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Studying the 'wings' of backyard tree seeds

Posted: Friday, February 04, 2011

Typical Juneau weather! We get a nice snowfall, for good skiing and snowshoeing, then it thaws and rains and makes the snow soggy. Then we get a hard freeze, so the footprints and postholes created by walkers during the thaw are frozen solid. After that happens, walking in those areas is a misery of lurches and ankle-turnings. The popular Dredge Lake trails are a good example. Then the cycle starts over again — lovely snow, then rain …

Photo by Kathy Hocker
Photo by Kathy Hocker

Before the last rains, however, the Parks and Recreaction hikers did manage to fit in one excellent junket to Hilda Meadows above Eaglecrest, on the one perfect day with good snow underfoot and more coming down throughout the day. Some of us were on skis, others on snowshoes. There was enough firm snow that it was easy to find places to cross Hilda Creek on our way to the chain of meadows.

Critters had been active overnight, so here and there we saw tracks of hare, squirrel and weasel, rapidly being covered with new snow. Trundling porcupines left numerous furrows as they wallowed in the fresh snow from tree to log to tree. One hiker inadvertently flushed a feeding ptarmigan, which flew complainingly off into the conifers. We all inspected its tracks; it had circled several small blueberry bushes, nibbling on buds.

The surface of the snow was dotted with wind-blown spruce or hemlock seeds. I wasn’t sure which kind they were, so I checked the forestry literature to see how to distinguish them. Western hemlock seeds are smaller, on average, than Sitka spruce seeds, but mountain hemlock seeds are larger. Foresters don’t usually measure individual seed weights; instead, they count the number of seeds per unit weight. So they estimate that there are 570 western hemlock seeds per gram, 400 Sitka spruce seeds per gram and 250 mountain hemlock seeds per gram. For those looking to convert those numbers to the English system of weights and measures, there are 28.35 grams per ounce.

I also collected some spruce and western hemlock cones and extracted the seeds. In general, spruce seeds look fatter and the wing seems a bit wider, but there is a lot of overlap in apparent size of the wing.

When seeds are released from cones, most of them land fairly close to the maternal tree. Many of the seeds that land near their mother or each other usually die, because seed predators focus on high densities of seeds. Or, if those seeds germinate, the crowded seedlings compete with each other, stunting growth and eventually causing mortality. Therefore, it is important to the tree’s reproductive success that some seeds disperse to greater distances. This can happen during strong winds or, as we saw on our way to Hilda, when fallen seeds are blown over the surface of the snow in a breeze.

The convenient name for the distribution of seeds around a parent tree is “seed shadow.” The seed shadows of trees are difficult to measure, especially for far-traveling seeds, and most studies have ignored the “tail” of tree seed shadows. With that limitation, a review of published studies of tree seed shadows suggests that Sitka spruce seeds may tend to fall closer to their parents than do western hemlock seeds.

It might be interesting to do a little experiment with our wind-dispersed trees (spruce, western or mountain hemlock, Sitka or red alder), all of which have winged seeds. Take seeds from ripe cones, an equal number of each species. One by one, drop the seeds from a standard height, letting them land on a clean surface. Mark the landing site of each seed. Then compare the average and maximum distances achieved by each species. Try it all again, in a breeze.

Are some wings better than others for carrying seeds away from a source? Does the “wing-loading” of each kind of seed differ and is that related to dispersal distance? (Wing-loading is the ratio of seed weight to area of wing.) Is the total weight of the seed-plus-wing important? How does the height of release or breeziness affect the outcome?

Prospective Science Fair students take note!

• Mary F. Willson is a retired professor of ecology.



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