Trees and Bees
Tree Biology
Rust Never Sleeps
Telial spore horns of cedar-apple rust, Gymnosporangium juniperi-virginianae, on eastern redcedar.
Aecial spore horns of Gymnosporangium sp. on serviceberry. Probably G. clavipes, cedar-quince rust.
Lesions caused by the aecial stage of cedar-apple rust, on crab apple.
Lesions on apple leaves caused by cedar-apple rust
Take a walk and smell the trees
Southern magnolia, Magnolia grandiflora, flower
Carolina silverbell, Halesia carolina, flowers
American holly, Ilex opaca, flowers
Sugar maple, Acer saccharum, flowers
Leaves Are Wearing Out
Insect and fungal damage on Shumard oak leaf
Sweetgum leaf with fungal and insect damage
Gender identity and the mystery of the persimmon fruit
What does gender mean? We know that human gender is a broad and fluid landscape within which people choose to identify themselves. This is increasingly recognized as a healthy and welcoming way to think about who we are. Gender fluidity is true in many animal species, perhaps the majority. But what about plants?
The photograph above is a mystery. Not its identity. It is a persimmon fruit lying on a driveway in Castlewood Park, Lexington. The mystery is the tree next to it. This is the only persimmon in the Park. I have known this tree for over 25 years and see it regularly, at least once a year. The mystery is that this is the only fruit this tree has ever produced, to my knowledge. And therein is the the tale of gender fluidity in a tree.
Persimmon, Diospyros virginiana, is a gorgeous tree with very distinctive bark. It is an important component of many forests from river bottoms to rocky hill slopes. In the Bluegrass, it is often found on farm hedgerows and the edge of woodlots. Persimmon is closely related to ebony (Diospyros ebenum and related species), the tree with dark brown or black wood famous for making the black keys on a piano, the pegs and fingerboard on violins, and fine clarinets. The heartwood of persimmon is also very dark, but most trees have only a little heartwood with a much paler sapwood. Persimmon was prized for making golf clubs known as “woods”, though today only a few artisans still make authentic persimmon clubs.
So why is this fruit so strange? As with humans, tree species often lie upon a spectrum of genders. This solitary fruit brings us to ask “what is the sex of this tree?” Many trees are monoecious, with male and female reproductive organs in the same individual tree. Others are dioecious, with male trees and female trees. A large number of trees lie somewhere in the middle. Some trees are mostly male with a few female flowers. Others are the reverse. Botanists have made up dozens of words to describe various expressions of sex, such as polygamodioecious (mostly one sex, some flowers of another), subgynoecious (mostly female with a few male or bisexual flowers). Charles Darwin devoted much of his career to the forms of sexual expression in plants, writing three books on the subject and recognizing dozens of different patterns of sexual expression.
For twenty five years, I have thought that this tree is male. Persimmon is one of the most strongly dioecious tree species in our flora. Trees are either male or female. Period. Yet here is a tree that has either been male the whole time I have known it, or has been asexual. Scan as we might, my friend and I could not see another fruit in the tree or on the ground. Is this tree in transition from male to female, and will make more fruits in subsequent years? Did one branch or one flower suddenly decide to be female for just a single occasion? We don’t know, but this single fruit certainly compels us to keep an eye on the tree and await further developments.
Gender identity in higher organisms is complicated, subject to evolutionary pressures that we barely understand. Gender identity in trees is complicated, difficult to understand, and fluid. Why should it be any different in humans? It shouldn’t.
A persimmon, Diospyros virginiana, in Castlewood Park, Lexington
Stem of persimmon tree in Castlewood Park
Death of an Oak
This beautiful old oak tree was struck by lightning last September. Within 2 days, it had completely wilted and showed no signs of life. Sudden death due to lightning is uncommon in trees, especially after the end of the growing season. Several experts recommended leaving the tree until spring to see if it showed any signs of recovery. Sadly, the tree did not leaf out this spring. Because the tree is in a public park next to a popular basketball court, it had to be removed promptly. The photos below show the tree before and after death and the process of removing it. We have collected a section of the trunk and will use it to determine the tree’s age.
Before it was hit by lightning, this was a remarkably vigorous tree with no signs of death or decline. Within days after being struck, the leaves had wilted, twigs were dead and the fungus Biscogniauxia atropunctata had popped out on the major branches. Biscogniauxia is an endophytic fungus, living quietly within the bark of healthy trees and fruiting rapidly when the tree is stressed.
The tree before being hit by lightning.
The tree after felling
Section of the tree for ring counting
The tree three days after it was struck by lightning
Slicing the tree to count rings
The gray material is the fungus Biscognauxia atropunctata
A Question of Timing
Leaf exchange in holly
We know that trees usually lose their leaves in autumn, which is why we sometimes call the season fall. A few trees do things differently, though. One of those is American holly, Ilex opaca.
Holly is a tree that we usually call evergreen because it stays green year-round. But if you look carefully at this time of year, you will see yellow leaves on holly just as the new leaves are emerging. Have a look at the pictures to the right. You will see the young developing shoot of this year’s new growth, but you will also see yellow fading leaves on the tree and on the ground.
This unusual pattern is called leaf exchanging. The new leaves are emerging as the old leaves die. The tree is evergreen in the sense that it is always green, but the leaves are replaced each spring.
