Most trees have rough, brown, or gray bark, but many species exhibit a striking green coloration on their trunks and branches. This distinct hue signifies a specific biological adaptation: the tree retains photosynthetic capabilities outside of its leaves. This survival strategy allows these plants to supplement their energy supply or survive periods when foliage is absent.
Why the Bark is Green
The green color in tree bark is a direct result of the pigment chlorophyll, which is present in the outer layers of the stem tissue. In these specialized trees, chlorophyll is retained within the inner bark, which is normally covered by non-photosynthetic cork in most species. Sunlight penetrates the relatively thin, smooth outer bark, initiating photosynthesis within these layers. This allows the trunk and branches to convert light energy, carbon dioxide, and water into sugars, supplementing the energy produced by the leaves.
Stem photosynthesis offers a significant advantage, particularly when leaves are shed due to environmental stress or dormancy. The process helps the tree maintain a positive carbon balance and provides an auxiliary energy source. Bark photosynthesis also plays a role in the internal oxygen balance of the stem, counteracting oxygen deficiency in the dense, metabolically active inner tissues. The specialized chloroplasts function efficiently with the limited light that filters through the outer layers.
Identification: Desert and Arid Species
Green-barked trees are most widely recognized in hot, arid regions, where this adaptation is crucial for survival. The Palo Verde tree, belonging to the genus Parkinsonia, is the quintessential example; its name means “green stick” in Spanish. Species like the Blue Palo Verde (Parkinsonia florida) and Foothill Palo Verde (Parkinsonia microphylla) are native to the Sonoran Desert in the Southwestern United States and Mexico.
The primary survival mechanism involves conserving water by dropping their small leaves during extended drought. By conducting photosynthesis through their vibrant green bark, these trees continue to produce energy even when leafless. This allows them to endure scorching temperatures and dry conditions that would be lethal to most other deciduous trees.
Identification: Temperate and Ornamental Varieties
Green bark is also found in cooler, temperate climates, where the adaptive strategy shifts from drought survival to energy production in low-light or cold-dormant conditions. The Snakebark Maples, such as the Striped Maple (Acer pensylvanicum) and Père David’s Maple (Acer davidii), are prime examples. These deciduous trees are often found as understory species in mountain forests, where their smooth, olive-green bark is frequently streaked with white lines.
The green bark allows these maples to photosynthesize during the early spring, late fall, or even throughout mild winters when their leaves have dropped. This provides an important energy boost when the trees are leafless. This is particularly beneficial for understory species that receive limited light during the summer growing season.
Other temperate trees, including certain dogwoods and birches, also possess photosynthetic tissue beneath their outer bark, especially on younger stems and twigs. While their mature bark may not be prominently green, the ability to photosynthesize in the stem ensures an early start to the growing season or helps recycle carbon dioxide respired by the woody tissues. This auxiliary energy source highlights a widespread biological phenomenon across diverse habitats.