Category: Plant parts

Trees produce buds for the next year’s leaves, flowers and shoots.  They are small and not easily noticed among the foliage of a fully leafed-out tree, which is why they are easiest to spot in winter.  Buds develop in late summer, when a tree has lots of energy from the sun, but is no longer growing new leaves and branches as the tree prepares for winter.

Tree buds remain dormant through the winter.  In spring, as daylight lengthens and temperatures start to warm, buds will burst open.  Buds contain the beginnings of a leaf, a shoot, or a flower.  Terminal buds, located at the ends of branches, grow into new shoots.  Lateral buds, growing along the sides of branches, produce either vegetative shoots or flowers.

Lateral buds that grow into flowers are well camouflaged and easy to miss.  Frequently, flowers are dark red or green in color and are quite small, easily blending into the foliage.  Many trees are wind pollinated, so there is no need for the flower to be showy.  Once the flowers are pollinated, they quickly die; many only live for a few days.

Bur Oak shoot, 23 inches last year, by DonArnold Feb 2022

Terminal buds appear at the ends of branches and will create new shoots, allowing the branches to grow longer.  These buds release a hormone named auxin that slows or prevents the growth of lateral buds on the same branch, so all of the energy for that extension of the tree goes into growing new wood.  Arborists may prune terminal buds before they open to stimulate lateral bud development, thus controlling the shape of a tree.

Many buds are covered with scales.  These are small, modified leaves that protect the bud in winter against cold, freeze damage, and drying.  Buds contain stored energy in the form of sugars and nutrients to be used as soon as the buds burst in spring.  This makes buds attractive as a winter food source for aphids, cedar waxwings, finches, squirrels and deer.  Trees can lose some of their buds, but excessive predation could stunt tree growth.

As you are out walking in the next few weeks, you will be able to see many trees with their buds getting ready to open.  You may want to take along a field guide that describes characteristics of tree buds that can be used in winter tree identification.  Two good ones are Winter Tree Finder by May Theilgaard Watts, and The Tree Identification Book by George W.D. Symonds.  Both describe tree bud shape, color, size, and orientation, in addition to other tree parts.

Bark provides protection for the tree from organisms trying to get under it to lay eggs and let larvae grow, to live out of the elements, or to feed.  Whether bark is thick or thin, there are areas of good protection and areas of weak protection.  Weak areas may include lenticels, cracks, branch junctions, wounds, broken branches, or open spots created by fire, frost, or other environmental actions.  Many trees are weakened by drought and poor nutrition increasing the likelihood that the bark will not provide protection over the entire exterior surface.

A mature tree can provide food and shelter for dozens of species, but not all visitors are harmless.  Dutch Elm disease is caused by a fungal bacteria found on non-native beetles introduced to the U.S. through the landscaping trade in the early 1930s.  Native elm trees had no defenses, and native elm beetles also picked up the fungus and spread it further. 

Insects that lay eggs under bark cause very little damage, but their larvae burrow through the cork layer, sometimes creating extensive tunnels that can damage the tree’s circulatory system.  Where tunnels break through the surface, decay sets in.  Carpenter ants feed on the decaying wood, opening the wounds further.  Woodpeckers, especially the pileated woodpecker, feed on the carpenter ants, excavating wood to get to more ants.  Tree sap, leaking from the damaged circulatory system, draws even more insects, and the cycle of damage continually worsens.

Many animals feed on sap and inner bark in late fall and winter when food resources are scarce.  This is also the time of year when the tree is least able to defend itself.  Red squirrels and porcupines bite through bark on beech and maple to get to the sap.  Beavers break through the outer bark of willow, aspen, poplar, and cottonwood to eat the inner bark.  Native Americans and settlers harvested the inner bark of hemlock and pine to make flour.  Deer, moose, and black bears strip bark to mark their territories.

Many organisms use significant parts of a tree without harm.  Peeling bark is used by birds for nest building.  Smooth patch disease, easily observed on white oak, is caused by a fungus eating the exterior bark, but not penetrating to the inner bark.  Insects move along bark cleaning up moss, lichen, and other material that may be clogging lenticels.  Their presence attracts ants, spiders, and other predators.  When it rains, water flowing down the bark carries minerals, nutrients, and organic matter from plants such as lichen and moss, plus fecal matter from bark dwelling insects, and washes it all into the root area providing additional resources to feed the tree.

