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Look at this boring picture above. It is kind of easy to overlook this occurrence. It’s a little too early for fall leaf drop, yet we’ve been getting a number of calls about it. Despite the Drought Monitor not classifying our region as undergoing a moderate drought, the trees indicate otherwise. I reckon they are a more reliable indicator of water deficit than the Drought Monitor’s models. This leaf drop is an adaptation to conserve water. As trees (and all other plants too) transpire, water evaporates from the leaves through stomata. Stomata function like an aperture, adjusting to meet the needs and environment of the plant. Because the stomata are also the point where the tree collects CO2 from, minimizing water loss can also partially restrict photosynthesis. This is a complex balancing act trees undergo all the time. When adjusting the stomata isn’t enough, the tree will rapidly abscise leaves (or actively shut them off), and hence the leaf drop. Because photosynthesis can’t entirely stop, dropping them off is the next best thing. In other words, leaf drop that we’re seeing is not the first attempt at addressing their water deficits. It is probably the second or third, behind wilting and light shedding (another cool adaptation where the leaves can bend away from the sun to minimize photosynthesis, and ergo, minimize water loss. I don’t have any photos of it though, I need to take some). In other words, the plant is functioning despite its stress. But there’s a maximum threshold, just like anything else.
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