well on sandier, poorer soils where most other trees grow poorly. Sugar maple and basswood prefer richer, well-drained soils with lots of nutrients. Other species, such as bur oak and quaking aspen grow well on a wide variety of soils. This variability is largely related to the amount of available nutrients in a soil, the nutrient demand of a particular species, and a tree’s ability to extract those nutrients.
Moisture
This is related to both rainfall and soils. The amount of available moisture varies during the year. High moisture levels during the dormant season will not help trees. Or usually hurt them. Saturated conditions from spring runoff or flooding does not hurt most trees because they are not actively growing. Some tree species are more tolerant of short periods of flooding during the growing season, such as bur oak or silver maple. Oddly enough, white-cedar is quite sensitive to rapid changes in moisture, either wetter or drier. Northern swamp tree species grow on small, dry microsites. They don't usually grow in the water.
Biotic Factors
These are the living parts of an ecosystem that trees interact with. Other plants will impact forests. Insects and diseases play a major role in forests. Animals like white-tailed deer, porcupines, and squirrels also have prominent roles. Not all of these impacts are negative. Many are beneficial. Insects pollinate tree flowers. Soil animals loosen soil. Birds and bats eat lots of insects. And of course, humans manage forests for a wide variety of reasons.
Pronounced "my-core-HI-zee", these are beneficial fungi to trees. The fungi are associated with tree roots in a symbiotic relationship. That's where both partners benefit from each other. The mycorrhizae increase a tree's ability to absorb water and nutrients. The tree supplies the mycorrhizae with a share of photosynthate. Sometimes, species of mychorrizae are only associated with a particular species of tree. The lack of proper mycorrhizae in the soil can prevent a tree from growing well, or maybe from surviving at all. It may be one of the factors that limit trees to a certain range. Scientists are learning more about these special fungi.
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Activity Suggestion PLT Plant A Tree |
Tree Regeneration Strategies
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There are four ways Michigan trees regeneration themselves.
All trees can reproduce by seeds. Each species has a unique set of requirements for seed production and germination. Seed dispersal strategies vary widely, from wind-driven seed to seeds carried by certain species of animals.
Sprouts and suckers are similar, in that dormant buds "come alive" to form new shoots of parent trees. Sprouts are shoots from stumps or the base of a tree. Suckers are shoots that originate from buds on the root systems. Often times, sprouts and suckers will not grow until the parent tree dies or becomes very sick. The buds are held in dormancy by hormones produced in the leaves. When these hormone levels drop below a certain point, the dormant buds will grow.
Vegetative layering is uncommon, occurring mostly in white-cedar and Canada yew (which most would not consider a tree!). When branches or stems come in contact with the soil, cambium tissue sometimes form roots. In this way, former branches of a fallen cedar might become trunks of several "new" trees.
Trees do not live forever, therefore cannot be "preserved." A forest condition, or forest type might be preservable (if managed), but not individual trees. While people know that all living organisms eventually die, often times this is not taken into account when people consider forests.
Tree longevity varies from about 70 years to over 1000 years, depending upon the species. Most trees do not live past 50 years (or 1 year, for that matter), if you consider attrition from the time of germination. Short-lived species tend to be successional "pioneers", or trees that first colonize an unforested site. Aspens, paper birch, cherries, jack pine are examples of short-lived tree species. They also tend to be intolerant of shade. Long-lived tree species tend to be more shade tolerant, occupy later stages of succession, and employ more "conservative" survival strategies. Sugar maple, basswood, beech, and white-cedar are good examples.
Note: "Succession" is explained in another place in this guide and is one of the most important concepts in forest ecology.
Most
Common Michigan Tree Species (by volume) |
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| Sugar Maple Red Maple Quaking Aspen Cedar Northern Red Oak |
200-300 |
Balsam Fir White Oak Eastern Hemlock Jack Pine Yellow Birch |
70-100 |
| Red Pine Bigtooth Aspen Basswood Paper Birch White Pine |
200-250 |
Black Cherry White Ash American Beech White Spruce Black Spruce |
150-200 |
Trees must have adaptations to survive the cold and drying conditions of winter. Trees cannot change their location or behavior like animals can, so they must rely on physiological and structural adaptations.
The height advantage of trees becomes a liability in the winter, as tissues are exposed to the severe weather. There are four basic strategies that trees employ.
1. Either leaf drop or
adaptations for leaf retention.
2. A physiological acclimatization process.
3. Resolution of water issues.
4. Methods of reducing mechanical damage from snow and ice.
Broadleaf trees (hardwoods) drop their leaves during the winter, avoiding the problems of maintaining foliage in cold and dry conditions. Conifers (softwoods) retain foliage and have special adaptations in order to do so (better stomate control and a waxy coating called cutin).
All trees go through an acclimatization process. Like leaf drop, the process is initiated by changes in photoperiod and is controlled by hormones and other chemicals. The process also exploits the physical properties of water.
Winter conditions make finding sources of liquid water and transporting water a challenge. Water loss is minimized in several ways. Water can be obtained from the ground, within the tree, or from the subnivean (under snow) micro-environment. Conifers have special cell adaptations to facilitate water transport whenever temperatures allow it.
Snow and ice accumulation can cause breakage, especially under windy conditions. Conifers have growth patterns that minimize the chances of damage occurring. Spruces and firs are much better than pines at bending under the weight of snow.
Dramatic loss in vegetation from animal consumption increases pressure on woody tissues, especially foliage, buds, and bark. Browse damage can be significant in many regions of Michigan. Lastly, pollutants from highways, particularly road salts and exhaust, can damage trees, especially those more vulnerable to these chemicals.
Click here for a more detailed explanation of winter adaptations of trees.
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This website was developed and created by Michigan State University Extension for the teachers of the State of Michigan. |