For the next couple of weeks, you should be able to see this pattern in our American holly and some of the ornamental hollies. [Scroll down for the story of a fly with good timing]
Leaf Exchange in American holly. The yellow and dark green leaves are from last year, the pale green shoots are this year’s growth
A Fly With Good Timing
The timing of leaf exchange in holly is interesting to us, but for a one insect, it is a matter of life and death. The native holly leaf miner, Phytomyza ilicicola, is a tiny fly. The fly larvae live inside the holly leaf, protected from enemies by the tough, leathery holly leaf. The problem for the female fly is how to get her eggs into the leaf. It is not possible for the fly to penetrate the mature leaf with her ovipositor, as it is too tough. She needs to deposit her eggs in the young leaves as they develop. Perfect timing is essential. The fly spends the winter inside the leaf, in the mine made the previous year. As soon as spring comes, the mature fly needs to emerge from the leaf, mate, and deposit eggs in the young leaves. If she emerges too soon, there will be no young leaves. If she emerges too late, the leaves will be too tough. Like the porridge in Goldilocks, the leaf must be just right.
Reference: Kimmerer and Potter 1986 Oecologia 71:548-551
The track of a native holly leaf miner larva in a mature holly leaf
Holly leaf with a leaf miner track
The Wolf Trees of Nashville
When you hike through the woods in Nashville, you don’t expect to encounter wolves. If you keep your eyes open, though, you may see some wolf trees.
Foresters used the term ‘wolf tree’ to indicate a very large tree with a broad crown and a short main stem. These trees were considered wolves because they were so large that they devoured sunlight that other trees might need. These trees were often killed because they had no market value.
Today, we have a changed opinion of these huge trees. They are now understood to be very important as wildlife habitat and also in regenerating a new forest after harvesting or damage. Harvesting rules, especially on state and federal forest land, often require that wolf trees be left in place.
Wolf trees serve another important purpose: they can tell us a lot about the history of a forest. On a recent walk through the Warner Parks of Nashville, I noticed something striking. Throughout these hilly, dense woods were many huge wolf trees. A closer inspection showed that these were species typical of the woodland pasture habitat: chinkapin oak, Shumard oak, blue ash, and kingnut (I have not yet seen any bur oak).
Their enormous size and low, spread branches tell us that these trees did not grow up in a forest, but in either an open woodland or woodland pasture. The level ground surrounding the Warner Parks has a large number of woodland pasture tree species and several intact woodland pastures. I suspect that this part of Nashville represented a continuum from woodland pastures on level ground to open woodlands and the slopes and denser forest on the upper slope.
The Warner Parks are about to lose their large population of white ash to emerald ash borer. I made a quick estimate of the stocking (timber density) of white ash in the Warner Parks and estimated it to be more than 20%. As these trees die over the next few year, conditions may favor the ability of the wolf trees to reproduce.
I look forward to learning more about this beautiful area.
A magnificent wolf tree – an old chinkapin oak in Warner Parks, Nashville
How Old is that Tree?
Everyone wants to know how old a particular tree might be. We will discuss this complicated and important subject in two stories. This month we will talk about unitary trees, and next month we will tackle the complex problem of aging clonal trees.
A unitary tree is like a person – a single, integrated organism. Unitary trees have one or a few stems on a single root system. The stem is the same age as the root system. Most tree species are unitary, although some, such as oaks, beeches, and red maples, will create a new stem or a few stems when the original one is lost so that the root system is older than the stem.Clonal trees are colonies of many stems, sometimes thousands, on a single root system. The entire clone, including all the stems and the root system, is a single organism. Over time, some root connections may be lost and the tree no longer functions as a single organism, even though the separated parts are genetically the same as each other.
Clonal trees are colonies of many stems, sometimes thousands, on a single root system. The entire clone, including all the stems and the root system, is a single organism. Over time, some root connections may be lost and the tree no longer functions as a single organism, even though the separated parts are genetically the same as each other.
As most of us know, trees in temperate climates generally produce a single ring of wood each year, growing fast in the spring, slowing down in the summer and producing a distinct ring. It should be a simple matter, then, to count the rings and find out the age of the tree. We can do this either by cutting the tree down or by taking an increment core, a narrow cylinder of wood from the bark to the pith at the center of the tree. This is not as simple as it sounds. Obviously, we don’t want to cut down ancient trees unless they are dead or in danger of falling and doing harm. Increment cores are very labor intensive both to obtain the wood sample and to prepare the sample and count the rings. Very large trees like our bur oaks are extremely difficult to core.
Determining the age of an old tree by carefully counting and cross-dating the rings using valid methods, followed by publication of the results in a peer-reviewed journal is the gold standard for determining and confirming the age of a tree. On that basis, the oldest known tree in the world is a bristlecone pine, Pinus longaeva, originally collected by Ed Schulman in the 1950s but only analyzed by Tom Harlan in the past couple of years. This tree, now 5065 years old is still alive and growing well in the White Mountains of California. Understandably, its location is kept secret. This tree is accepted as the oldest unitary tree known in the world.
In the Bluegrass and Nashville Basin, the majority of our very old trees are hollow. You can’t count the rings in a hollow tree unless you take a sample above the hollow part, and then you may miss many years of growth.
