Why do some trees keep their leaves later into the fall?

The intuitive answer is that a healthy, vigorous tree will keep on photosynthesizing late into the fall, while a weak, sick tree will give up and go dormant earlier in the season. In fact, it’s exactly the opposite.

When it comes to the problem of how long leaves should be held onto, a tree faces a dilemma. On the one hand, a tree can only produce food and store energy while it still has its leaves. But on the other hand, as winter creeps closer, having lots of green leaves is a big risk – the leaves could be killed suddenly by a frost, or they could catch flakes in an early snowfall, causing weight to pile up on the tree and increasing the likelihood that branches will break.

In the normal course of things, a tree wants to store all the energy that its leaves have produced, reabsorb the chlorophyll and other useful chemicals from its leaves so they can be used again the following year, and then jettison the leaves and ride out the winter in a dormant state. Ideally, all of this will happen just before frost arrives, thus maximizing photosynthesis and minimizing the risk of frost damage.

Of course, the tree doesn’t know exactly when the first frost will arrive, and so it has to take a bit of a gamble with its timing.

A healthy tree that has stored up plenty of energy for the winter will play it safe, losing its leaves well before cold weather sets in, and relying on its reserves to last until spring. Conversely, a tree that is struggling will run the risk of being injured by a night of freezing temperatures in order to squeeze in a few more days of photosynthesis, in the hopes of storing enough food to make it through to the next growing season.

Thus, trees that settle in for winter early are doing well, and trees that cling to their leaves are desperately trying to survive.

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Why do some trees keep their leaves later into the fall?

Why do trees lose their leaves and branches?

The common understanding is that a tree’s leaves die at the end of the summer, and then fall off. This isn’t quite accurate. It’s more correct to say that the tree kills its leaves.

What’s really happening is that the tree builds a layer of cells inside the leaf’s stem to create a seal that blocks the flow of nutrients to the leaf. Imagine putting a tourniquet around your arm and leaving it there: eventually your arm would die and fall off, and that’s exactly what happens to the leaf.

A similar thing happens with branches. Humans, like most other animals, start their lives with a fixed number of limbs, and don’t get any more. For this reason, we’re very interested in keeping our arms and legs intact. But a tree can grow a new limb pretty much whenever and wherever it wants. When a branch isn’t pulling its weight, the tree will kill it – by creating an internal seal and starving it to death – and then grow a new branch that contributes more to the tree as a whole.

When a leaf or branch that has been killed in this way falls off, it doesn’t do the tree any harm. The tree wasn’t using that body part anymore, and the internal seal prevents any infection from entering the living parts of the tree. In contrast, when a leaf or branch is broken off suddenly – perhaps due to a hungry herbivore, pruning, or a powerful windstorm – the tree loses a healthy, productive body part, and acquires a wound through which insects or fungal infections can easily enter. Because trees don’t do anything very quickly, it can take them years to seal even a relatively small wound – more than enough time for a serious infection to set in.

It’s therefore inaccurate to say that cutting off a tree’s limb doesn’t hurt the tree. Of course, it doesn’t hurt the tree the same way that it would hurt a human, because the tree can regenerate. But the loss of a productively photosynthesizing branch, coupled with the energy demands of sealing a wound and growing a replacement branch, can put serious stress on a tree. While pruning may be desired for other reasons, the idea that pruning is good for a tree’s health just doesn’t cut the mustard.

Why do trees lose their leaves and branches?