Tree bark is the outermost covering of the stems and roots of woody plants, including trees. It functions as a protective layer, shielding the tree from its external environment. This complex outer tissue varies greatly in appearance among different tree species.
The Layers of Tree Bark
Tree bark is composed of distinct layers, each with specialized roles. The bark generally consists of an inner bark and an outer bark. The inner bark, known as the phloem, is a living tissue responsible for transporting sugars and other nutrients produced during photosynthesis from the leaves to the rest of the tree, including the roots.
The outer bark is primarily composed of dead cells and is referred to as the periderm or rhytidome in older trees. The periderm itself comprises three layers: the phelloderm, cork cambium (phellogen), and cork (phellem). The cork cambium is a meristematic tissue that produces new cork cells towards the outside and phelloderm cells towards the inside. Cork cells are impregnated with suberin, a waxy substance that makes them impermeable to water and gases, forming a robust protective barrier.
Essential Roles of Bark
Bark serves several functions important for a tree’s survival. Its primary role is protection against physical damage, such as impacts and browsing animals. It also acts as a barrier against pests and disease-causing organisms like fungi and bacteria. The outer bark, with its waxy composition, significantly minimizes water loss from the tree’s stems by preventing excessive evaporation.
Beyond physical defense, bark provides insulation against temperature extremes. Some tree species possess bark that offers protection against fire damage, allowing them to endure wildfires.
Diversity and Development of Bark
Tree bark exhibits a wide range of appearances, varying in texture, color, and pattern among different species. Textures can range from smooth to deeply furrowed, scaly, or peeling. Colors vary from shades of brown and gray to red, green, white, or even striped patterns. These diverse characteristics can be used for tree identification, especially when leaves are absent.
Bark develops and changes as a tree ages and grows. As the tree’s trunk expands in girth, the outer bark stretches and may crack or split. In many species, this leads to the shedding or exfoliation of old, dead bark layers, revealing newer bark underneath. This process is a natural part of the tree’s growth cycle and helps maintain healthy lenticels, which are pores that facilitate gas exchange between the tree and the atmosphere.