In Europe and Asia, the age of very old trees may be known from historical records, such as the date a yew tree was planted in a churchyard, or a sacred bodhi tree was planted at a temple. We rarely have that luxury in North America with its very short written history. A number of trees planted by Thomas Jefferson, William Bartram and other botanists and enthusiasts survive today and we can date them precisely. William Bartram planted a yellowwood (Cladrastis kentukea) at his garden in Philadelphia in 1805. Badly damaged by a wind storm in 2010, the tree recovered and is thriving today at age 210.
Any other method of determining the age of a tree is very imprecise. The relationship between tree size and age, especially in our open-grown hardwoods, is not a strong one. Trees growing slowly in poor conditions often outlive trees in better conditions.
Over time, a number of biologists have developed criteria to assess whether a tree is very old. Neil Pederson summarized the most important characteristics of very old hardwood trees: 1) balding bark – bark smoother than younger trees of the same species; 2) low stem taper – the main stem is cylindrical, not narrowing with height; 3) sinuous, winding stems and branches; 4) crowns with few, very large and twisting branches; 5) low crown volume relative to the stem diameter; 6) low leaf area to trunk ratio. For the ancient trees of the Bluegrass and Nashville Basin, two additional characteristics are 7) leaves tufted at the ends of branches; and 8) main stem rarely present, with the top of the tree damaged and decayed. While these sound complicated and difficult to analyze, with a little bit of practice, it is not difficult to separate our oldest trees from younger ones.
Using these criteria and a knowledge of the terrain and land use history, we can usually sort our trees into categories. For example, an extremely large bur oak growing at the top of a slope that shows the above characteristics was probably growing before the area was settled. That would make it somewhere in the range of 300-500 years old, possibly older (we have found some older trees but have not yet published the data). A bur oak of similar size, growing along a creek with good access to water and showing fewer of the old-tree characteristics may be half the age of the one at the top of the hill.
I know this answer frustrates a lot of people anxious to know the age of their favorite trees, but the plain answer to “how old is my tree?” is usually “I don’t know but I think it is several hundred years old. Based on the limited scientific evidence that we have, plus some historical data, I am often comfortable saying that our biggest old trees are “probably 300-500 years old, maybe older.”
And as to all the claims of the great age of trees that we see on the Internet, we should treat them with a great deal of skepticism. That 6,000-year-old baobab? It might be 1,000 years old. The Angel Oak said to be 1,500 years old? More likely to be 400-500 years old. These trees are still very old and deserving of our respect and veneration, and there is really no good reason for people to make such wild claims. And what about that 9,000-year-old spruce called Old Tjikko, the subject of thousands of Facebook claims and blog posts? As we will see in our next story on old trees, that claim does not hold up either.
Veteran’s Park Oak in Lexington, a very old unitary tree
National champion smooth sumac, Rhus glabra, Anacardiaceae
A chinkapin oak that was growing in Lexington before 1779. This is at 14′ off the ground because the base was hollow.
Hollow trees cannot be aged with accuracy. Hackberry.
A Deep Green Autumn
A deep green autumn is rare in Kentucky. By this time of year, our trees usually look forlorn and bedraggled. Dry summers, high temperatures, insects and diseases all take their toll. Not in 2015, though. Our frequent rainfall and cool temperatures have allowed many trees to maintain lush green leaves and to continue growing. Some trees are still producing new shoot growth.
Leaves are a bit like tissue paper, designed for use for only a short time and then discarded. From the time a leaf is formed, a tree invests a limited amount of resources into maintaining it. Shade, drought, insects and fungi all take their toll, so by August, a lot of trees look pretty sad and pale. This is especially true in the Bluegrass and Nashville Basin, where our karst topography leads to water stress even in modest droughts.
The last three years (2013-2015) have been different. Tree growth has extended well into the fall. Today, the first day of meteorological autumn, sees many trees still growing, and leaves of some trees staying dark green.
2015 is even more exceptional. I have rarely seen trees still growing in September, but some of our bur oaks, even some very old trees, are still producing new flushes of growth.
See what the trees in your neighborhood are doing.
Bur oak leaves in August
Eastern redbud leaves, Cercis canadensis
The Old Schoolhouse Oak in August 2015
September flush of growth in bur oak
How many trees species?
When I worked in Borneo, I was fascinated by the immense diversity of trees in the jungles where I worked, even though I could only identify a few species. I did not even know how many different trees I was seeing. They look so much alike, with some exceptions, that I could walk by a rare, unrecognized tree without knowing it. And that raises the question that has long puzzled biologists: how many kinds of trees are there?
We know that in North America there are about 1,000 tree species, and similar numbers in temperate Asia. Europe has a more limited tree flora, with 250-500 species. The number of tropical trees has never been known with any accuracy.
Now, a worldwide study lead by Prof. Ferry Slik of Universiti Brunei Durussalam, along with 170 colleagues from all over the world, has come up with the most accurate estimate to date, and it is quite astonishing. The problem is that most tropical species are rare, and easily missed in species counts. Instead of analyzing species lists or herbarium specimens, the researchers analyzed a database of over 200 intensively studied forest plots worldwide.