Trees have their own chemical and mechanical defenses to repel, isolate, and kill various organisms.  Resin seals wounds to prevent desiccation and infection.  The inner bark of black cherry contains hydrocyanic acid to repel bacteria, a chemical long used in cough drops.  Tannin refers to a variety of compounds found in many plant species that is used to deter epiphytes, insects, and fungi.  Trees may grow inner bark tissue to form a walled off area that isolates an organism that has breached the exterior bark.  This area, covered by callous tissue sealing off the wound, is known as a canker, and results in a visible patch on the outer bark.  Burls forms from abnormal wood growth around these areas. 

Bark is an important and integral part of all trees, as well as a complex subject with much information still to be learned. There are several good resources available for further study, including Bark, A Field Guide to Trees of the Northeast by Michael Wojtech.  As you are walking the woods this year, take some time to think about the role of bark not only in tree  identification, but as a resource for the woodland habitat.

Ecology is the study of the relationship between organisms, each other and their environment.  Bark is a highly visible part of a tree.  By observing the color, thickness and function of the bark we can learn about the environmental factors that affect trees.  These may include food production, fire, water availability, sunlight, wind, weather, and temperature. 

Food production occurs through photosynthesis in tree cells that contain chlorophyll.  These cells are most often associated with green leaves, but many trees with smooth, thin, or peeling bark have these cells in the cork skin layer immediately below the outer bark layer.  Sunlight can penetrate thin bark even on days where temperatures are below freezing.  Warmth generated by sunlight on south or southwest sides of a tree will start photosynthesis.  Trees growing in habitats with less than ideal growing conditions due to too much shade, a shorter growing season, or higher altitudes can depend on this secondary source of food production. 

Bark’s outer layer is made of dead cells filled with air.  When combined with thickness, color, and density, these factors determine the effects of temperature fluctuations occurring from several sources.  In winter, melting water from ice or snow can flow into any opening in the bark.  When temperatures fall, rapidly drying and cooling bark refreezes the water causing it to expand and put pressure on the bark to create or enhance cracks or to pry the bark loose from the tree.  Normally this promotes additional bark growth, but when repeated often over a short period of time, the tree’s appearance will become rougher with an increasingly uneven surface.  Similarly, lightning changes the moisture content of bark into instant steam, splitting and blowing off sections of bark.  Smooth bark species are much less affected as the energy follows the sheet of water down the smooth bark and into the ground.

Sunlight is another factor both positively and negatively affecting a tree’s appearance.  In winter, southward facing bark exposed to direct sunlight can heat to over 70 degrees.  As the sun sets, dropping temperatures can cause bark to shrink around a still warm and expanded inner core resulting in stretching or cracking.  This may also occur in trees subjected to hot sun and cool nighttime temperatures after bark has been dried out due to drought, reducing its ability to stretch.  Some species with thin bark will reflect sunlight rather than absorbing it.  Bark of the quaking aspen rubs off in powdery form that reflects sunlight well enough that Native Americans used the powder as sunscreen.

Fire is a familiar environmental hazard.  The best protection from fire comes from bark composed of several layers.  Ridges, scales, plates, and furrows also contribute to keeping temperature fluctuation small on the inside of the tree.  They have an uneven surface that retains moisture and makes them more resistant to the effects of fire.  Trees growing in hotter, drier climates and more exposed to fire grow bark faster than other species.  Pitch pine inhabits drier areas of the northeast U.S. and has bark that thickens at an early age and contains epicormic buds that sprout into new branches after exposure to fire.  Some species have a different strategy for fire.  Quaking aspen has  thin bark that burns easily, but the trees resprout quickly and often from old rootstock.  Paper birch also burns easily, but puts out a much greater quantity of seed annually than other species in the same habitat.

The environment places many stresses on the external layers of trees.  Form, function, and appearance is initially determined by DNA, but is continually being modified by location and external forces.  As you get outside or look out your window, try to identify the environmental factors affecting your trees.  Next week, our final look at bark will deal with impacts from other organisms on trees. 

It has been cold outside for the last few weeks; and whenever we go outdoors, we tend to bundle up with warm coats, gloves, hats, boots, and long underwear.  Animals and plants have their own strategies for living through the winter, including trees.  Bark is an essential part of any tree.  Bark provides protection, insulation, and plays a role in getting food to the rest of the tree.  Bark can be used to identify tree species from the way it grows, its color, and its texture.  However, bark can look different from tree to tree, and from branch to branch on the same tree.

