Science of the Seasons: Little seeds, big impact

By Dr. David Wartinbee, for the Redoubt Reporter

Photos courtesy of Dr. David Wartinbee. Alder seeds as seen under a microscope. The individual seeds are only about one-eighth of an inch wide. Their small size and lateral wings help them get carried long distances from the parent tree.

There is still plenty of snow in my yard but the blanket is steadily shrinking, and looks like it could use a good laundering.

As our snow slowly melts from the top down, debris from the surrounding trees is clearly visible. There are assorted branches and twigs lying everywhere from the various windstorms during the winter. There are also little snippets of feathery lichens and any number of spruce cones scattered about the yard. When I look closer, there are small spruce seeds embedded in the snow that look like miniature maple seeds. Once these seeds are released from the cones high up in the branches, wind carries them long distances from the parent tree.

Once down on my hands and knees, looking at the surface of the snow, I can see a large number of smaller, seedlike structures scattered all over the snow. They are smaller than a BB, light brown in color, and somewhat flattened. At first I suspect they might be seeds from the birch trees, since they are found all around the yard. But those should have dropped well before the snow arrived. The only other common trees in our area are alders.

This is going to take a little more investigation, and maybe a dissecting microscope to figure out for sure what these are.

Once I viewed them under the microscope, I was able to rule out birch trees as the source.

Female catkins hang from an alder tree this winter.

These were small seeds with little, winglike structures facing laterally. It is obviously a wind-dispersed seed, like maple and spruce seeds, but quite a bit smaller. Now to my bookshelf to see if I can identify the seeds in one of the tree books. While I was pretty sure I knew the source, there was another way to verify what was littered all over the snow.

I went over to a stand of alders and removed a number of female catkins that were still hanging on the branches. They were spread open to release their seeds and looked pretty well empty. But, under the microscope, I was able to find more than a dozen seeds that were still attached. They were identical to those I had been finding on the snow.

Returning to my tree books, I was surprised to find that there was no concurrence on what species I had found. Most likely, I had found seeds from an American green alder (Alnus crispa) or perhaps a Sitka alder (Alnus sinuata), but I need some leaves this summer to tell them apart. Either way, at least I know the source of the tiny seeds, even if I don’t know its exact name.

Alders are found all over the country and are especially common in Alaska. They are

Alder seeds litter the snow along with a larger-winged spruce seed and a dried alder leaf.

particularly common in ravines where they can get adequate water. These trees are known for their ability to invade disturbed or barren ground before many other plants.

This pioneering ability comes from their association with a variety of nitrogen-fixing bacteria. Alders produce root nodules where a couple different bacteria can thrive. The bacteria are provided with carbon, water and a protective root-covering while they, in turn, extract nitrogen from the air and fix it into a nitrate or nitrite form the plants can use. The fixed nitrogen is the same as found in garden-variety fertilizers, so the presence of alders improves the local soils.

Alders provide food for some local birds, like pine siskins and redpolls, who feed on these tiny seeds. Some of our smaller rodents, like red-back voles, readily consume alder seeds, too. The rest of the alder tree can be food for a variety of browsing herbivores. Moose will feed on alder buds and twigs from time to time, although it isn’t on their favorites list. Sometimes humans will consume the male catkins in the spring, since they contain a fair amount of protein. However, it is often noted that this is a survival food and not particularly tasty.

Our local Native populations have used alders for a variety of medicinal purposes. A tea from the green female catkins is said to be curative for diarrhea or internal bleeding, and a poultice from green leaves has been used to sooth insect bites. A mild bath made with alder leaves and dried bark is said to be soothing for children with measles or skin rashes.

Some common medicinal compounds are found in alders. The leaves contain salicin, which gets metabolized into salicylic acid that is an active ingredient of aspirin. The fresh inner bark of alders is known to be an emetic and has been used to cause vomiting after ingestion of poisons.

Perhaps the most widely known use of alders is from smoking salmon and trout. The oily leaves and bark impart a pleasant and distinctive flavor to the drying fish. Another use of alders is making dyes from the inner bark layers. These colors vary from tan and yellow to rich, reddish browns and were commonly used on skins and baskets.

From the minute seeds to the leaves and roots, the alder tree is an important member of our local flora. The trees provide food, nutrients, flavorings, colorings and medications for local animals and humans alike.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.


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Filed under ecology, science of the seasons

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