Their conclusion is that there are 40,000-53,000 species of tropical forest trees in the world. This is much higher than any previous estimate or calculation, which typically miss rare trees.
This number, and the high frequency of rare trees, has important implications for conservation. If we are to prevent extinction of large numbers of tropical tree species as the world warms up, we need to change our conservation approach. Instead of focusing on conservation of individual species, as is commonly done with wildlife species, tree species conservation will only succeed if we can set aside very large areas of tropical forest. This could allow rare species to maintain self-sustaining populations. Climate change may result in the loss of large amounts of tropical moist forest, and this will inevitably cause extinction of many rare tree species. Addressing climate change and tropical forest conservation are critical if we are to avoid the loss of a large percentage of tropical tree species.Similar principles can apply to temperate forests. Conservation of large tracts of forest land could help prevent the loss of species as the world warms.
Quercus sumatrana in Sumatra
Tropical forests of West Kalimantan in 1983.
The Great Flush of 2015
Have you seen the Great Flush of 2015? In Central Kentucky, trees are growing very fast right now, producing new growth that is noticeable for its pale green or yellow colors. Late flushes are complicated responses to changes in soil moisture. This is the first of several stories about the Great Flush of 2015, but also about weather patterns and, perhaps, about climate change.
What is a flush? Think of a bud as a package of tiny leaves waiting for spring. The leaves were made late last year. As the weather warms in spring, the little package begins to open and out come the leaves, quickly growing to capture sunlight. As the stem grows, the distance between leaves increases. Soon, growth ends for the year. It is surprising that, in most trees, shoot growth ends by June, long before the growing season ends. A new bud sets and the leaves for next year begin to develop. Here are couple of examples in kingnut and American beech.
This is not the only style of shoot growth. Some fast-growing trees like yellow-poplar continue producing and growing new leaves throughout the summer until shortening days or drought bring an end to their growth.
But what happens in a year like 2015? We are having an exceptionally wet July, setting records in many counties throughout Kentucky and adjacent states. We are usually in mild to moderate drought at this time of the year, but right now soil is exceptionally moist. That means that growing conditions for trees are excellent.
How do trees respond? Instead of waiting out the year and growing again next spring, many trees produce a new flush of growth. Suddenly, it looks like spring again. We don’t know exactly how this works, and why some trees like dogwood ignore the high moisture and snooze until spring.
For oaks, hackberries and many other trees, the Great Flush of 2015 is easy to see. The pale color of the second flush contrasts with the deep green of the first flush. Some people see these colors as an early sign of autumn, but the opposite is true: it’s a sign of a renewed spring.
The amount of growth can be astonishing. Here is a bur oak produced a moderate first flush but an huge second flush:
Why are the second flush leaves so pale? Here are two pictures that provide a simple explanation: On the left is a bur oak, on the right is a black locust. Both are continuing to produce new leaves but the locust leaves are dark green. Black locust is a nitrogen-fixing tree species, continually producing nitrogen in root nodules. The bur oak depends on nitrogen from the soil and can’t make its own. In the spring, the soil contains lots of nitrogen from decay over the winter. The tree also contains a lot of nitrogen stored in the form of protein, which it uses to make new leaves in spring. By mid-summer, both the internal reserves of nitrogen and soil nitrogen are depleted. So, the new leaves don’t have enough nitrogen to make an adequate supply of nitrogen.
In the next part of the story, we’ll talk about the trees that don’t quite growth so early. In the third part, we will talk about the weather and climate factors that great great flushes.
Scenes from the Great Flush of 2015
New leaves on a defoliated bur oak
A couple of weeks ago, we told you about a magnificent, ancient bur oak that was suddenly defoliated overnight. We said that it would recover quickly and that we would update you. With 10 days of mild weather, the tree has leafed out very quickly. Most trees maintain large reserves of starch and protein in their stems, and they can quickly mobilize the stored material to make new leaves. Here are a couple of before and after pictures. The upper picture is the tree on May 21, right after it was defoliated. The lower picture is the same tree on May 31, showing how quickly the tree has leafed out. We still haven’t found the culprit, but this old tree does not seem to have suffered from early spring defoliation.
Flowers, Pollen and Allergies
Is that tree causing your allergies? That pretty tree with the white flowers? That pine tree covering your car in green film? Nope. It’s the trees you don’t see that are getting you.
This is the height of allergy season. You can feel it in your sinuses and see it on your car windows. Huge amounts of pollen are flying through the air, seeking out female flowers with which to mate. There are many misconceptions about pollen, tree flowers and allergies.
In my experience, many people are confused about what trees cause allergies. The beautiful showy flowers of spring trees like black locust or flowering crab are not the cause of allergies. These flowers are designed to attract insects, hummingbirds and other pollinating animals. They do not toss their pollen into the air, but wait for animals to carry pollen from tree to tree.
It is the tree flowers we don’t notice that are the culprits. Oak, Osage-orange, hickory, and lots of other trees produce long male flowers called catkins that drop huge amounts of pollen into the air. You may not notice the flowers, but your respiratory system does.
Pine, spruce and other conifers don’t produce flowers, but they do toss huge amounts of pollen into the air. However, their pollen is so large that few people have problems with conifer allergies.
So, if you have to curse at a flower this spring, don’t pick on the pretty ones.
