Why do leaves fall from trees?

Answer 1

1- Trees drop leaves when they enter dormancy when the whole tree carbon balance shifts. For growth to continue there must be more photosynthate (carbohydrate) present than is required to support basic life functions, root growth 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 actual canopy loss but the plant continues to live and respiration continues to support this minimal function during dormancy. This can as true for regions where the limiting factor is water rather than cold because there are trees that display periodic canopy drop in the tropics also.
2- Trees in regions with freezing periods must protect themselves from damage. Water is the problem in cold. Water expands when it freezes so the plant has to protect itself from this. If a plant cell freezes, it does what a frozen water pipe does: it ruptures. Leaves are not salvageable unless modified as they are with evergreens.
3- Another reason to drop leaves is to shed pest loads. Eggs laid on leaves will be shed with the leaves. Once in the leaf litter the eggs are likely to become prey to soil dwelling predators.
4- Another possibility is to shed metabolite waste products. They will be shed with leaf turnover.
5- Growth, as the tree matures the new upper branches shield lower tier branches from sufficient light. Once the leaves no longer return photosynthate but become a drain on resources the leaves are dropped. This is very common in mature forests and the tropics.
6 Leaf longevity is limited because UV repair is finite. Leaf senescence and turnover occurs regularly to support continued photosynthesis.

Answer 2

A plant will abscise a part either to discard a member that is no longer necessary, such as a leaf during autumn, or a flower following fertilisation, or for the purposes of reproduction. Most deciduous plants drop their leaves by abscission before winter, while evergreen plants continuously abscise their leaves. Another form of abscission is fruit drop, when a plant abscises fruit while still immature, in order to conserve resources needed to bring the remaining fruit to maturity. If a leaf is damaged a plant may also abscise it to conserve water or photosynthetic efficiency, depending on the 'costs' to the plant as a whole.

As most deciduous plants "hybernate" during winter, it has no need for leaves in the winter when there is less sunlight. Think of the leaves as the factories that conduct photosythisis - make food for the tree during the summer. When winter comes the tree stores what it has and goes essentially dorment until spring. Then it makes new leaves.

Imagine a layer of cells that weaken as fall progresses and they eventually fall away as the cells disintegrate.

Abscission occurs after the formation of an abscission zone at the point of separation. Within this, a thin layer of cells, the abscission layer, becomes weakened and breaks down through the conversion of pectic acid to pectin.

Answer 3

A tree is kind of like a human. As the years pass, it starts to die and weaken. The tree can no longer go through photosynthesis. As a result of the tree dying. The leaves start to change color because it can no longer produce chlorophyll. After a while, the leaves begin to fall.

Answer 4

gravitational properties