Have a look at the leaves of trees around you. Chances are pretty good that they don’t look healthy. Fading leaves with holes and other insect signs, mold and other fungi and a generally bedraggled look are normal at this time of year. What is wrong with the tree? Probably nothing.
Leaves look bad at this time of year for one reason: they are disposable. Leaves are the botanical equivalent of Kleenex – made to be used for a short time and then thrown away. As a result, trees have a limited ability to repair leaves, and the injuries from weather, pests, and pathogens continue to accumulate through the growing season.
These sad-looking leaves are actually the beginning of a complex series of steps that trees take to get ready for winter. The most important process in getting ready for autumn is for the tree to withdraw critical nutrients into the stem or roots. For a tree to toss away leaves rich in protein and other forms of nitrogen is a waste of resources. While the bugs chew and the fungi grow, trees are busy pumping all the important nutrients out of the leaves and into the stem and roots. These changes are subtle at first, but will soon be visible as autumn colors begin to develop.
This is not to say the trees are free of disease. Many serious problems, especially wilt diseases, show themselves at this time of year. These diseases are often fatal, though usually slowly. It sometimes takes a degree of expertise to figure out whether sick-looking leaves are benign or signs of a serious problem. Wilt diseases that limit the access of leaves to water are usually fatal and largely untreatable. Sometimes, the services of a plant disease diagnostic lab are needed to figure out whether a tree is truly sick.
Close observation of trees at this time of year can be rewarding. The development of autumn colors often starts very early. Some trees, such as walnut and hackberry, have already begun dropping leaves and turning yellow.
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
The emerald ash borer, a shiny green beetle, is emerging from ash trees all over the eastern US and Canada. The beautiful little beetle, part of a group called “metallic wood boring beetle” was introduced in packing material from China around 1990. Carried throughout the region by people moving firewood, the beetle threatens to eliminate white, green and black ash. There is some hope that parasitic wasps introduced from China by the USDA may spread quickly enough to slow the beetle, but this is far from certain. Below you will find a slide show of the beetle and the damage done.
A female emerald ash borer feeds on leaves and will soon deposit eggs on the bark of an ash tree.
A female ash borer ready to lay eggs
The adult borers eat ash leaves before they lay eggs on the bark.
The borer larvae make wandering galleries in the cambium, xylem and phloem. This is what kills the tree.
In spring, the adult beetles emerge by tunneling through the bark, leaving little piles of sawdust.
The adults emerge from the stem, making a characteristic D-shaped exit hole.
The borer begins at the top of the tree and works its way down, sometimes taking several years to kill the tree.
When the beetle is done, cities and forests alike are left with dead trees.
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.
Many trees are full of showy flowers at this time of year. A close look at these flowers can be quite rewarding. Flowering dogwood is one of our most popular flowering trees, but those beautiful white blossoms hide a secret. Take a close look at a dogwood tree and you will see that the showy white “petals” are not actually petals. Instead, these are white bracts that direct the attention of a pollinator – bee or moth – to the center. Now look at the center and you will see a cluster of tiny green flowers.
Each of these tiny green flowers is a “perfect” flower, containing both male and female organs. Pollinators are attracted by the showy bracts, but it is the tiny flower that is pollinated. Soon, you will see a cluster of small fruits developing, though dogwood usually loses all but a few fruits before they ripen.
Trees are huge and easy to look at from a distance, but they also reward a closer look.
Inflorescence of flowering dogwood. The white bracts (not petals) surround a cluster of flowers
Flowering dogwood inflorescence. The arrow points to a single flower
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.
All over Kentucky, there is an explosion of the amazing white flowers of black locust, Robinia pseudoacacia. This is a tree that is both loved and hated. While it is in flower, I thought we’d take a few moments to talk about the virtues and sins of black locust.
Here are the things we love about black locust:
We love its flowers. Everyone in black locust country raves about the beautiful white clouds of flowers and the intense, sweet fragrance. I personally think it is the best-smelling flower of all.
Bees love its flowers. Beekeepers usually allow clover and locust honey to be mixed. However, in years when locust flowers before clover, it is possible to harvest a large quantity of the palest, clear, intensely sweet honey ever. The years when I have been able to keep the locust honey flow separate from the clover have been my favorite beekeeping years.
Black locust is a nitrogen-fixing pioneer tree on disturbed sites. It will colonize cut banks, abandoned coal mines and other very poor sites and enrich the soil for other species.
For early settlers, it was a tremendously important timber source for masts, tool handles and fences. The hard, decay-resistant wood was prized.
