Guide to Trees


What Is a Tree?

Trees grow, reproduce, and respond to their environment. Like all plants, trees manufacture their food through photosynthesis. Trees are in the family of woody plants which have cambium, a special layer of cells that allow the tree to increase in girth and are self supporting with a single stem. Trees are perennials and can live for many years. The oldest known tree species is the Great Basin bristlecone pine, which is believed to reach ages of over 5,000 years old!

Food for trees is produced through a complex system starting with the leaves. Leaves produce sugar as a result of photosynthesis which combines carbon dioxide and sunlight. The by-product of photosynthesis that Trees produce is oxygen—a vital resource for other living organisms. Sugar that is produced runs down through the tree, under the bark down the trunk and to the roots of the tree. The sugar combines at the roots with minerals and water in the ground to move back up the tree trunk and under the bark back to the leaves to which starts the cycle of photosynthesis again. Along the way, minerals, water and sugar stimulate the growth and development of the tree or assist in its reproduction.

Tree Types

Trees can divided into two categories: deciduous and coniferous.

Deciduous trees are also known as broadleaf trees because the leaves are generally larger and wider than those of conifers. The larger leaf size means a greater surface area for photosynthesis, but it also means the leaf is too fragile to withstand winter conditions. Therefore, most deciduous trees drop their leaves in autumn.

Coniferous trees keep their leaves throughout the year, shedding only the oldest leaves. Usually these leaves are lower down on the tree and do not receive as much sunlight as newly developed leaves higher up. Some of the best-known members of the conifer family are pines, spruces, firs, and hemlocks. The cones of the conifers are its flowers.

Parts of a Tree

Tree parts diagram showing crown, leaves, branch, trunk, root collar, and roots

The crown of the tree is made up of the leaves and branches.

The trunk of the tree supports the crown and serves as a highway for food made in the leaves to travel to the roots and for water and nutrients from the roots to travel to the leaves.

The root collar is located at the base of the trunk just above where the roots join. Also called the root flare, due to the flared shape on some tree species, the root collar must always remain above ground when planting and mulching.

The roots of the tree support the trunk and crown, and also anchor the tree in the soil. They serve as a storage facility during the winter for the food produced by the leaves during the growing season. The roots also absorb water and nutrients from the soil for use by the tree.

Many wildlife species depend on trees for food, water, cover, or places to raise their young. You can help wildlife by planting trees and creating habitat in your community!

Get Involved

Where We Work

More than one-third of U.S. fish and wildlife species are at risk of extinction in the coming decades. The National Wildlife Federation is on the ground in seven regions across the country, collaborating with 53 state and territory affiliates to reverse the crisis and ensure wildlife thrive.

Learn More