There are several layers to a tree.  The innermost layer is called wood and is made up of dead cells from previous year’s growth.  Wrapped around the wood is the vascular cambium where new cells are growing inward to become the xylem layer that transports water and nutrients from the roots upward, and new cells growing outward to become the phloem that transports sugar and nutrients from the leaves to all parts of the tree.  During winter, the need for food and water is much less as the tree enters a non-growing, dormant state, but circulation continues enough to keep the tree alive and hydrated. 

Bark consists of several layers starting with the phloem, also known as the inner bark.  The periderm makes up the outer layers of bark protecting the phloem and other inner layers from environmental elements.  The cork skin is the first layer of the periderm, containing cells to store food and waste products including tannins and resins.  The cork cambium is the second layer where new cells are growing to add to the outside and final layer, the cork.  Cork, mostly hollow, dead cells, prevents desiccation, provides insulation, and is a protective barrier from fungal and bacterial diseases. 

Bark’s appearance is caused by both internal and external factors including growth, gas exchange, and  the environment.  At what rate are new cells being added to expand the tree?  Does the bark stretch?  Are there cracks or breaks in the bark?  Bark thickness may vary from one-quarter inch on a mature beech to over an inch on a mature oak.  Are there visible openings in the bark?  Lenticels are specialized cells in the bark allowing an exchange of gases with the outside environment.  Is temperature speeding or slowing growth?  Has fire influenced the area?  What affect has the wind had?  Has the tree been attacked by bacteria, fungi, or animals?  Have other plants used or damaged the tree? 

Six bark appearance categories are commonly used to distinguish trees:

1. Smooth: Cork maintains a constant thickness throughout the life of the tree, and old cells slough off easily.  An example is the American beech, Fagus grandifolia.
2. Horizontal peel: Expansion causes the external layer to peel away in strips when it is still quite thin.  An example is river birch, Betula nigra.
3. Visible lenticel: Often a different color than the bark, lenticels can be oval, round, linear, or diamond-shaped.  Some retain their shape over time, while others stretch as the tree grows.  An example is pin oak, Quercus palustris.
4. Vertical cracks: Cracks often begin at lenticels, and they may be a different color depending on the color of the phloem layer that is exposed below.  An example is pignut hickory, Carya glabra.
5. Scales, plates, or vertical strips: Each of these structures are separated on one or more sides from one another, and appear as overlapping sections of a similar size.  Examples include black cherry, Prunus serotina, and American sycamore, Platanus occidentalis.
6. Ridges and furrows: As trees with hardened cork expand, the cork begins to crack.  Sections are held together by fibrous tissue, creating furrows between ridges.  Examples include Eastern cottonwood, Populus deltoides, and bur oak, Quercus macrocarpa

As you walk in the woods this season, consider taking along a tree guide to see how many trees you can identify by their bark.  In a future installment on ecology, we will take a closer look at the role bark plays in the relationship between trees and their environments.

Villages, public parks, and forest preserves in our area contain thousands of trees for our enjoyment.  Being able to identify some or all of these can make our walks more fun.  In the last blog, I identified several tree characteristics that can be used in tree identification in the winter.  Here are some of the more common trees you may find in this area, along with their winter identification characteristics.

Silver maple, Acer saccharinum, is a native Illinois tree with bright yellow/green fall leaf colors.  When looking at a twig, the terminal and lateral buds will all appear slightly reddish in hue and rounded or pointed.  Remember that lateral buds are where new flowers or leaves will grow.  If the buds are clustered, this is where flowers will bloom on the twig.  The twig is slender and gray to red in color, with buds arranged opposite one another.  If you scratch the twig with a fingernail and take a sniff, there is an unpleasant odor.  The bark is smooth on young trees, becoming shaggy on older trees.

The hackberry, Celtis occidentalis, a favorite street tree, is a large, fast growing tree that can reach heights of 40 to 60 feet.  Native to Illinois, the hackberry may live for 150 years or more.  Twigs in winter lack a terminal bud, but have lateral buds that are brown, flat, and triangular in shape.  Lateral buds display an alternate arrangement  along very thin twigs.  As each new twig grows at a slight angle from the bud, branches take on a zig-zag appearance.  The bark, smooth and light gray, is covered with light colored lenticels, and is often times covered with wart-like bumps, their density being greatly variable.  Cutting open a twig reveals a banded pith with visible cavities.

