The most visible brown part of a tree is commonly called bark, which refers to a complex system of protective and structural tissues. This brown hue, characteristic of the tree’s entire woody body, is a byproduct of biological compounds that provide strength, defense, and support. These brown structures are not a single material but multiple distinct layers, each with a specific function in the tree’s life and growth.
The Outer Protective Layer
The outermost layer of the trunk is the bark, which functions as the tree’s primary shield against the environment. Botanically, the rough, dead exterior is called the rhytidome, a layer composed of multiple sheets of cork and dead tissue. This exterior is constantly pushed outward and replaced from within as the tree’s girth expands, providing a physical barrier against mechanical injury, temperature fluctuations, insects, and fungal pathogens.
The distinctive texture of bark, whether deeply furrowed, scaly, or smooth, depends on how new layers form beneath the rhytidome. Cork cells within the bark contain suberin, a waxy compound that makes the tissue highly impermeable to water, preventing excessive moisture loss. The color of the bark also comes from various organic compounds, which can range from light gray to deep reddish-brown or nearly black across different species.
The Wood Beneath the Surface
Beneath the bark and the thin living cambium layer lies the wood itself, the main brown structure of the tree. This internal mass is divided into two major components: the lighter, outer sapwood and the darker, central heartwood. Sapwood, also known as the functional xylem, is the living part of the wood responsible for transporting water and dissolved minerals from the roots up to the leaves. Its lighter color is due to its active role and high moisture content.
As a tree ages, the inner rings of sapwood cease transport function and transform into heartwood. The cells in this central zone die, and their lumens become plugged with various organic compounds, collectively known as extractives. These extractives, including resins, phenols, and tannins, preserve the heartwood and make it resistant to decay and insects. The accumulation and oxidation of these compounds give heartwood its darker, richer brown color, distinguishing it from the surrounding sapwood.
The heartwood’s function shifts entirely from transport to structural support, acting as the central pillar that enables the tree to stand against gravity and wind. The heartwood’s dead, plugged cells contain less moisture and provide superior mechanical strength compared to the moist sapwood. This difference in color and resistance to rot makes the heartwood of many species, such as cedar and black walnut, prized for its durability and aesthetic qualities.
Brown Parts Beyond the Main Trunk
The brown coloration and woody structure are characteristic of all perennial parts of the tree, extending well beyond the main trunk. Branches and twigs are covered in the same protective bark layer and contain the same internal arrangement of sapwood and heartwood, growing wider each year. These structures provide the necessary rigidity to hold the leaves up to the sunlight and serve as conduits for the movement of water and nutrients. The root system, which anchors the tree firmly in the ground, also develops a brown, woody exterior as it matures. Like the main stem, these roots contain inner wood that provides mechanical support and a pathway for the absorption and storage of necessary resources.