Why do leaves fall off trees in the autumn?

Question

Why do leaves fall off trees in the autumn?

Answer 1

Also the tree needs to save energy so the leaves drop off. because they are useless

Answer 2

Seasonal dryness or cold short days ensure some plants have a synchronous leaf drop instead of a continuous replacement like some tropical species. However all trees and shrubs, even evergreens, at some point in the year, in all habitats, shed and grow new leaves. Leaves age and produce less food with time, they harbor eggs of harmful insects, leaves accumulate damage from insects, UV and storms, leaves may be where the tree deposits metabolic waste material, as well as the habitat having enough seasonal variation that the plant cannot support a leaf canopy year round.

Temperate deciduous trees drop leaves when the leaves cost more to keep than the leaves produce in photosynthesis. We call this the whole tree carbon balance. For growth to continue there must be more photosynthate (carbohydrate) made than is used to support both the tree's basic functions including below ground and in the trees woody parts plus reserves for spring bud break. With less light and a decrease in temperature to between 5 & 10 C the tree struggles to support foliage. So there is a break-even point between available light energy and temperature that predicts the actual canopy loss trigger point. They plant drops leaves but the plant continues with basal respiration  to support  minimal function until conditions adequate to support photosynthesis return.

Some plants drop the leaves off as soon as the leaves die and shed insect eggs that might survive the winter. A tree protects itself from snow  accumulations on leaves. Shedding leaves avoids the weight of ice & snow that might build up on the broad leaves adding incredible weight that could lead to limbs breaking and other damage to the tree.

But some trees, like oak and beech, do not shed their dead leaves until spring. This may protect low branches from foraging deer. The withered, non-nutritious leaves may deter deer from feeding on the tasty live twigs and buds under the dry leaf layer. Another thought is the leaves are frost protection in winter. Just as gardeners wrap tender plants to shield them from sudden thaws so the tree wraps itself in a still air layer.

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Some trees growing in habitats that lack a cold or dry season also grow in flushes despite constant light and warmth. They have resting leaf buds between successive flushes of leaves, flowering, and fruiting with no obvious correlation to seasonal factors they experience. These growth rhythms are perhaps timed to migrating animals needed for pollination or seed dispersal. So leaves fall when the tree can best afford the cost of new healthy leaves.

Answer 3

In autumn, there is less sunlight because the days are shorter. Because sunlight is a crucial ingredient in the leaf "factory", the leaf is unable to sustain itself any longer. It becomes weak, breaks from the branch, and floats to the ground.