White oak, Quercus alba, is one of our most popular native hardwood trees.  The state tree of Illinois, white oak is used for building dozens of items including cabinets, watertight barrels, floors, caskets, and pianos.  In winter, look for bark that is light gray to silver in color.  The bark can be quite variable from tight, shallow ridges to broad, loose flaky plating.  Many trees exhibiting different bark formation on different parts of the trunk.  Twigs  display small, rounded, and reddish buds.  Lateral buds are alternate  on twigs that are slender, smooth and slightly reddish in appearance.  Terminal buds are clustered with broad, hairless scales.

Black walnut, Juglans nigra, is a slow growing tree with a large, shady canopy and rich, dark-brown hardwood.  The wood will not warp, can be polished to a high luster, and is highly prized for furniture and veneers.  The buds are gray with a light, fuzzy coating of hairs and arranged alternately along the twig.  The leaf scars are heart-shaped and contain a very distinct bundle scar that looks like a “monkey face”.  The pith is chambered, a distinctive trait found only in black walnut and butternut trees.  The bark on this tree is dark brown, with high ridges and deep furrows often arrayed in a diamond-shaped pattern.

Bitternut hickory, Carya cordiformis, also a member of the walnut family, is a medium sized tree, growing 50′-80′ tall with a narrow canopy.  This wood burns with an intense heat and is often used for smoking ham, bacon, and other meats due to the distinctive flavor imparted by the smoke.  Twigs are thin, shiny, and greenish- to grayish-brown.  The alternately arranged buds are bright yellow to sulfur-yellow, covered by two to four large scales that join one another along the edges without overlapping.  The bark is light gray, appearing in a diamond-pattern with very shallow ridges.

These trees are some of the most commonly planted in urban communities and found in area forest preserves.  All are native to Illinois, are hardy in urban settings, and provide food and shelter for many native wildlife species.  Winter identification characteristics are easily visible and a great place to learn how to use a key.  Take a walk and get started soon!

Many people can identify an oak tree or maple tree during the summer, but how can you ID a tree without leaves?  There are several basic characteristics used in tree identification.  Many of these are readily visible in winter.  When looking at a tree, inspect several of the twigs, the smallest branches on a tree, to rule out any anomalies that may be present on only one.  Different characteristics may appear on more than one tree, but when taken all together, a unique combination of traits will identify a single species.  As you read the following, a drawing of a twig and all its relevant parts can be referenced at https://mekwamooks.wordpress.com/winter-id/.

The terminal bud, located at the tip of the twig, is where new growth will start from in the spring.  Note its size relative to the twig, the bud’s shape, and whether or not it is covered with scales.  Buds may be naked, not having any scales, or covered with overlapping scales, or scales may meet at their edges.  Lateral buds are arranged along the length of the twig, and they are the site from which a new leaf or flower will grow next year.  They will appear on the sides of the twig as you move along it.  The arrangement may be opposite, when two buds are at the same point on either side of the twig, or alternate, when a bud on one side is spaced a few inches along the twig from the next bud on the other side.

Lenticels are specialized cells where gas exchange takes place during the growing season.  They appear as light or dark spots along the twig.  Note the color, size and amount found on a twig, or whether there are none.  Running your fingers over the surface of the twig allows you to tell if the lenticels are smooth or rough and raised.

When leaves fall off the tree, they leave behind a leaf scar.  Take note of the size and shape of a scar.  Is it round, oval, heart-shaped or some other shape?  Inside the leaf scar is the bundle scar, where the phloem and xylem layers that transport water and food entered each leaf.  The arrangement of the bundle scar in addition to the size and shape of both scars is very helpful in determining the tree species.

The pith inside the twig is another part to use in identification.  You will need to break off a twig and slice it lengthwise with a sharp pocketknife to see the center structure of the twig.  It can be solid, hollow, spongy or chambered.  This structure is found in young branches and is used to store and transport nutrients throughout the plant.  As branches mature, storage and transportation will move to the xylem and phloem layers found just underneath the bark. 

Bark, another highly visible feature, comes in a variety of colors from gray to tan to dark brown to black.  Many species may have a hint of red tones.  Identifying the texture of bark is very important.  Is it smooth to your touch?  Is it covered with warty patches?  Some trees have bark that peels off, and it is important to note how the peel starts.  Is it the top and bottom ends of each strip that are loose, or the middle that is pulling off to the right or left?  Is the bark paper thin and coming off in loosely curled sheets?  Thicker bark has a much rougher appearance.  Does the bark run in ridges and furrows?  Does it appear to be a diamond-shaped pattern on the tree?  Some bark looks blocky, with no discernable pattern.  Are the blocks flat to the tree or are there deep furrows between the blocks?