The Sap Is Rising
“The sap is rising” is an often heard description of early spring. If you cut into the stem or branch of certain trees – sugar maple, birch, hickory, walnut, or sycamore – on a cool spring day, you may see sap dripping from the cut end, or an icicle of sap forming. The sap is slightly sweet and is the source of maple syrup. Yet if you cut a tree a few weeks earlier or later, nothing happens. What is going on?
During the winter, when trees are dormant, the stems store large amounts of starch, a polymer made of long chains of glucose, a simple sugar. The starch is stored in living cells in the wood, called parenchyma. When nights are cold and days a bit warm, enzymes in the stem break down the long polymers into the simple sugar sucrose. Suddenly, the number of molecules in the parenchyma cells goes from a small number of starch polymers to a huge number of small sucrose molecules. The sucrose creates an osmotic potential that causes water to flow into the parenchyma cells. This is the simple and familiar principle of osmosis. As water moves into the cells, the pressure inside the cells rises. Some of the sucrose is pumped out of the parenchyma cells into the dead xylem cells. Water continues to flow from the soil, raising the pressure in the stem. When a branch breaks or is cut, the pressure causes the sap to flow out.
In maple syrup production, a metal spile is driven into the stem and connected to plastic tubing, allowing the sap to flow from the tree to the sugar shack. Maple syrup production is only commercially practical in places with the right weather – cold soil, cool nights and warm days. The cool nights promote the conversion of starch to sugar, while warm, sunny days allow water to flow from the soil into the stem.
In Kentucky, where I live, commercial maple production is not practical. The spring tends to warm up rapidly and there is usually little snow to insulate the soil, making the sap flow season too short to make any money. A year like 2015, though, could be a good sap year. I have already seen bleeding sap in a number of maple trees.
The positive pressure inside the stem lasts for only a few days to weeks. Trees do not push water up the stem, they pull it from the top. For the entire growing season, the stem is under strong negative pressure (<0). Negative pressure is not something familiar in everyday life: it is not possible to create a pressure less than zero (a vacuum) in a gas. But it is possible in tightly constrained narrow columns of water in the xylem of a tree.
In the best years for sap production, spring weather produces cold soil, cold clear nights and warm days. Sudden spring warming, as happens more frequently as climate change take hold, reduces sap yields.
Early Spring Trees
You may feel we are in the throes of winter, but for many trees, it is already early spring. How can that be?
In the late summer, trees begin to enter a stage of deep sleep called dormancy. They don’t stop growing because it is cold, they stop growing because a combination of long nights and cold sets up a complex hormonal response. If you take a twig off a tree in late fall and bring it inside, giving it plenty of light, warmth and water, it will probably do nothing. It is not simply asleep, but dormant, and only one thing will wake it up – more cold.
By now, in most North Temperate regions, some trees have gotten enough cold to overcome dormancy. They are poised to begin growing as soon as the weather improved. Other trees are not ready yet – they need more cold to overcome the dormant state.
If you are a careful observer, you should begin to notice buds beginning to swell, bud scales changing color, and even a few trees beginning to show flower parts. These trees are ready for spring and only need a few warm days to begin growing. In my area, red maple, silver maple, and elms are ready to go.
If you don’t want to wait, try bringing some twigs inside. Set them in water like a flower arrangement, and see what happens over a few days. Some trees will do nothing – they are not yet ready and need more cold days. Oak, beech and hickory often require longer periods of cold. Other trees will pop in a few days.
Why do trees hold on to their leaves?
Why do trees hold on to their leaves in winter? We get asked this question quite often. The short answer for the impatient reader is sex, or more specifically, puberty.
In some years, cold weather comes on so suddenly that leaves are killed by frost. Last year, a sudden cold snap came on the heels of an exceptionally warm Autumn, killing the leaves. In a more normal year like this one, trees gradually prepare for winter by creating a corky layer in the petiole that cuts off the leaves after nutrients have been moved into the stem for storage.
Although most leaves have fallen, many trees are still holding onto leaves in the lower part of the crown. This is most commonly seen in oak, beech and sugar maple.
We botanists love fancy words, and there is one for this: marcescence (from a Latin root meaning ‘to shrivel’). Here is what happens: the corky layer that cuts off the leaves fails to completely grow, leaving the leaf still attached, if tenuously, to the tree. Over the course of the winter, the leaves gradually wear out and are dropped. Any remaining leaves are pushed off as growth resumes in the spring.
If you search the internet you will find erudite explanations of why trees have marcescent leaves, but they all fall into the category of “Just So Stories.” Rudyard Kipling wrote a volume of Just So Stories for kids in which he provided fanciful, often silly, explanations for things kids might see in nature. You may remember reading these, including the most famous one “How the Camel Got its Hump.” (Answer – through laziness). Today, biologists use the term “just-so stories” for flights of fancy that are not supported by any actual information. Just so stories are most commonly told about the supposed adaptive significance of a certain trait with little or no evidence. Telling stories about supposed adaptations is not necessarily a bad thing – it can lead to a hypothesis which can be tested. There are a lot of just-so stories about trees because we know so little about their biology.
