Great fall outdoor activity: Make a nature collection at the arboretum
Photo: Fiona Cohen
Kids are great collectors.
Whether it’s trading cards or bottle caps or seashells, they like to accumulate a variety of specimens, and then sort them and arrange them by category or type. And fall is a good time for those disposed toward collecting, with trees dropping leaves and seeds, and sometimes other things. Though these should only be temporary collections. Few of the things you find in the woods will will stay the same after you collect them. The brilliant colors of leaves fade, and seeds shrivel and lose their gloss. If you want a keepsake, it's best to take photographs, and leave the leaves and seeds behind to add to the soil of the forest floor.
And one of the best places to make a fall collection is the Washington Park Arboretum. For one thing, it’s a collection itself. Its 239 acres are packed with thousands trees from all over the world, gathered by generations of University of Washington horticulturists. Where better to get a look at nature’s variety?
As you look around, here are some things to keep in mind:
You know how when you are painting a picture, you mix colors to get just the right shade? Well, leaves are a mixture of colors, too. As they grow on the tree, most leaves are a mixture of three kinds of colors: There’s the green chlorophyll that it uses to turn sunlight into food energy, there are some yellowish colors called carotenoids, and there is the brown of the cell walls.
In the fall, as the weather gets colder, the green chlorophyll breaks down, so that we can see the yellow color of the carotenoids. Eventually, after the leaf falls, the carotenoids break down too, leaving a brown color. That’s what happens in many leaves.
But in some, there is a fourth color at work: anthocyanins that turn leaves vivid red and purple.
What do these red pigments do for the tree? That’s a question that tree scientists are figuring out. Maybe it helps the tree hold on to nutrients. Maybe it acts like a kind of sunscreen. Maybe it sometimes keeps pests away.
As you pick up leaves, take a look at them. Can you find which of the different kinds of colors are showing? On a given tree, are all the leaves the same color? If they aren’t, why do you think that would happen?
Seeds come in all kinds of shapes and packages. The reason is that they need to travel. Underneath the parent tree is not a good place for a new tree to grow. So plants shape their seeds to go places.
Some seeds can be carried on the wind. Maple seeds are each attached to a big wing. When you drop one, the wing goes in a circle, like a helicopter blade. The twirling makes the seed fall more slowly, so if it is windy, it can travel farther.
Other seeds rely on animals to carry them places. Strawberry trees have bright colored fruits that are eaten by birds, and the birds poop out the seeds later. (If a wild fruit is brightly colored, it means birds are the creatures transporting the seeds. Most mammals don’t see color very well.)
Other trees, such as oaks, rely on animals that stockpile seeds for the winter. Squirrels and jays gather acorns and stash them in hiding places for later eating. The animals have excellent memories, and most of the seeds will get eaten. But maybe they’ll hide away more food they can eat. Or maybe some of the squirrels will die before the eat their winter’s stores. That’s enough for a few acorns to become trees.
Other trees that rely on seed-eating animals: horse chestnuts. Horse chestnuts, a common street tree, has spiky fruits that contain richly brown, beautifully smooth seeds. British people call them “conkers” and kids in Britain play a game with them. You might want to check it out if you have a young anglophile. Horse chestnuts are poisonous to a lot of mammals, including humans and horses, but squirrels do eat them.
Douglas firs have a combination strategy: The seeds have wings and can sail on the wind, but more often animals cut the cones down and store them to eat later.
Shorepines take another tack. Some of the seeds fly on the wind year to year. Others are in cones that stay on the tree, gummed up with tree sap. If there’s a fire, the cone will open up, releasing the seeds.
Around the arboretum, you can find a variety of odd seeds.
When you find a seed, take a look at it. See if you can figure out what strategies the trees are using.