MICHIGAN FORESTS FOREVER TEACHERS GUIDE
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 weather. There are four basic strategies that trees employ.
1. Either leaf drop or
adaptations for leaf retention.
2. A physiological acclimation process.
3. Resolution of water issues.
4. Methods of reducing mechanical damage.
Leaves are a major source of water loss and are difficult to protect from winter conditions. Most trees drop leaves in the autumn to avoid these problems. Conifers are the exception and will be llooked at in the next section.
Annual leaf and flower expansion requires tremendous inputs of water and nutrients. Trees must produce and store a sufficient amount of reserves during the four to five month period when leaves are photosynthesizing. Buds are usually set by the end of July. Above-ground growth also ceases in late summer.
Physiological processes, including leaf drop, are stimulated largely by changes in photoperiod. The lengthening dark period changes the production rates of a number of chemicals and hormones. The most important is probably the increase in abscisic acid (AA). AA slows protein and RNA (ribonucleic acid) production. Both are keys parts of growth. AA also increases cell membrane permeability, which is important in the acclimation process.
Chemical breakdown of the green chlorophyll molecules reveal the pigmentation of the yellow and orange carotenes and xanthophylls. The scarlet colors are enhanced by hard frosts affecting residual sugars and anthocyanins.
Abscission layers between the leaf stem and the twig are formed. Cells along this line expand at different rates, and enzymes degrade tissue. As a result, a physical line of weakness develops. Scar tissue is formed over the attachment point that prevents water loss. Gravity and wind cause leaves to drop.
Not all trees lose their leaves at the same time. Black ash is usually the first to drop leaves. Some species will retain brown leaves well into the winter, especially oaks, ironwood, and beech. In May of each year, leaf growth can be tracked by species. Black ash and bigtooth aspen are among the last tree species to leaf-out.
Adaptations For Leaf Retention
In the north temperate forest, all broad-leafed trees lose their leaves. Each year conifers also drop leaves, similar to broad-leafed trees, they just don’t shed them all. Most conifers retain needles for two to three years before shedding them. Although conifers require the resources to produce new needles each year, they gain a large measure of economy by using a set of needles for more than one year. The coniferous exception to this needle-retention strategy is the genus Larix, the tamaracks and larches.
Conifer needles have a thick, waxy coating of cutin that significantly reduces water loss. Needles also have much tighter stomatal closure. Stomata are the pores that allow air and water to pass in and out of the needle. Lastly, tissues undergo an acclimation process, similar to other living tissues in trees.
Retaining needles allows trees to extend the length of the photosynthetic season. It also potentially allows trees to take advantage of winter thaws and, perhaps, even to permit slow rates of photosynthesis during cold weather. However, needle retention presents serious challenges in terms of water loss, water re-supply, and snow-loading.
Loosely analogous to animal hibernation, trees undergo changes that allow them to survive the cold, dry conditions of winter. This process occurs at the cellular level and exploits the physical properties of water.
All trees have a measurable “killing temperature”, the temperature where ice crystals form within cell structures resulting in cell death. Killing temperatures vary among different species, between populations of the same species, and even among different tissues. In some cases, killing temperatures are limiting factors for species ranges.
Tree Species With Northern Ranges Limited By Killing Temperatures
Tree Species With Killing Temperatures Below Average Low Temps
Tree Species With Adjustable Cold Tolerance
Source: Marchand (1996).
Like leaf-drop, acclimation is prompted by changes in the photoperiod. Abscisic acid (AA), once again, plays a key role. Physiological changes include:
1. An increase in AA
2. Lipids (soluble fats) unsaturate.
3. Lipid concentration within cells increases.
4. Proteins de-polymerize.
5. Cell membrane permeability increases.
Solute concentrations within cells increase, slightly reducing the freezing point. Therefore, as temperatures drop, water outside cells freezes first. Freezing water releases small amounts of heat energy, which in turn, helps cell fluids remain unfrozen. Twig temperatures actually rise several degrees during this process. Water moves out of cells attracted to the ice crystals in the pore spaces. This process effectively reduces the freezing point of cell water to the killing temperature. Colder temperatures will begin to result in cell freezing and death.
Water is lost primarily from above-ground biomass. Bark and buds are fairly water-tight. Drastically lower levels of photosynthesis and respiration reduce water demand and subsequent loss. Conifers, however, have huge surface areas of living tissue in their needles. Any photosynthesis that might occur will increase water demand and risk of loss.
Water vapor moves from areas of high concentration to low concentration. Concentrations are usually higher inside needles, so the tendency is for water to be lost from foliage. Needles have advanced structures to present a barrier to water loss, but cannot eliminate it. In addition to tight stomatal closure and cutin, reduced air movement around needles will contribute to lower vapor gradients. Air “boundary layers” act like insulation. The dense foliage of conifers, especially stands of conifers, serves to mediate micro-environmental conditions somewhat. The fuzzy undersides of evergreen broad-leaf shrubs serve to increase this zone of “insulation”.
Oddly enough, warm, sunny days present the greatest water retention challenges for conifer foliage. Dark needle coloration readily absorbs heat and raises needle temperatures significantly above ambient air temperatures. Metabolic rates rise and internal vapor pressure increases. Despite thicker air boundary layers, the net effect is greater water loss.
Cloudy, windy days are actually better for conifers. Clouds block warming solar energy and wind readily removes heat from the needles, reducing the vapor pressure gradient.
Conifers have larger winter water demands than most broad-leaf trees (some hardwoods have photosynthetic bark and branches, which increases water demand). Without re-supply sources, trees would die from water loss. However, freezing temperatures and frozen water would make re-supply seem impossible.