Let’s ignore the just so stories and focus on the one thing we do know about why trees hold their leaves. It has to do with sexual maturity. Many trees hold on to their leaves when they are not sexually mature. In trees, sexual maturity is defined by flowers (or other reproductive organs). Before a tree starts flowering, it is immature. Flowering may begin at anywhere from a few years to many decades, depending on the species and the growing conditions.
OK, but then why do some very big trees hold on to their leaves? It turns out that, for reasons we don’t know, trees keep some branches – especially those low in the crown and close to the main stem – in an immature condition. This juvenile zone is easy to see if you watch a tree over a year or so: the parts of the tree that hold onto leaves through the winter will not bear flowers or fruit the following year.
So it is simply sexual maturity that determines whether the leaves fall off or not. Why should the immature parts of the tree hold on to their leaves? We have no idea, but if you’d like to make up a just so story, feel free. I suspect (and perhaps this is my just so story) that the juvenile zone has different plant hormone balances than the mature zone. Since hormone balances have a lot to do both with sexual expression (flowering) and leaf abscission, it may simply be that immature branches lack the proper hormone balance to complete the abscission layer.
One practical use of this knowledge is in plant propagation. It is very difficult to root cuttings from mature parts of a tree, but often much easier to root cuttings from the juvenile zone, so observing which branches retain leaves is a quick indicator of the juvenile zone.
Persistent leaves in American beech
Persistent leaves in Shumard oak
How the Camel Got Its Hump. Drawing by Rudyard Kipling, 1902
Dead trees and what they can tell us
Dead trees are fascinating because they provide us with a permanent record of their lives. The annual rings that record the tree’s experience with drought, nutrients and temperature are familiar to most of us. Somewhat less familiar, but easy to see, is the record of all the insults, accidents and stresses of life as a giant, long-lived organism.
The photos in on this page are from a very old blue ash (Fraxinus quadrangulata) that died and fell over a few years ago. As the bark decayed and sloughed off, the wood was exposed, showing us the most recent wood made by the vascular cambium. The cambium is an amazing tissue. It is a continuous sheath of stem cells that lie between the wood (xylem) and inner bark (phloem). All the wood in the world is made by the cambium of trees. The stem cells of the cambium are constantly adjusting their cell division to make the appropriate amount of wood and phloem, and to react to changes in their environment, including gravity, wind and wounds.
I think that the cambium serves as a pressure sensor – the pressures created by the wood to the inside and phloem to the outside, combined with pressure changes due to wind and gravity, determine what kind of cells the cambium produces.
The wood just below branch junctions is heavily wrinkled. This represents the combined influence of gravity and wind. The branch constantly flexes in response to wind, and the cambium below constantly adjusts to stabilize the main stem. On the main stem, the ripples in the wood are also probably due to wind and gravity, but are more subtle. These surface ripples are reflected deeper in the wood, and lead to the figure so highly prized by woodworkers.
Once the tree is dead and down, and the cambium is dead, other processes begin. Wood is under tremendous stress and strain in a living tree. As soon as the tree dies, the wood begins to crack as its moisture content changes. The cracks provide easy entry for decay fungi, protists and insects. The holes in the wood are made by beetles that feed on dead wood. Many of these beetles carry fungi with them that accelerate decay. Over time, insects, bacteria, protists and fungi gradually take the tree apart, converting the wood to energy and biomass. How fast this happens depends on temperature and moisture: in moist tropical forests, a tree may disappear completely in a few months. In drier or colder forests, complete decay may take centuries or may never happen.
The process of death, decay and recycling is critically important to forest health and stability. Without decay, nutrients would not be returned to the soil and be made available to the next generation of trees. Wood decay enriches the soil.
A magnificent dead blue ash tree
Ripples in the wood below a branch are due to gravity, wind and pressure change.
The ripples are due to the production of wood by the cambium. The cracks are from the internal stresses as the dead tree begins to dry. The holes are from beetles that bring in fungi to decay the wood.
The plight of butternut
Butternut trees, Juglans cinerea, are beautiful relatives of the more common black walnut. They get the name ‘butternut’ from the rich, fatty nuts, which taste better than walnuts. Another name for the tree is ‘white walnut’ because the wood is lighter in color than black walnut wood. You may already know butternut from eating its nuts or working with its wood, but the tree is disappearing before our eyes.
Butternut trees were very common in New York and Wisconsin when I was in school. In Wisconsin, forested slopes often had many butternuts mixed with other upland hardwoods. Beginning in 1967, though, butternut canker disease has been spreading through the entire range of butternut. In some areas, more than 80% of butternut trees have died from the disease. In Central Kentucky, most of the trees that I saw in the woods in the early 1980s are gone, and the ones that are left are mostly infected.
Butternut canker is caused by a fungus with the unwieldy name of Ophiognomonia clavigignenti-juglandacearum (Oc-j for short). The fungus infects butternut trees through small cracks or wounds and causes a canker – a dead area of tissue under the bark. The tree fights against the infection by creating a chemical compartment around the infected site and growing new tissue around the compartment. However, the fungus can create multiple cankers. Once these cankers encircle the stem, death is certain. Oc-j can also infect other Juglans species, especially heartnut, but black walnut is quite resistant.
