I wondered today why the live oak might be one of the largest and longest-lived trees in the American South. I did some research. It turns out that the live oak is not a deciduous tree, but something else known as a broadleaf evergreen.
The live oak is a broadleaf evergreen. That puts it in a remarkable category — though "category" is almost the wrong word. Deciduous and evergreen aren't taxonomic distinctions, like the difference between birds and mammals. They're strategies. They describe what a tree does, not what a tree is. A tree's answer to the question: is it worth the cost of maintaining leaves year-round, or should I shut down seasonally and wait out the winter? Different trees in completely different families have landed on the same answer independently, based on their climate and circumstances. Among the broadleaf evergreens you'd recognize: magnolias, hollies, wax myrtle — and, surprisingly, a number of oaks.
Which makes the live oak's story even more interesting. Most oaks went deciduous. The red oak, the white oak, the pin oak — they all drop their leaves. The Spanish oak on the other side of the lot drops its leaves. Within a genus that is almost universally deciduous, the live oak made a different choice. It held on.
And the way it holds on is a remarkably well-coordinated system. This is the time of year here in Texas when these trees are simultaneously shedding their old leaves while sprouting new ones. If you think of photosynthesis as the way a tree eats, the live oak makes sure it never misses a meal.
The tree keeps about half its leaves and sheds the other half. Where the old leaves fall, new ones begin to sprout — bright green and fresh against the dark green of the older foliage. The tree waits until those bright new leaves have unfurled and begun feeding it before releasing the older ones. It is a remarkably well-choreographed handoff. Once the older leaves fall, the remaining new growth fills in, and the full canopy of fresh bright green leaves settles in for the year ahead.
In this way, the live oak is never leafless. That may be why these trees grow so large, and live so long.