The growth and longevity of trees depend entirely on a thin layer of living tissue known as the cambium. This layer is the engine of girth expansion, allowing a tree to increase in diameter throughout its life. Cambium is responsible for all of the tree’s secondary growth, producing the dense wood that provides structural support and the bark that offers protection.
What Cambium Is
Cambium is classified as a lateral meristem, a type of tissue composed of undifferentiated cells that actively divide to produce new cells. This cell division enables the plant to grow in width, a process called secondary growth. The cambium forms a continuous, thin cylinder running along the entire length of the tree’s stem and roots. It is situated between the inner wood and the outer bark layers of the tree.
The cambium layer is often only a few cells thick, but its activity is constant during the growing season. This layer acts as the generative source for the plant’s vascular system, which transports water, minerals, and sugars. The new cells produced by the cambium differentiate into specialized tissues, continually creating new pathways for nutrient movement and providing structural strength.
The Role of Vascular Cambium in Tree Growth
The primary form of this tissue is the vascular cambium, which is solely responsible for producing the tree’s water and food transport systems. This meristematic layer generates secondary xylem cells toward the center of the stem, which accumulate to form the wood. Simultaneously, the vascular cambium produces secondary phloem cells toward the exterior, making up the inner layer of the tree’s bark. The production of secondary xylem is significantly greater than that of secondary phloem, which is why wood forms the bulk of the trunk’s mass.
The increasing girth created by the secondary xylem provides the necessary mechanical strength to support the growing canopy against gravity and wind. The newly formed phloem, or inner bark, moves sugars produced during photosynthesis from the leaves down to the rest of the tree.
Understanding Cork Cambium
The second major type of lateral meristem is the cork cambium, also known as the phellogen, which is located closer to the tree’s exterior. This tissue creates the protective outer layer of the tree, distinct from the vascular cambium. As the trunk expands in diameter, the outer layers of the stem are stretched and often rupture, requiring the cork cambium to develop and replace the original epidermis and cortex.
The cork cambium produces cork cells (phellem) toward the outside and the secondary cortex (phelloderm) toward the inside. This accumulation forms a protective layer called the periderm, which constitutes the outer bark. Cork cells are dead at maturity and are filled with suberin, a waxy substance that makes them impermeable to water and gases. This waterproof, insulating layer shields the underlying living tissues from physical damage, harsh weather, and pathogens.
How Cambium Creates Tree Rings
The yearly activity of the vascular cambium is directly responsible for the visible phenomenon of tree rings, or annual rings. In regions with distinct seasons, the cambium’s rate of cell production fluctuates predictably throughout the year. When the growing season begins in the spring, the vascular cambium is highly active, producing large cells with thin walls. This rapid growth forms the lighter, wider band known as earlywood or springwood.
As the season progresses into late summer and fall, growing conditions typically become less favorable, causing the cambial activity to slow down. During this period, the cambium produces smaller cells with thicker walls and a higher density. This slower, denser growth forms the darker, narrower band known as latewood or summerwood. The sharp contrast between the dense latewood of one year and the less dense earlywood of the next year creates the distinct boundary recognized as a tree ring.