Where did this fungus come from? Nobody knows for sure. The rapid spread of the disease and the high rate of death of butternut suggest that this is an introduced disease. Genetic analysis of Oc-j shows very little genetic variation. Apparently, the fungus consists of three genotypes, or clones, that reproduce asexually. The sexual form of the fungus is not known. The low genetic variation of the fungus is also evidence that this is an introduced fungus.
Without action, butternut trees may disappear. Although butternut has been proposed for listing as a threatened or endangered species under the Endangered Species Act, the Fish and Wildlife Service has taken no action. Listing of the tree as threatened or endangered would trigger some federal and state action and may provide financial support for finding resistant trees.
Several universities are trying to identify resistant butternuts. The University of Tennessee has an active program to collect butternut seeds and plant them in plots underneath infected trees. The seedlings that survive under these conditions may be resistant, or partly resistant. Once resistant seedlings have been selected, they can be crossed to create a new generation of trees which can then be tested for resistance. As you might imagine, this is a process that takes a very long time and with an uncertain outcome.
The plight of butternut illustrates a major problem in trees all over the world. International trade, on the rise for hundreds of years, is bringing trees into contact with insects and diseases that they have never encountered before. You have heard of the big ones – chestnut blight, Dutch elm disease, emerald ash borer – but you may not have heard of all the hundreds of other foreign organisms that trees are being exposed to. And it’s not just in North America – new diseases are cropping up throughout Europe as well.
We can’t stop international trade, though we can do a better job of requiring treatment of imported wood and plants. What we need to do, then, is to ramp up our ability to respond more quickly to threats, and to begin breeding and selection programs for resistance. We don’t seem to learn this very well – our response to emerald ash borer has been too little too late.
I am optimistic that as long as funding is available for butternut canker disease resistance programs, the species will survive. But its glory days are behind it.
Giant Cane in the Bluegrass
Giant cane, Arundinaria gigantea, was abundant in the Bluegrass before the area was settled. Bison herds maintained cane, grazing on it but then leaving for long periods, allowing the cane to recover. When bison were replaced by cattle, sheep and horses, the cane quickly disappeared. Natural stands of cane are today quite rare in the Bluegrass. Cane was abundant, dense and tall. Josiah Collins tells of stepping off the buffalo trace that is now Harrodsburg Road into a tall stand of cane, getting lost, and coming out three days later near Ft. Boonesborough. Filson’s map from 1784 and tales like Collins’ tell us that cane was abundant, dense, and tall.
In spite of its density, cane did not prevent the Bluegrass tree species from regenerating. All the important Bluegrass trees are tolerant of shade when young. It is common in cane stands to find tree seedlings growing slowly in the shade of the cane.
Today, cane is found at scattered locations in the Bluegrass. Some cane stands are recently planted, while others are remnants of larger stands from the past.
In contrast to the vanishing cane, invasive species like bush honeysuckle are taking over forested areas of the Bluegrass. Honeysuckle forms a dense understory that prevents trees from becoming established. Removing honeysuckle is often ineffective, or temporary, because the plant can regenerate from stump or root sprouts, and because birds carry the abundant seeds everywhere. For honeysuckle removal to be effective and long-lasting, something needs to be planted in the understory or mid-story to replace it. Various native shrubs such as rusty blackhaw or spicebush can be effective, but it is difficult to plant shrubs densely enough to keep honeysuckle out.
Giant cane could be an effective replacement for honeysuckle along woods margins, where honeysuckle is most successful. Cane won’t grow under the deep shade of a forest canopy, but will grow well on the canopy edge where moderate sunlight is available.
Giant cane mixed with trees creates excellent wildlife habitat, and is quite attractive. It can be a bit aggressive in spreading out, but mowing around the edges is an effective control.
There are several problems that make giant cane difficult to establish and maintain. It is notoriously difficult to transplant, requiring considerable skill to be successful. Few nurseries carry it, and propagating by dividing roots of existing plants often fails. Like other bamboos, giant cane is monocarpic – it flowers and bears seeds only once, then dies. This only happens at long intervals of 10-20 years. That means that seeds are rarely available and that, periodically, an entire stand will die. Usually, a stand that dies has produced enough seeds for a new generation of cane to take over from the previous one.
In the next couple of years, Venerable Trees will initiate experiments on some farms in Horse Country to restore cane and use it as a management tool to control honeysuckle. One potentially effective way to use cane to reduce maintenance costs would be to fill tree pens with a mixture of native trees and cane. The cane would eliminate mowing costs and keep the honeysuckle out. The photo to the left of Shumard oak and black walnut with giant cane resembles what we have in mind for tree pens – just add a plank fence.
A farewell to the autumn colors of ash trees
Ash trees, especially white ash, are among our most reliable and beautiful trees for fall color. White ash shows rich shades of red, purple, yellow and green, all within the same trees and the Biltmore ash variety is cloaked in yellow and orange hues. Green ash takes on an astonishingly bright yellow hue.
Sadly, the emerald ash borer is wiping out this brilliance. In some central Kentucky neighborhoods, most of the ash trees are already dead. The city of Lexington is beginning a program to remove dead and dying trees – a program that should have begun five years ago.
Some ash trees are being treated by their property owners, and these will remain in the landscape for some time. I suspect that many homeowners will eventually give up treating their trees, and the rest will die.
