ENHS Nature Trails February 2003 Article 2

Natural History and You - The President's Forum
"Skeletons in the Closet"
by Dave Wagner

This is the time of the year that I notice skeletons in the back yard. Being a botanist, what I'm noticing are leaf skeletons, not animal bones. The leaves I find are those of deciduous, broadleaf trees. The deciduous trees drop their leaves in the fall after one season of activity. All the ones in our area are flowering plants. On the east slope of the Cascade Mountains there are western larches, Larix occidentalis, which drop their needles every fall. That there are such trees is why we mention broadleaf to distinguish them from the needle bearing trees that are deciduous. A few larches are cultivated in town but not many because they do not thrive in wet climates.

Another needle bearing tree that is deciduous and which you can see around town is the dawn redwood, Metasequoia. It drops not only its needles but entire branchlets every fall. In my mind it is paired with another ornamental tree, the ginkgo. Ginkgo is also deciduous and not a flowering plant. Ginkgo is a dinosaur among trees, its early relatives having populated the earth a hundred million years before any flowering plant. Both dawn redwoods and ginkgoes used to grow in Oregon during the tertiary (ca. 60-6 million years ago). Their fossils are among the most frequent in Oregon sedimentary deposits of Miocene age. They disappeared when our dry summers developed, even before the ice ages. Now they grow only where they get watered in the summer. With summer water, they grow beautifully.

Although they left good fossils, dawn redwood and ginkgo leaves do not leave skeletons. The skeletons I find are produced from trees that have a differential decomposition of their leaf tissue. The soft tissue between the veins decomposes rapidly, perhaps being grazed upon by micro-arthropods after first being degraded. The veins, meanwhile, resist degradation from the same decomposers and thus produce the leaf skeleton I love to admire. If a leaf has a fine venation pattern, it is as beautiful as the most delicate Belgian lace!

Although most leaf skeletons are produced by partial decomposition, there are other natural processes that produce a leaf skeleton. I mentioned micro-arthropods grazing on the soft tissue of leaves. I think that sometimes they are the primary mode of skeletonization. ENHS member Marge Zane found that the aquatic snails we call periwinkles skeletonize leaves that fall into the streams where they live. Marge did some experimentation, putting different kinds of leaves into the stream behind her house. She'd pin a branch down with stones. It was remarkable how quickly the periwinkles reduced a leaf to a skeleton, sometimes overnight. The end of the experiments came when Marge put a small branch of oak leaves into the water. The periwinkles did indeed graze much of the soft tissue away, but they stopped before finishing the job and wouldn't touch it again. Nor would they touch anything Marge put in the stream after that. I guess they thought she was poisoning them with the tannins of oak leaves!

Some trees are much more prone to skeletonization than others. The most commonly found ones are those of the Oregon ash. Anywhere the ash leaves have lain untouched all winter is a likely spot to find leaf skeletons. It is not difficult to find nearly completely skeletonized leaflets of the Oregon ash. I've found that blueberries produce leaf skeletons almost as readily and just as beautiful. My favorites are the leaves of the vanillaleaf (Achlys). It's necessary to go up to the mountains to find it, and now is the time. The skeletons of this are rather fragile. If you wait until the spring flush of foliage emerges from the ground, it will be too late. The emerging grass and other stems quickly grow through the tender skeins and it is impossible to extract them after this happens. Further decomposition takes its toll too.

I have been keeping a collection of skeletonized leaves. The common ones I have mentioned here. I am always on the lookout for ones which do not get skeletonized so often. Illustrated here is a rare one, the leaf of the red alder. Alder leaves decompose rapidly and usually completely. This one has lost some of the fine structure but it makes for a nice image to reproduce.

So here's another way to enjoy nature: searching for and gathering skeletonized leaves. It adds a new dimension to the way we walk in the woods. The leaf skeletons usually dry quickly in a few folds of newspaper put under a brick or board. I like to keep them in transparent picture pages such as used for photographs so I can hold them up to the light and see the patterns. Notice, too, how nicely they reproduce on a scanner.

David Wagner

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