Black locust is now the most widely planted North American tree in the world. It has been planted all over Europe, temperate Asia and southern Africa. It is used for livestock fencing, fuel and tool handles.
Here are the things we hate about black locust:
Black locust is impossible to get rid of. If we cut down a tree in our yard, sprouts will come up for decades.
It takes over pastures very quickly and is hard to control.
The sharp little spines hurt! I got one buried in my head while clearing fields in North Carolina when I was a kid and I still have a little bump in my head.
It is invasive. Black locust is regarded as a nuisance or invasive tree on every continent except Antarctica.
Have you seen the sugar maples in flower? In Kentucky, they are flowering right now, but you need to look closely to see them. The flowers are not very show, their pale green blending with the green riot that is spring. These interesting flowers will reward the careful observer. Sex in maples is complicated. Even though sugar maple flowers are perfect – they have both male and female parts – most of the flowers low on the tree, as in this picture, are functionally male – the female parts don’t work. High on the tree, the opposite is true, and many flowers are functionally female. For a wind-pollinated tree like sugar maple, this division of labor may prevent pollen falling directly on the stigmas of the same flower. Maples are self-infertile, so the pollen of one tree can’t fertilize the flowers of the same tree.
A drop of sap from a fresh cut on a sugar maple branch.
“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.
When starch is converted to sucrose, water flows into the cell.
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.
An icicle of maple sap
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.
What good are knees on a tree? Baldcypress famously has these odd structures that stick out above the water line. Botanist Francois André Michaux saw knees on baldcypress trees and, in 1819, said “No cause can be assigned for their existence.” Nearly 200 years later, what do we know about the function of this strange structure? We’ll get to the evidence shortly, but first lets take a closer look at this remarkable tree.
Baldcypress, Taxodium distichum, is in the cypress family, the Cupressaceae. This family includes some of the most ancient lineages of trees, dating back to the late Jurassic, about 150 million years ago (mya). The cypress family includes the world’s tallest tree, coast redwood (Sequoia sempervirens); the world’s largest tree in volume giant sequoia (Sequoiadendron gigenteum); and some of the longest lived trees. Many members of the family are relicts, species that were once widespread but are now confined to very small ranges, often with unusual climate or soil conditions. Because the family is so ancient, the distribution of modern species are best explained as a result of plate tectonics, especially the breakup of the supercontinent Pangaea 100 mya.
Baldcypress knees and a buttressed stem the autumn when the water line is low. The white lines show different water lines during the spring and summer.
The modern members of the cypress family are mostly restricted to harsh or special environments where they don’t have to compete with the more efficient hardwood (angiosperm) trees. Baldcypress is found in the swamps of the Atlantic and Gulf Coastal Plains, or on soils that are waterlogged at least part of the year. In that habitat, it is a slow-growing tree that can live thousands of years.
One of the biggest challenges to life in a swamp is the variable water level. If the water is too deep, the tree will die from oxygen stress. If the soil dries out, drought stress and competition from hardwoods like red maple may eliminate the baldcypress. In between is where we find baldcypress growing the best. And it is here that we find the two characteristics of baldcypress – the buttressed lower stem and the knees sticking out of the water.
Baldcypress on a swamp margin with a strongly buttressed stem but few knees. The white lines show seasonal changes in the water level.
Lots of trees make buttresses, especially in the tropics. Buttressed stems and roots are a response to very poor soils that are either low in nutrients or low in oxygen. As a result, the root system remains near the surface of the soil. The buttress allows the tree to stand up to the stresses of gravity and wind without having a deep root system.
But the knees are still a problem because almost no other tree has them. We see occasional looped roots and other odd growths in other trees, but nothing like the knees.
Early botanists were quite certain that they knew what the knees were for and they gave the knees a fancy name “pneumatophores,” meaning air roots. They thought that the purpose of knees was to allow the root system, deep in the anaerobic swamp water, access to air. In the absence of any evidence, this seemed reasonable. You still see many descriptions of knees that assert that they play a gas exchange role.
In spite of the certainty of early botanists, there is no evidence that knees play any role in gas exchange. Plant organs that are adapted for gas exchange have several important features – lenticels and aerenchyma. Lenticels are areas of the bark or epidermis that have a lot of air space, allowing oxygen and carbon dioxide to exchange through the bark. Aerenchyma consists of special cells with a lot of air space in between the cells. The problem is that baldcypress knees have no lenticels or aerenchyma.
Lenticels (red) in the bark of lacebark elm help the stem to exchange carbon dioxide and oxygen with the atmosphere.
A few scientists have measured gas exchange between baldcypress knees and the air. Paul Kramer, the renowned plant physiologist at Duke, and his students found no evidence of significant gas exchange and concluded that knees do not play a significant role in bringing oxygen to the root system.
