Trees with strikingly white trunks often stand out, prompting curiosity about their unique appearance. Understanding the specific types of trees that exhibit this characteristic, along with the biological processes behind it, reveals aspects of tree biology and adaptation. This phenomenon is not only aesthetically pleasing but also serves various functions for the tree’s health and survival.
Iconic Trees Known for White Trunks
Many tree species are recognized for their distinctively white or light-colored trunks, each achieving this appearance through different mechanisms. Among the most recognizable are birch trees, particularly the paper birch (Betula papyrifera), widespread across northern North America. Its bark is known for peeling off in thin, papery strips, revealing a fresh, chalky white layer underneath. This exfoliation gives the trunk its characteristic bright appearance.
Another prominent example is the Himalayan birch (Betula utilis var. jacquemontii), native to the Himalayas, which boasts an exceptionally bright, almost luminous white bark that does not peel as extensively as the paper birch. Instead, its bark remains smooth and clean, offering a consistent white surface. These birches thrive in cooler climates and are often planted ornamentally due to their striking visual appeal. Their smooth, unblemished white surface reflects light efficiently.
Aspen trees, such as the quaking aspen (Populus tremuloides) and European aspen (Populus tremula), feature pale, often greenish-white or grayish-white bark. The bark of young aspens is smooth and light-colored, becoming furrowed and darker with age. Their lighter bark is partly due to the presence of chlorophyll, allowing them to photosynthesize through their bark, particularly in colder months when leaves are absent. Quaking aspens are found across North America, often forming large clonal colonies.
Eucalyptus species, commonly known as “gum trees,” display white trunks due to their shedding bark. The ghost gum (Corymbia papuana) and snow gum (Eucalyptus pauciflora) are examples, found in Australia. These trees shed large plates or strips of old bark, exposing a smooth, new, intensely white or cream-colored layer beneath. The newly revealed surface is very smooth, contributing to its reflective quality and white appearance.
The American sycamore (Platanus occidentalis) exhibits a white trunk, especially on its upper branches and newer growth. Its bark exfoliates in irregular, thin plates, revealing lighter inner bark that ranges from cream to greenish-white. As the tree matures, the older, darker bark peels away, creating a mottled appearance with patches of lighter inner bark exposed. Sycamores are commonly found in riparian areas across eastern and central North America.
The Science Behind White Tree Trunks
The white appearance of certain tree trunks is primarily a result of the unique composition and behavior of their outer bark layers, known as the periderm. This protective tissue, which replaces the epidermis in woody plants, consists of three main parts: the phellogen (cork cambium), phelloderm, and phellem (cork). The phellem, or cork layer, is the most visible component, and its characteristics largely dictate the trunk’s color and texture. Its cells are dead and filled with air, which contributes significantly to light reflection.
One major mechanism contributing to whiteness is the exfoliation or shedding of older, darker bark layers. As trees grow in girth, the outer layers of the periderm crack and peel away. This exposes the newer, lighter-colored phellem beneath, which has not been subjected to weathering, oxidation, or the accumulation of pollutants and microorganisms that can darken older bark. The exposed, fresh bark appears brighter.
Some trees, like birches and aspens, possess a waxy or powdery coating on their bark, known as a glaucous bloom. This whitish wax layer is composed of tiny wax crystals that scatter and reflect sunlight efficiently. This reflection helps reduce heat absorption, protecting the tree from sunscald in intense sunlight.
In some species, the bark may contain chlorophyll. This green pigment allows the trunk to perform photosynthesis, contributing to the tree’s energy production, especially when leaves are absent during winter. While chlorophyll is green, the overall appearance can still be whitish-green or pale due to overlaying protective layers and how light interacts with the bark’s surface.
The “white” appearance is less about a specific white pigment within the bark cells and more about how light is reflected and scattered by the bark’s surface structure. The dead, air-filled cells of the phellem, combined with waxy coatings or exposed fresh layers, scatter all wavelengths of visible light. This scattering is what our eyes perceive as white, giving these trees their distinctive trunks.