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By Root 817 0

fully leafed out. That night, it started to rain, and it got colder at the same time. By nightfall, temperatures dipped slightly below the freezing point, and the rain turned to snow. As more snow fell, it stuck to the already wet leaves and froze on. Throughout the night, temperatures continued to hover around the freezing point, so that the snow did not melt but was wet enough to stick. By morning, critical snow loads had accumulated, and I heard loud crashing throughout our woods as huge limbs snapped and came thundering down. All the large-leafed (deciduous) trees were damaged, but no trees and branches that had no leaves, and none with needles (small, nondeciduous leaves) were affected. Half a foot of snow on the ground at this time certainly did not seem usual, but it may not have been unknown to the 200-year-old trees. It would have been common in the long evolutionary history that shaped them.

These natural “experiments” demonstrate what may seem obvious: trees need solid scaffolding to hold their leaves up to the light. The costs and risks that are involved are a necessity resulting from competition. Most mature forest trees have dropped millions of viable seeds and perhaps produced thousands of seedlings in their lifetime, but of course on average, in a stable population, only one seedling can grow into another tree to replace the parent. The seedlings derived from any one tree are in an intense race to put on growth, and only one of them may grow large enough to break through the canopy and capture enough solar energy to produce seeds as well. Given such competition, one might suppose that trees would continue growing to the very end of summer. Instead, stem growth is usually completed in June, near the midpoint of summer when the ends of the twigs become capped with buds and stop growing. The buds then stay dormant through the warm weather of late summer, through the fall, and through the winter. Generally, they become ready to unfurl their leaves and flowers only in the warmth of the next year. Why do the trees stop growing taller with at least three months of warm weather still to go, long before low temperatures can put a damper on further growth?

I do not have an adequate answer to this question. However, I speculate that backup support has something to do with it. I have noticed that vines, such as Virginia creeper, grape, and blackberry, continue to grow throughout the summer, long after trees stop putting on height. During intense competition with neighbors there is a race to get the leaves up, to get priority in grabbing the sunlight, and the leaves pop out and the twigs lengthen in a short sprint early in the summer. But the tree then has to route resources to thicken the trunk and limbs—the solid scaffolding to support the new leaves and twigs. I measured the girths of five different trees of different species throughout the season and indeed found that they did not increase in circumference until the leaves came on in June, and they stopped putting on girth by mid-July or August. (This conflicts with the idea of light and large and dark and narrow supposed summer versus winter ring growth. Is it summer versus fall?) Bushes such as honeysuckle put on as much length per season as trees such as oaks, but the first grows to be only ten feet tall whereas the other grows to 100 feet. The reason is that bushes put out shoots in all directions, and these shoots die off as fast as new ones are created; but trees grow in only one direction, and one year’s growth adds to the next.

The geometry of trees is also an important aspect of leaf abscission. Deciduous trees spread their branches out and up in all directions to capture as much light as possible from above. However, long limbs, when snow-loaded, exert a huge torque that pulls them down until they may break or split the trunk. This configuration is great in the summer, for capturing lots of sunlight, but it compromises the ability to shed snow. In contrast, both spruces and firs have tough and generally short branches that bend down under a load, so that