Knees could play a structural role, making the root system stronger in the face of high wind, but there is no evidence for this either. It would be quite difficult to measure the mechanics of a tree with and without knees. Since trees with and without knees grow in slightly different habitats – with knees in shallow water, without knees on dry land or deep water – it’s hard to see how one could design a proper experiment.
And there we are, exactly where Michaux left us nearly 200 years ago. We do not know what baldcypress knees are for. Big trees are hard to study, and field work in swamps is difficult and expensive. There are not a lot of research funds to allow scientists to pursue esoteric subjects like the function of knees. I suspect that we may be using the same explanation as Michaux for a long time to come.
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.
Second flush of growth in the wet growing seasong of 2013. Bur oak. Click for a gallery of late season shoot growth
Trees are generally very conservative in their growth habits. They often go to extremes to avoid rare events such as an early frost or severe drought. This is especially true of shoot growth. Although we think of trees as growing all summer, most trees only grow in height for a few weeks. Flowering dogwood sets a bud and quits growing in May or June, and so do most maples and oaks. In 2013 and 2014, conditions were good enough for many trees to pause for a while and then produce a second flush of growth. Click on the photograph for a gallery of trees with late-season flushes of growth.
Late season shoot growth in yellow-poplar. Click for a gallery of late season shoot growth in yellow-poplar.
Fast-growing trees like yellow-poplar, Liriodendron tulipifera, have a different pattern, one that is much riskier but allows for faster growth. In the spring, yellow-poplar produces a flush of growth from the bud, just like other trees but then, instead of setting a new bud and pausing, yellow-poplar continues to crank out new leaves without a bud. This is a fast-growth strategy, one taken by the majority of fast-growing trees. This year, yellow-poplar leaves are beginning to turn color as the days get shorter, but the trees just won’t quite making new leaves. This may have to do with the unusual buds of yellow-poplar – each leaf is encased in a clam-shell arrangements of bud scales (actually stipules) so that even if a newly-developing leaf is exposed to cold, the next leaf is protected. Look carefully at yellow-poplars, cottonwoods and other fast-growing trees and see if they are still growing. Click on the photograph for a gallery of yellow-poplars with continuous growth.
Although we might expect trees in tropical moist forests to have the same continuous growth pattern as yellow-poplar, the vast majority, in my experience, have recurrent flushes of growth with a pause in between This may be a strategy to avoid having young, tender leaves constantly available for hungry predators.
Look at the two leaves below. The oak leaf has flat, rather dull drops. The redbud leaf has round, shiny drops. If you look around at leaves after a rain, you will see some leaves with round, shiny raindrops like the redbud, while other leaves will have flat drops like the oak, or will shed all the drops and appear dry. Scroll down past the pictures to learn more.
Water drops on Shumard oak leavesWater droplets on redbud leaves
The diagram below shows the difference between the leaves. Leaves are covered in a waxy cuticle. The structure and chemistry of the cuticle on the redbud leaves repels water more strongly. As the water is repelled, it pulls away from the leaves and forms an almost spherical ball, which reflects light. The angle between the leaf and the water is the contact angle – the more obtuse the angle, the higher the water stands from the leaf. The oak repels water with less force, leading to a more acute contact angle – more of the water is in contact with the leaf, and the flattened pool of water reflects less light. Scroll down past the pictures for more.
A. Oak leaf with raindrops; B. Redbud leaf with raindrops; C. Contact angle of water on oak leaf. The less waxy oak leaf allows more of the water drop to be in contact with the leaf surface; D. Contact angle of water on a redbud leaf. The waxy redbud leaf repels water, pushing the water into a tighter ball with less contact with the leaf.
Why the difference between leaves? A very water-repellent cuticle reduces water loss from a leaf. The difference in the ability of the cuticles to repel water is mostly due to chemical differences in the wax, but also in the structure and thickness of the cuticle.
Waxy leaves are often more drought resistant than less waxy leaves, and redbud is certainly very drought tolerant. But that is too simple an explanation. Other factors, such as leaf thickness and density of hairs, are important in drought tolerance. And leaves may be waxy for reasons other than drought tolerance. A heavy coating of wax can protect the leaf from UV radiation, insects, and pathogens, such as fungi, bacteria and viruses. Most plant structures have evolved to play multiple roles, so we can’t say that the redbud leaves are more water repellent only for drought tolerance.
Whatever the reason, leaves with waxy cuticles like redbud produce very attractive patterns on the leaf surface after a rain, making them more interesting to photograph.