For now, we can enjoy the glorious colors of this iconic tree. Nothing can be done to slow the spread of emerald ash borer or to protect trees in the forest from the beetle. In urban areas, high-value trees can be protected. Foresters are now on a search for living white ash and green ash trees in places like southern Michigan, where the beetle has killed nearly all of the trees. If some living trees are found, it might be possible to select for resistance to the beetle,and, over the long run, return ash trees to their former glory.
Blue ash trees lack the spectacular colors of other ash species, their pale green fading to a mottled, subdued yellow. Although perhaps not as pretty a tree, blue ash are very abundant in the Bluegrass and very long lived. They are the most abundant of the Venerable Tree species that create our woodland pastures. The good news is that research in Michigan suggests that blue ash trees are largely resistant to emerald ash borer. It appears for now that we are likely to keep this very important tree.
Leaves in a Warm Autumn
In a warm autumn, as we are having in 2016, leaf color change is slowed down. Instead of a quick display of color, we often see slow development of color, muted colors, and mottled colors within a single leaf. We had the same conditions in 2014.
Trees use photoperiod, or day length, to determine when to begin casting off leaves. The duration and color intensity of autumn is determined by temperature, with shorter duration and more intense colors when night temperatures are low. Photoperiod does not vary from year to year, but weather does.
In a warm autumn, leaves do change color and eventually fall off, but they do so more slowly and with more subtle colors than in a cooler autumn. As the world continues to warm, autumns like this one will be more common.
Click the picture for a slide show of leaves in a warm autumn.
What is your favorite autumn tree?
What is your favorite autumn tree? Mine changes on a daily basis, but my current favorite has to be sassafras, Sassafras albidum. I like trees like sassafras and sweetgum that display many colors in a single tree.
Sassafras is a great urban tree in fairly large spaces such as parks. It reproduces from root sprouts, so one tree will lead to many.
Click on the sassafras picture below for a slide show of the splendor of sassafras in autumn.
Make a comment below or visit our Facebook page to tell us about your favorite autumn tree.
Ginkgo Trees Are Lovely – Let’s Stop Planting Them
Cities are heating up because of a combination of climate change, the urban heat island effect, and a loss of urban tree canopy cover. We can mitigate some of these effects and increase the resilience of urban environments if we plant more trees. The wrong choice of trees, though, can reduce urban biodiversity and may make cities less resilient.
There are few trees more glorious than ginkgo in the autumn. It is easy to overlook how abundant ginkgo trees have become in our cities until we see the blaze of yellow up and down the streets.
As most people know, ginkgo is an ancient tree, found worldwide in the fossil record but today only found in cities and in a tiny area of China, where its survival has probably depended on Chinese monks and botanists for thousands of years (See Peter Crane’s marvelous book Ginkgo: The Tree That Time Forgot for the whole story).
Over the last 150 years, ginkgo has found a place for itself in cities throughout the world’s temperate zones. It is tolerant of air pollution, which was probably the attribute that made it so popular. It also handles other urban stresses well, from soil compaction to excess salt. Ginkgo is almost entirely free from the insects and diseases that beset other trees in urban environments.
And therein lies the conundrum of ginkgo – it seems to be such a perfect urban tree that we plant them by the millions. In my city, Lexington Kentucky, we almost seem to have an obsession with ginkgo. All the pictures on this page were taken within a few blocks of the center of town (click the pictures for a gallery).
If all we cared about was shade, ginkgo might be a good choice. But urban biodiversity is also important. Trees in urban areas can support many other species – birds, small mammals, insects. In fact, a highly diverse urban canopy can appear to many organisms as a forest. Cooper’s hawks are forest birds that have become quite abundant in cities, and there are several nesting pairs in my urban neighborhood.
Ginkgo may not be a good neighbor for other species. I have carefully inspected a number of ginkgo trees in my neighborhood this summer. It is remarkably difficult to find insects on ginkgo trees. Douglas Tallamy, in his excellent book Bringing Nature Home, says that an urban oak may host over 500 species of caterpillars while ginkgo hosts only one. Ginkgo seeds, with their strong odor that many people find offensive, are probably adapted to be distributed by carnivorous animals, but in urban area, only squirrels will eat ginkgo, and it is not a preferred species for them. Acorns, hickory nuts and other native fruits are much preferred.
Not surprisingly, birds avoid ginkgo as well. Most resident birds spend time in trees where food is available. Since there are no insects in ginkgo, birds tend to avoid them. This summer, I only saw birds in ginkgo trees as casual visitors, flitting through on the way to somewhere else.
The genetic diversity of planted ginkgo trees is also low. Ginkgo in China appears to originate from only a few remnant populations, suggesting that the species has been through a genetic bottleneck of very small populations in only one or a few geographic areas. In urban areas, the requirement for male trees, to avoid the offensive odor of females, means that only a few clones are planted, further reducing genetic diversity. A city that contains thousands of ginkgo trees may in fact contain thousands of copies of a few individuals. While ginkgo is currently not troubled by pests and pathogens, having only a few clones is a risk for future problems.
I love ginkgo trees, as most of us do. But ginkgo is not helping to create diverse, resilient cities. Instead of endless planting of ginkgo, we urgently need to diversify our urban forests with diverse plantings of seedlings of many species.
Let’s resolve to enjoy our urban ginkgo trees, but not to plant any more.
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