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March-April 2015

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The Sex Life of Floodplain Trees

By Pamela Eyden
With special thanks to Carol Jefferson
March-April 2015 Big River Magazine

These silver maples leafing out in the backwaters are lined up along the 'strand line,' where seeds end up when floodwaters recede. (Steven Katovich, USDA Forest Service, Bugwood.org)


In late winter, when the river is full of ice and the woods are brown and black and white, knowing something about tree sex could brighten your walk. While your cheeks turn red from the cold, you can look for voluptuous reddish-purple tree flower buds. Red pigment shields sex parts from damaging ultraviolet light.

Trees use a variety of strategies for reproducing themselves. Many use both asexual and sexual reproduction. Asexual reproduction happens when a tree sprouts a clone of itself from its roots or from the base of its trunk, or when branches and twigs break off and float away and become rooted elsewhere, like willows and cottonwoods do. For sexual reproduction, a male gamete fertilizes an egg of the same species to create a seed.

Tree gender can be flexible and even changeable — it's not so much a matter of hard-wired destiny as a matter of roles. Some trees may consistently take on a male or female role. Some may have male and female flowers on different branches on the same tree. Some may have some male flowers and some (hermaphroditic) flowers with both male and female parts. Plants generally don't have sex chromosomes, but have multiple sex-determining genes.The gender of most tree flowers may be determined by their environment, maturity, neighbors and where they are on the tree.

Silver maples (Acer saccharinum) are common floodplain trees that grow fast and live long — about 130 years. Their range stretches the length of the river, from the headwaters at Lake Itasca to the Gulf.

Silver maples are the first tree to bloom in the bottomland forests of the Upper Mississippi River. They face a dizzying array of possibilities: A tree may produce all male flowers, all female flowers, mostly male with a few female flowers or mostly male with hermaphroditic flowers. Also, tucked inside every cluster of female flowers are males that won't develop, and inside male clusters are female flowers that won't develop.

Winona State University ecology professor emeritus Carol Jefferson says to look for blooms in late February or early March, unless the winter is bitter cold and long, like the winter of 2013, when silver maples bloomed a month late.

"When people say, 'Oh, look, the leaves are already coming out,' it's usually the flowering parts they see," said Jefferson. Leaves come later.

Why do silver maples flower so early?

“They have to flower before the floods come because they need to get food resources up from their roots and they can't do that when soil is saturated,” Jefferson said. After that, silver maples have a high tolerance for flooding.

Use binoculars to look for tiny reddish-yellow flowers on the top branches of the trees. These are male flowers. Squirrels depend on them for food in the hard times of late winter. As they mature, the male parts extend and turn pale before shedding their pollen to the wind. Silver maples need spring winds to blow the pollen around. In the river valley, people's late-winter allergies are often caused by wind-borne silver maple pollen.

Female flowers are usually on the lower branches of trees that support both male and female flowers. They bloom later, timed to prevent trees from self-pollinating. Twenty-four hours after flowers catch the pollen, seeds begin to grow inside the ovary. Three weeks later the winged “heli- copter” seeds mature and twirl to the ground or water, about the same time floods are receding.

“Seeds float on floods, and when water goes down the seeds are left in strand lines,” Jefferson explained. “That's why you see long lines of silver maples in the backwaters.”

The great American elm (Ulmus americana) may have nearly disappeared from city streets and boulevards, but there still are a lot of elms in the backwaters, according to Randy Urich, La Crescent, Minn.-based forester for the Army Corps of Engineers.

Once they were the dominant tree in the backwaters, but they don't get as big as they used to. Now they tend to grow large enough to seed, then they succumb to Dutch elm disease, caused by a fungus carried by elm bark beetles.

Elms flower right after silver maples. Look for elm flowers before the leaves are out. Each tree has both male and female flowers. Like silver maples, they use timing to avoid self-pollination — on any given tree the male pollen is ready before the female flowers.

Elms release their pollen to the wind, which carries it to female flowers. After pollination, it takes about 30 days for the seeds — those pale green, fringed disks that hang in clusters — to mature. Seeds can be carried on the wind for a quarter of a mile, although they can be carried by water for miles before getting waterlogged. Seed fall is usually complete by early June. If seeds have landed in good spots, they will germinate in just a few days.


Top: Male catkins of a swamp white oak. (Katy Chayka, minnesotawildflowers.info)

Above right: Swamp white oak acorns mature from female flowers pollinated by male catkins high in the tree. (Paul Wray, Iowa State University, Bugwood.org)

Top: Female elm flowers.

Right: Male elm flowers. (both Paul Wray, Iowa State University, Bugwood.org)


The Dutch elm disease epidemic has benefited the hackberry (Celtis occidentalis), another grand tree of the floodplain, which has filled the gaps left by elms in many areas. Hackberry flowers appear in early spring and are quite tiny. Male and female flowers occupy the same branchlet, males at the bottom and females — which look like the heads of green caterpillars sporting soft green antennae — at the very top. These trees, too, are wind pollinated.

Top left: Female cottonwood developing seed. (Dave Powell, USDA Forest Service)

Top right: A male cottonwood catkin before opening. (Bill Cook, Michigan State University, Bugwood.org)

Above: Cottonwood male catkins open to spread pollen on the wind to female flowers. (Dave Powell, USDA Forest Service)

Cottonwood trees (Populus deltoides) have a different strategy. Trees are either male or female. A cottonwood's gender is determined by many factors, including proximity to another tree. If two trees grow close together, the older one will be male.

Reddish male catkins develop earlier and are generally bigger than female flowers. The male catkin grows longer, opens and lets pollen fly on the breeze, which carries it to the yellow female catkins, which then swell with seed and burst open, casting seeds embedded in white cotton that float on the breeze and have a bad reputation among people with screened porches. Wind may carry seeds a couple hundred feet.Water can carry them farther. Receding floodwaters leave behind strand lines of cottonwood seeds.

Cottonwoods are prolific — one tree may produce as many as 48 million seeds and cast them off gradually over the course of a month. This is a good strategy in case there's no wind, or if rain damps the seeds into the ground in bunches.

Cottonwoods grow on natural sand levees, but because of the irregular flood cycles of recent years, natural levees aren't forming as they once did, and cottonwoods, while plentiful, are a matter of concern for foresters looking at the future of bottomland forests.

Beavers eat lots of young, three-to-four-inch cottonwood saplings.

Swamp white oak (Quercus bicolor) is another tree that stands tall in the floodplain forest. Male and female flowers appear on the same tree, in the spring when leaves are about one-third developed (from May to June). These oaks bloom for about two weeks in mid to late spring, and rather high in the tree, so they're not so easy to see. The male catkin is long, conspicuous and hangs down. The female bloom sticks up, a bud or two on a stem.

Acorns mature in fall. The sweet meat of the acorn is prized by birds and squirrels. In fact, the now-extinct passenger pigeons apparently came in greater numbers in the years when acorn production was prolific (about every third year). Indians in the river valley ate both acorns and passenger pigeons. They warned the steamboat crews not to cut the swamp white oaks, but the crews put their own energy needs before those of the Indians and pigeons.

March-April 2015 © Big River Magazine
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