Why do leaves fall and change color?

04 Oct, 2025

Ok so a chain of questions here: why do trees make leaves? why do they shed these leaves? do all trees shed their leaves seasonally? why do these leaves change color? what determines the colors of the leaves?

Trees

A tree is a perennial plant, meaning that it survives at least 2 years, and have a big stem, that being the trunk. Typically these trunks with support branches. Trees are so tall because a taller tree will reach sunlight before a shorter one sharing space with it, and this is much more desirable than being in the shade, so an evolutionary competition for sunlight. The tree's trunk is usually made of tissues namely Xylem and Phloem.

• Xylem is the tissue that makes up lots of wood. It largely functions to transport water up from the roots of the tree, which reach outward under ground to provide much needed stability to the almost comically precarious above-ground setup. Of course, standing in it's way is gravity, but thanks to some clever engineering, chemical reactions elsewhere it its duties provide pressure at important parts of the system, forcing the water upwards, where it is needed.These systems are vital - and are a product of past growth. The growth leaves behind dead pipe like tissue which is crucial to any sort of movement of water. Since dead tissue is used so much here, this proccess is not very energy intensive.
• Phloem's primary responsibility is to distribute organic compounds made through photosynthesis, like sugar sucrose, around the tree. Phloem transports this stuff in the form of sap, and it does so mostly to parts of the tree unable to photosynthesize their own, like the roots. The sap can move in both directions, unlike in xylem. It seems as though leaves are big sources of these photosynthesized resources - these leaves need to make a large surplus of these resources, and they are used for many things, general upkeep as in the roots, keeping it in storage organs, or producing seed-bearing organs, like fruit, which is not cheap! The main method of transportation is also pressure, but of a different sort. Phloem is in kept in the inner layers of the bark, and it's growth makes the trunk girthier, not taller, and it does this with cork and bark. A security concern here is that, since the phloem so crucial, and on the outside of the inner tissues, it could be separated, and the tree would die - this is seen with beavers, and also something humans might take advantage of to 1) efficiently kill a tree or 2) grow large fruits - if all other fruits of a branch are inaccessible, all spare relevant resources will be funneled to the one fruit. Populations desperate for nutrition have been known to eat the phloem of trees!
  From Wikipedia: "Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, it is not surprising that their form should be similar to pipes." These tissues are usually protected by a layer of bark. The bark has 2 layers- inner, living bark, and outer, dead bark. One of bark's primary functions is providing a wall against diseases, like fungi.¹

Ok, so some hints as to what leaves are for- they produce a lot of valuable resources through photosynthesis. Maybe they are like solar panels?

Leaves

Leaves are typical of vascular plants, like trees, and are typically grown above ground, and are specialized for photosynthesis. Most leaves have a distinct upper and lower, the upper being better for photosynthesis - as such, the upper and lower often are different colors. Variegated leaves have the typical green look about them, but with some specks of lighter colors, which indicate abnormal levels of pigment. The shape of leaves in retrospect is very flat, wide and tall - optimizing surface area (or even surface area of one side)...

• Photosynthesis is a process in which a plant (and other things) turn energy from the sun to chemical energy, which, in collaboration with energy from other sources, like foods, can sustain life! Most plants photosynthesize in a manner which outputs, when it takes the Hs out of H2O (supplied by the xylem). Not all forms of this process release oxygen, though - some end up producing sulfur. Apparently, this sect of photosynthesis was dominant in the Boring Billion The final product of this energy use is an array of sugars, starches, and cellulose. -- Regardless, chloroplasts are an organelle common in plants, and which lots of the pigment chlorophyll. These pigments are very important! The absorb the red and blue human visible light, taking in the energy, and at the same time the green light bounces off of them! This energy taken in is used to strip electrons of some substances like water, which produces oxygen. The hydrogen produced this way is then put under some other transformations which gets it to a form that is digestible by the plants? (anyhow, these supplies are handed off to the phloem)
  • Cellulose is an organic compound, and makes up 90% of cotton and 40% of wood! As for modern uses, it is used to produce most any paper product.
  • The Boring Billion was from .8 to 1.8 billion years ago. The time period is characterized by its distinct lack of oxygen just about anywhere! Because of this, rather than the typical reactions which take place today, bacteria possibly relied on reactions with vaguely similar purposes, but involved no oxygen, and a bi product of this would be that the oceans may have been colored black and "milk turquoise"!

Abscission is the process of getting rid of leaves. This happens in 3 phases:

1. Resorption - The plant sucks all of the remaining nitrogen out of the leaf
2. Layer formation - A layer of cork is grown where the leaf stem currently is
3. Detachment - The leaf actually falls off. This can happen in the layer formation step, but sometimes the leaf hands on, and its connection inside the tree will suck up enough water to swell to the point where the stem will fall off.

Now, why do this? The obvious answer for seasonal plants is that leafs are an expense and not worth it over the winter, with less sun time and harsher weather. Wikipedia glosses over this so I'll assume this is the case. In this case, when the leaves are shed for seasonal reasons, the process of cutting off the leaf with a layer of cork kills off the chloroplasts which host the pigments that make the leaf green, revealing other pigments, which may turn the leaf new colors, like red, orange or yellow!