The second flush of growth
spring
When is a flower not a flower?
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
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.
In Praise of Black Locust
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.
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More tree sex – sugar maple
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.
We have lots of other articles about the fascinating biology of tree sex.
Sycamore in Spring
American sycamore (Platanus occidentalis) flowers in late spring and creates the well-known sycamore ball that kids are fond of throwing at each other (not quite as painful as sweetgum balls). The fruit is unusual because it stays on the tree all winter as a hard, dormant ball. The following spring, the ball begins to expand and soften, soon releasing thousands of feathered achenes – tiny fruits, each containing a single seed. These achenes can float long distances in the air or on the water, and quickly germinate when they find damp soil. (Note: Sycamore in North America is called plane-tree or buttonwood in the United Kingdom, while the word sycamore is used to refer to a maple, Acer pseudoplatanus. Confusing? That’s why Latin names are important.)
The smell of spring
On my walk yesterday, the world was filled with the smell of spring. The smell of flowers and barbecuing meat will be along soon enough, but early spring is dominated by the earthy fragrance after rain and the wonderful, evocative odor of new-mown grass. Lawnmowers were whining away last weekend mixing the smells of new mown grass and damp earth.
Where do these smells come from, and is there a deeper meaning to these odors than just a pleasant spring fragrance? Many of the wonderful smells of spring are of semiochemicals – chemicals produced by an organism that can affect the behavior of other organisms.
The new-mown grass smell is mostly from four closely-related six-carbon molecules that are produced by the breakdown of fatty acids. Together, they are called green leaf volatiles (GLV). As a cow or mower takes a bite of grass, these molecules are quickly produced by leaf enzymes and just as quickly evaporate. In pure form, each GLV has a pleasant, spicy fragrance, but it doesn’t remind me of the smell of new mown grass. It is the combination of the six-carbon molecules, and probably some others as well, diluted in the air, that combine to give us that wonderful smell.
It is what happens next that is quite remarkable. We are not the only creatures that can smell green leaf volatiles. We can think of green leaf volatiles as alarms, warnings and invitations. Entire communities respond to the GLV chemicals wafting through the air. For nearby plants, GLV is the equivalent of Tolkein’s Horns of Buckland sounding “Awake, Fear, Fire, Foes, Awake” and plants that smell the GLV immediately mobilize their defensive chemicals to ward off herbivores. I use “smell” here in the sense that receptors in the plant detect the chemicals, just as receptors in our nose do.
Insect herbivores that want to eat the plant can also smell the GLV. To them, it may say “someone is already eating this plant,” and they will lose interest and go elsewhere. Other insects may be attracted by GLV. For a predatory or parasitic insect looking for another insect to eat, the odor says “Hey, there are bugs eating a plant, c’mon over.” Many wasps, for example, are attracted to GLV and will attack insects that are eating the plant. So in two ways, GLV act as plant defenses: they tell nearby plants to prepare for herbivores, and they tell nearby predators and parasites to attack the herbivores.
So, while you are smelling the wonderful odors of new-mown grass, there is an entire community of plants and animals that are responding to the GLV.
And what of the earthy smell? That is another interesting story of chemical communications. Earthy fragrance comes from several sources, but the two major odors are geosmin and 2-methylisoborneol (2-MIB). Geosmin is made by an important group of soil bacteria in the genus Streptomyces. You may recognize that name as being related to the antibiotic streptymycin, and in fact many antibiotics come from soil bacteria. Geosmin and 2-MIB are also associated with poor-quality water and wine.
What might geosmin communicate to other organisms? Prof. R. Meganathan suggests that geosmin might be a cue to the availability of water. Camels
are thought to be able to smell water in an oasis from 50 miles away. Since water has no smell, it may be the geosmin that lures the camel. What does that do for the Streptomyces? Camels drinking from an oasis might carry the bacteria from one oasis to another. Of course, this is one of those Just So Stories that is hard to validate. In humans, geosmin may warn us of low-quality water or wine. The smell is often associated with fish raised in aquaculture ponds, but it can be eliminated by rinsing with lemon juice, which breaks down the geosmin.
As you take your spring walks and inhale that heady mixture of damp soil and new-mown grass, look around and think of all plants, insects and other creatures that might be sniffing the air as well.
Fringetree in Flower
Fringetree is one of the most elegant small trees in our flora. Here is a slideshow about fringetree at The Lives of Trees.
Time for Services
Serviceberry trees are blooming all over the Appalachians. Although other trees like birch and alder may flower earlier, the first showy flowers are serviceberries, coming a couple of weeks before the dogwood. The slopes of all the forested hollows in eastern Kentucky are dotted with these elegant, slender trees.
In the old days, before good roads and easy transportation, serviceberry flowers played an important cultural role. You see, when a person died in the winter, it wasn’t possible to hold a service and a burial. The ground was frozen solid, and a proper grave could not be dug. With deep snow and poor roads, it was difficult to travel from one holler to another.
Serviceberry blossoms provided the signal, more reliable than the calendar. The haze of white flowers in the hollows meant that the ground was thawing and it was time to go to funerals. The body in its yellow-poplar or chestnut casket, would be stored in the barn until the serviceberries bloomed.
That is, of course, how this tree got its name. It has lots of other names – the number of names of a tree is, I think, a measure of its usefulness. Sarvis is a variant of Serviceberry. Shad, or shadbush or shadblow was another set of names having to do with telling time – when the shadbush blooms, the shad are running in the creeks and it is time to go fishing. Pieberry or pietree, which I have only seen a few times, is an indication of something later in the calendar – the berries are delicious in jams or pies if you can beat the birds to the fruit. I have three serviceberries in front of my house, but if I wait long enough for the fruits to fully ripen, the birds get them first.
There are several species of serviceberry. Our common tree-sized serviceberry, Amelanchier arborea, common or downy serviceberry and Amelanchier laevis, Alleghany serviceberry, are difficult to tell apart except when the leaves are just breaking bud. Then, the leaves are downy, as the name suggests. There are several other less common species, but by far the most popular ornamental is the shrubby Canada serviceberry, Amelanchier canadensis. Although not native to Kentucky, it is abundant in states to the east.
Serviceberry flowers in the spring when the soil is thawed
Ripe fruit of Canadian serviceberry
Flowering schedules in alpine flowers are changing
A 39-year record of wildflower blooming in the Rocky Mountains shows that climate change has altered the timing of blooming for most species in an alpine meadow. (Link is to University of Maryland press release. Original paper published in PNAS).
The blooming season now runs from late April to late September instead of late May to early September. The response to climate change is complicated: some plants flower earlier, others for a longer time.
The complex changes in flowering times are producing what ecologists call “no-analog” communities that are not closely related to existing communities. In the case of the meadow flowers, hummingbirds can no longer rely on a huge spring peak in blooming when rearing their young. Instead, the same number of blossoms are spread out over a longer period of time. This reduces the amount of nectar available each day.
Phenology is the study of the scheduling of events in an organisms life. Long-term plant phenology studies are pretty rare. David Inouye, a distinguished conservation biologist at the University of Maryland, started this project in 1974 because of his interest in nectar sources for hummingbirds and bumble bees. He has continued this project every year since, along with his students and post-docs.
This study is important for several reasons:
- It shows the value of very long-term studies. Long-term research is hard to sustain and even harder to fund. Yet without long-term studies like this, we would detect only the most severe and catastrophic effects of climate change.
- It shows that the impacts of climate change are complex, with each species responding differently, and with relationships among species changing radically.
- It demonstrates the importance of studying mountain habitats, where the effects of climate change on the lives of plants and animals can already be seen.