There are three potential sources of water; 1) the soil, 2) internal tree reservoirs, and 3) subnivean (below snow) vapor absorption.
Soils are not always frozen. In fact, much of Michigan’s soil remains unfrozen for all or part of the winter. The insulating effects of snow can result in ground thaws or prevent freezing in the first place. This means that liquid water is available. Transportation above-ground becomes an issue, discussed later.
The sapwood of trees and branches contains water. Oftentimes, this water is frozen and unavailable. However, differential warming (solar insolation) and winter thaws can melt the sapwood water, making it available for transport.
Lastly, conifer branches below snow-level might benefit from higher water vapor concentrations outside the foliage. Potentially, this absorbed water could be transported to other locations in the tree.
Given that liquid water sources exist during the winter, the problem of transport remains. Water cannot be moved while frozen, so temperatures along a transport line from source to sink must be near or above freezing.
Water is moved within a tree through the xylem, which consists of cells that make up long tubes, called tracheids. The strong cohesive properties of water permit continuous columns of water to be “pulled” through the tracheids. If a water column is broken, it is virtually impossible to restore the column.
When tracheid water freezes, two things potentially break the water column. Ice crystals stop the flow. More importantly, as ice forms, dissolved gases are expelled and form gaps in the column. Upon thawing, these air gaps remain, rendering the column unusable for water transport. Hardwood (broad-leaf) trees grow new xylem cells in the spring to re-establish the water transport system.
Conifers have some fascinating adaptations that overcome the problem of broken water columns.
Within the transport tubes, conifers have tiny “check valves” between each tracheid. Ice formation and volume expansion increase pressure within a water column causing the “float” within the check valve to seal the ends of each tracheid making up a tube. The float is called a “torus”. The expelled gas is held under pressure within the tube by the incredibly strong tracheid walls. Measurements have demonstrated that the tracheids can hold pressure up to 900 psi. When the ice crystals melt, the gas is forced back into solution, pressure returns to normal, the tori migrate back to the middle of the check valve, and the water column is restored.
Water column restoration in conifers can occur multiple times during the winter during warm periods or when solar insolation is high. Foliar water stress caused by those warm, sunny winter days can be alleviated by restored water columns supplying water from any of the available water sources.
In addition to the clever adaptations of conifer xylem cells, there is evidence suggesting that water can also be diffused from cell to cell via the phloem, in both hardwoods and softwoods (conifers). Diffusion is slow but may be sufficient to meet the water demands of dormant hardwood species that appear to have no other winter water transport system. This may help explain why some hardwood species, such as paper birch, can survive winter conditions right up to the treeline.
Other Winter Season Challenges to Survival
Conifers have higher leaf densities than hardwoods. This means snow can quickly accumulate to the point of stem and branch breakage. Ice storms can be even more detrimental. To offset this snow-loading problem, conifers display alternative growth and branching patterns.
Conifers have a single leader or main stem (determinant growth), as opposed to the many leaders of hardwoods (indeterminant growth). The subsequent cone shape more effectively sheds snow. Conifer branches grow at more obtuse angles to the main stem. This allows branches to reach snow-shedding angles with less bending. Longer wood fibers also generally provide more flexibility.
Denser conifer foliage offers greater wind resistance, potentially leading to breakage, especially when foliage is loaded with snow and ice. Trees on the perimeter of conifer stands take the brunt of wind damage, but the dense foliage also protects those individuals internal to the stand. This factor creates typical stand shapes in mountainous terrain, but is not as pronounced in Michigan. However, Michigan conifers sometimes do display “flagging” in the direction away from prevailing winds. Tall white pine are particularly noteworthy in this regard.
Many conifer species become targets for animal browsing during the winter. Foliage contains some of the better sources of nutrients, although they are poor compared to summer food availability. In many parts of Michigan, high deer densities have eliminated the regeneration of most tree species (hardwoods and softwoods), along with other plant forms. High moose densities have had tremendous impacts on the vegetation of Isle Royale.
Porcupines, rabbits, and mice find sustenance in the living bark and phloem tissue of trees. If the bark is chewed all the way around the stem, the girdling will kill the tree beyond that point. Girdling is an especially severe problem in certain conifer plantations and young trees in old fields.
Many birds feed on the rich flower buds of trees. Ruffed grouse are particularly well-known for their affinity for the male flower buds of quaking aspen. However, flower bud browsing has seldom, if ever, resulted in significant damage. Plantations grown for fruit or seed production may be an exception.
The final winter challenge for trees is human-caused. Trees along major roads may eventually show signs of poisoning from road salts and vehicle exhaust. Some species are more resistant to these pollutants. The more vulnerable species include; white pine, red pine, hemlock, basswood, ironwood, sugar maple, and red maple.
Guillion, Gordon W. 1984. Managing Northern Forests for Wildlife. The Ruffed Grouse Society. Coraopolis, PA. 71 pages.
International Society of Arboriculture. 2001. Website [http://www2.champaign.isa-arbor.com/].
Marchand, Peter J. 1996. Life in the Cold. University Press of New England, Hanover and London. 304 pages.
Raven, Peter H., R.F. Evert, and H. Curtis. 1976. Biology of Plants. Worth Publishers, New York, NY. 685 pages.
Salisbury, Frank B. and C.W. Ross. 1978. Plant Physiology, second edition. Wadsworth Publishing Company, Belmont, CA. 422 pages.
Winchester, A.M. 1969. Biology and Its Relation to Mankind. Van Nostrand Reinhold, New York, NY. 717 pages.
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