As you accumulate clues, you can match them against identification guides.  A special type of guide, known as a key, will help you identify many of the species found in a given geographic area covered by  the key.  Keys present a series of questions, each with a limited number of choices to select from.  An example is “Winter Tree Finder” by May Theilgaard Watts and Tom Watts, “for identifying deciduous trees in winter.”  This book uses a series of questions about twigs to lead the user to a correct identification.

There are many keys and field guides available, but remember to choose ones that pertain to the time of year you are in.  Keys for flowers and leaves will not be of great value in winter, and many of the characteristics discussed above, such as scars and buds, will not be available to view in summer.  I urge you to get out and try this fun activity before spring.

Publications to get you started:

Autumn officially began just a few short weeks ago, but we are already able to observe the changing colors and leaf fall in the woods.  There is much about nature that we can appreciate at this time of year, with observation and a little help from books, blogs and research.

Leaves on trees produce food for the rest of the tree.  They do this by a process called photosynthesis which combines carbon dioxide, water, pigments and energy to create sugars and oxygen.  The sugars are used to feed the rest of the tree.  The oxygen is a by-product that is released into the surrounding atmosphere.  The various pigments include chlorophylls, which allow a plant to absorb energy from light; carotenoids, which also assist in energy absorption; and xanthophylls, which protect the photosynthesis process from the toxic elements of light.  All of these pigments give leaves certain colors.  During spring and summer, chlorophylls are present in the highest concentrations, and they give leaves their green color.  In autumn and winter, as the number of hours of light per day gets shorter, the photosynthesis process slows down and finally ceases.  The amount of chlorophyll decreases and color from the other pigments starts to show through.  Carotenoids are orange and yellow, while xanthophylls are yellow and brown.  Another pigment that is only present when there is more sugar being produced than used by the plant is anthocyanin, and it colors the leaves red and purple.

Colors in autumn may be brilliant in some years, or more muted in other years.  This intensity is determined by the weather conditions during late summer and autumn.  Carotenoids and xanthophylls are always present in the leaves during food production season.  Dry periods in late summer reduce the amount of sugar being produced.  Thus, the red and purple hues from anthocyanin may be muted or not present at all, allowing more of the orange, yellow, and brown pigments to be displayed.  Colors may be more muted with less moisture in the leaves and the length of time the leaves remain on the tree may be shortened.  

Color is also affected by temperature.  When days are warm and sunny, leaves produce a lot of sugar.  At night, in cooler weather, leaf veins constrict and limit the amount of sugar flowing to the rest of the plant, thus creating an excess of sugar remaining in the leaves each day.  This is when we see lots of red and purple hues from anthocyanins displayed.

While leaves are falling, trees are preparing for spring in other ways.  During senescence, that time of year when leaves grow old, carbohydrates, nitrogen, sulfur, phosphorous and potassium are reabsorbed by the tree in great quantities from the leaves.  None of these elements are lost, but they are stored in twig, stem and root tissues.   They will be used in springtime to fuel the beginning of the next year’s growth. 

Another way for a tree to get a head start on next year comes from bud growth.  Buds are formed in late summer or early fall, and are covered with modified leaves called bud scales.  These will seal them against the cold and wet weather of winter.  Most of the buds that you can observe are leaf buds containing tightly packed, immature leaves.  In spring, when the weather gets warm enough for sap to start flowing, these buds will unfold into the first leaves of spring.  Larger size buds may be flower buds, depending on the species and age of the tree.  Flower buds do not change much in the winter months, but they will grow a bit larger as we get near bud burst in spring.  You may also observe terminal buds, found at the ends of branches.  Oak trees, as well as other species, add length to existing branches when these buds begin their spring growth. 

There are numerous good field guides about trees available, but I would like to mention four  that I use.  “Winter Tree Finder” by May Thielgaard Watts and Tom Watts instructs you in how to look at a twig and its structures.  Then the book guides you through an examination of a twig to identify the genus and species of deciduous trees in winter.  “Trees of Illinois” by Linda Kershaw is another book organized using keys with excellent pictures of leaves, buds and fruit for each species, along with ranges and characteristics of each.  “The Tree Identification Book” by George W.D. Symonds provides a wide variety of black and white photographs for every part of a tree including thorns, leaves, flowers, fruit, twigs, buds, bark and needles.   Peterson Field Guides presents “Ecology of Eastern Forests,” with chapters describing the plants and animals encountered in different types of forests, and how they all function together.  Other chapters talk about how forest patterns change with the various seasons.