Introduction
The weather is not the only process affecting the foliage. In fact, the weather is a distant second in importance to the biology of the trees and leaves.

Trees
There are many species of trees and shrubs, each with different characteristics. There are two types of trees, evergreen and deciduous. Evergreen trees (e.g., pine, spruce, fir, etc.), sometimes referred to as conifers, keep their leaves (needles) year round. Deciduous trees however, shed their leaves during the winter or during prolonged droughts.

Leaves
The leaves are responsible for producing food and nutrients for the trees and shrubs. Deciduous trees produced enough nutrients, which are stored by the tree in the trunk and roots, to last throughout the winter. The leaves also allow the trees and shrubs to release excess moisture. The process through which leaves produce food is known as photosynthesis. Photosynthesis breaks down carbon dioxide (CO₂) into organic materials such as carbohydrates (e.g., sugars and starches). In order for photosynthesis to take place, energy is required. For plants, this energy is provided by sunlight.

Pigments
To use the sunlight, the plant must have a mechanism for absorbing the light. Plants use chlorophyll and carotenoids, both chemical pigments, to absorb light. Light has color components collectively known as the spectrum. The spectrum consists of red, orange, yellow, green, blue, indigo, and violet. The green light portion of the spectrum is not effectively absorbed by chlorophyll in plants. The green light is either reflected by or passed through the leaf. This is why leaves generally appear green.

Carotenoids represent a collection of two groups of pigments, carotenes, and xanthophylls. Carotenes are similar to vitamin A and are responsible for the coloration of carrots (the name carrot is derived from this pigment), pumpkins, and yellow and orange leaves. Xanthophylls are yellow pigment responsible for the coloration found in dandelions, sunflowers, corn, egg yolks, and yellow leaves.

In addition to carotenes and xanthophylls, three other chemical pigments play a role in foliage coloration. Anthocyanins produce blue, red, and violet colors. If the plant cell fluid is acidic, the coloration will be red. Conversely, if the fluid is basic, then the color will be blue. Tannins produce brown colors often seen in tea, chestnut bark, and in oak leaves in the fall. The final pigment is a group known as flavones. They are also yellow and are found in sumac, horse chestnut, tea and onions.