Agriculture and Natural Resources Blog

Matthew Orwat
County Extension Agent – AG/NR
Email: matthew.orwat@ag.tamu.edu

The Oak Tree: A Keystone Tree

Article written by Master Gardener Volunteer Stephanie Suesan Smith for the Agriculture and Natural Resources division.
August 5, 2025

Oak trees are a keystone species.  This applies to any type of oak tree.  Without oak trees, our world would look very different.

Keystone Species = Vital

A keystone species is any organism that is so vital to the ecosystem that it would fail without this species.  Without this species, the ecosystem would be so altered that the present ecosystem would cease to exist.  The name comes from masonry.  The keystone is the last stone put in the very top of an arch.  The keystone holds the arch together.  If it fails, the arch fails.

Evolution of Oak Trees

Oak trees (Quercus) evolved from one species in a small area roughly 56 million years ago to roughly 435 species on five continents.  They only grow in the Northern Hemisphere.  Approximately 60 percent of Quercus species grow in the Americas.  Oak trees account for more biomass than any other plant in North America and Mexico.

Live Oak in Neighborhood

Shumard Red Oak in Neighborhood

In the Americas, oak trees are either red oaks or white oaks.  Red oaks have bristle-tipped leaves.  Their acorns take a full year from when pollen lands on the female flower until the seed is pollinated.  Squirrels prefer red oaks because they last longer when buried instead of germinating quickly and ruining the acorn.

White oaks do not have bristle-tipped leaves.  The pollen fertilizes the seed the same season as it falls on the flowers of the white oak.  In addition, white oak tree leaves contain more nutrients and make the soil more fertile when they fall then red oak trees.  White oaks are able to plug the tubules that carry water efficiently with tyloses, balloon-like structures, to keep fungal diseases from spreading all over the tree.  This ability makes them the preferred species for ships and barrels because their vessels hold water more effectively than red oaks.

Individual species of oak trees can be hard to tell apart from closely related species.  Because species freely hybridize, telling what species a specific oak tree is can be difficult without a DNA test.

Ecological Importance of Oak Trees

Oaks as a group support more life forms than any other North American tree genus.  White oaks are considered the most important tree genus in the hardwood forests of North America.  They live up to 200 years, with some oak trees living to be 500 years old.  During that time, they provide food, shelter, and habitat for thousands of species.  In addition, oak trees sequester large amounts of carbon in their extensive root system and large amounts of wood.

Food

When we think of oak trees producing food, most of us think of acorns.  Acorns are a perfect package of calories that are used by an amazing number of life forms.  Squirrels hide them, as do some woodpeckers and jays.  Deer, turkey, and bears paw through the fallen leaves to find the acorns.  While tannins make acorns taste bitter, white oak acorns have less of them red oak acorns.  Indigenous peoples carefully processed acorns to remove the bitter tannins, dried them, and ground them into a nutritious flour. 

In the spring, deer eat tender buds and new shoots from oak trees, while rabbits eat twigs and the bark from seedlings and saplings.

Shelter and Habitat

Mature oak trees develop cavities that are used for nests by birds, racoons, and squirrels.  Fruit eating bats also use the cavities to live, as well as under loose bark on the trunk of the tree.  Oaks are home to more butterflies and moths than any other type of tree.  The larvae and adult insects attract hungry birds and other animals who use them as food.  In fact, birds depend on caterpillars to give their babies enough protein to grow and thrive.

Biodiversity

Oaks play a critical role in ensuring biodiversity in the forest.  Without oak trees, many species of birds, animals, insects, and plants would become extinct. 

Environmental Benefits of Oak Trees

Oak trees shape the environment of the area they live in important ways.

Soil

The large roots of an oak support the soil and prevent it from being eroded away.  The leaves that fall onto the forest floor provide habitat for many organisms.  Most of the caterpillars that feed on the oak drop to the ground to pupate before turning into moths and butterflies.  Oak leaves act as a mulch around the trees to conserve moisture and return nutrients to the soil.

Water

Rain is slowed by the extensive canopy of the oak tree.  The rain tends to soak into the soil because it hits the ground slower.  The roots of the oak tree are extensive and soak up hundreds of gallons of water over the course of the year.  This water is used in photosynthesis.  Oaks emit oxygen and water vapor as a part of this process.  Oaks also filter pollutants out of water.

Air

Oak trees are giant air filters.  They take polluted air and clean it.  The oxygen they produce is essential for the birds and animals that live near them.

Climate

Oak trees keep the air humidity stable by emitting water vapor.  They sequester carbon in their wood and roots.  They provide shade in the summer, cooling the area around them significantly.

Threats and Conservation

Oak trees are declining in nature.  Development, invasive species, and the slow growth of many oak species put them at risk.  Oak trees have extensive root systems and need room to spread out to grow well.  White oaks grow slowly and are easily outcompeted by other species of seedlings and saplings for sunlight. When a forest fire or timber cutting removes the older trees, white oaks may not be able to grow back unless helped by humans.  Without oak trees, hundreds of plants and animals would disappear. 

You can help keep oaks healthy by protecting them in your landscape.  Provide them the food and water they need to thrive.  Plant oak trees when you are landscaping your home.  If the trees need pruning, or are sickly, hire a certified arborist to take care of them.  We all depend on oak trees in one way or another.  It is in our interest to keep them around.

Further Reading