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Treebeard's Stumper Answer
21 Nov 97

Fall or Spring?

Fall colors and winter rains are coming together this year. Native deciduous trees like willows, maples, and Valley Oaks are just starting to turn color. Soon they will drop their golden leaves and stand bare for the winter. At the same time, the hills and fields around school are starting to turn green as next year's wildflowers sprout after the recent rains, finally ready to grow after our long, dry summer. Here's a real stumper: why do the trees go dormant just when most other plants start to grow?

Note: this stumper probably only makes sense in coastal California. Here in the Santa Ynez Valley, we have a classic Mediterranean Climate with two main seasons, a hot dry summer and a cool wet winter. At this time of year, deciduous trees turn color and drop their leaves just as they do across the country at this time of year. But this is also the start of our growing season for many other plants, and the hills are greening up after recent rains. That's the stumper!


Why do our trees drop their leaves just when the hills turn green with new growth? Why indeed? We have a classic Mediterranean Climate here in coastal California, with a hot dry summer and a mild wet winter. But it hasn't always been like this. I think it's easier for life to find a new habitat than to evolve a new adaptation. Willows are sub-arctic plants that go dormant to survive a harsh winter. This doesn't help them here, but they survive by finding a summer habitat with adequate water in canyons and river beds. This is nature that works, not design.

Addendum: Sometimes existing adaptations bring new benefits when a plant takes to a new habitat. Willows and Cottenwoods grow on flood plains, and our local rivers flood during wet years. If the trees kept their leaves all winter, they would no doubt wash away because of the extra drag. This happened during the "March Miracle" rains of 199_, when a late rain scoured the Santa Ynez River after the trees had leafed out. Is this a kind of pre-adaptation?

Our winter-deciduous trees are mostly riparian, but not all. Willow, Cottonwood, Alder, Black Walnut, Sycamore, Maple, and Box Elder are all deciduous trees that grow along local streams and canyons. But Valley Oak (Quercus lobata), Blue Oak (Q. Douglasii), and Black Oak (Q. Kelloggii) don't. Valley Oak at least needs a high water table, but the others grow in exposed places away from obvious water, which I don't understand at all.

We also have several true summer-deciduous shrubs, such as Chapparal Current (Ribes malvaceum) and Buckeye, and many others such as Poison Oak and Elderberry that drop their leaves early in summer during dry years. This makes more sense.

I stand by my statement that this is nature that works, not design -- like the Panda's thumb. For more information on the Santa Barbara flora, check out Treebeard's Flora, my would-be flora of local trees and shrubs.

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Marc Kummel / mkummel@rain.org