How Trees Grow

Trees are long-lived, woody, perennial plants. Each year, a tree grows new living tissue on top and outside of the previous year’s growth. As new tissue grows over older layers, the tree grows in a cone shape. It becomes both wider and taller, with the root system forming its wide base. The living layers draw water and essential nutrients from the soil, through the roots, upward toward the canopy, while carrying carbohydrates created in the leaves through photosynthesis down the branches and trunk.


Tree roots prevent soil erosion - the loss of soil to weather and other factors - by holding soil in place. Soil provides tree roots with nutrients, oxygen, and water. Roots thrive in the uppermost 6 to 12 inches of soil, also called topsoil. If the topsoil becomes compacted by the pressure of feet or cars, roots may be prevented from accessing the water and oxygen they need.

Deeper down, the air pockets, minerals, water, and temperature all decrease, making a less hospitable environment for roots to grow. This is known as subsoil.


Roots store essential food reserves needed by the tree to produce leaves in the spring. They soak up water and minerals from the soil and transport them to the rest of the tree.

Root system diagram
Roots spread two to four times further than the canopy. Image courtesy of ISA.

Tree roots spread horizontally through the topsoil. It takes a few years for a tree to complete its roots system, growing surface roots horizontally and only a few anchoring roots vertically.

Absorbing roots extend up and out from the larger roots. Only about 1/16-inch in diameter, these roots seek minerals, water, and oxygen closer to the surface. A fully established tree has roots that spread two to four times wider than the tree’s crown.

Damage to roots is a major cause of decline, death, or physical failure of trees. Roots can be injured by flooding, drought, soil compaction, soil removal, being covered with subsoil, or being severed. Trees that have been damaged in this way often must be removed.


Decorative image of barkBark is the outermost layers of a tree's trunk, branches, and roots. All trees have two layers of bark. The outer bark is hard and firm and shields trees from pests, disease, storms, extreme temperatures, and sometimes even fire. This dead outer layer also rids trees of waste by absorbing and locking them into its dead cells and resins.

The inner bark is a living layer that the tree uses to pass water and nutrients up and down the trunk and along the branches. It is soft and moist.

Trees seal; they don’t heal. When a tree is injured, the cells in the surrounding area change chemically and physically to prevent the spread of decay. New cells then line the cut area to create a callus that covers and seals the injured area. Over time, this callus will blend in with the rest of the tree's bark.

Trunk and Branches

Tree cross section diagramThe trunk is the main stem of a tree. It supports the branches and is supported by the roots. The trunk layers include heartwood, sapwood, and cambium. The heartwood is the innermost layer. Technically dead tissue, heartwood is hardened and resistant to decay. Sapwood, the middle layer, is softer living tissue. Cambium is the most active layer, actively creating new tree cells. The thickness of each layer varies greatly by tree species.

Branches are woody extensions that grow from the trunk to support and display leaves. Branches usually have the same overall composition as the trunk, but the layers will be of different thicknesses. In thin branches, the heartwood may be missing altogether. Large branches that grow directly from the trunk are called limbs.

Leaves and Canopy

Decorative image of leaves

Leaves capture solar energy from sunlight. The tree uses that energy, plus carbon dioxide from the air and water from the soil, to create sugars and cellulose. The tree then uses the cellulose to build new layers, and the sugars to feed its cells. This is known as photosynthesis.

Leaves clean the air and water by filtering out dust, particles, and pollutants. They also release oxygen into the air as a by-product of photosynthesis. Leaves cool the surrounding air temperature by evaporating water, lessening the heating effect of pavement and buildings in cities. The bigger the trees, the greater the effect.

The canopy is the outer layer of a tree’s leaves. Tree canopies catch rain water, stockpiling some to use later. Canopies reduce glare and reflection along streets. By providing shade, canopies can reduce the temperature of buildings by as much as 20 degrees in the summer.

Fallen leaves can shade grass, inhibiting photosynthetic activity in the late fall. Thin layers of leaves can trap humidity and increase the likelihood of snow mold during winter. Thicker layers can smother and completely kill the grass. Using fallen leaves to mulch trees is a green and cost-saving solution.