Why do only leaves fall, but no needles?

Deciduous trees lose their leaves in autumn. The reason for this has to do with water. A tree supplies its leaves with nutrients and water. It draws the latter from the soil with its roots. In winter, however, this is not possible because the ground is frozen. Therefore, in the fall, the tree withdraws its nutrients, stores them in bark and roots, and takes a rest. Since the leaves no longer receive water, they wilt and fall off. Now the tree can spare itself to produce new leaves in the spring.

Conifers are different, although needles are also leaves. But needles are tougher and often covered with a thick layer of wax. In many needles, moreover, the small stomata through which water normally evaporates are sunk deep into the surface. Both features mean that they lose very little water in winter. In addition, the needles accumulate antifreeze substances so that they do not freeze. The trees only part with the needles when they have become too old for efficient photosynthesis. Then they fall off, and new ones grow back.

There are also exceptions, for example, the larch. Although it belongs to the conifers, it sheds its needles in the fall. Why? Because the stomata of its needles are not sunken like other needles and are therefore poorly protected in winter. In addition, larches grow in the high mountains. It gets bitterly cold there in winter. Only one tree can withstand this cold: the larch. But since the ground is frozen, it must get rid of its needle before the cold sets in so as not to die of thirst.