Many people imagine a tree growing by stretching its entire trunk and branches upwards, like a living tower extending towards the sky. While trees certainly increase in height, the actual processes behind their growth are more intricate and reveal clever biological strategies.
How Trees Increase in Height
Trees grow taller through a process called primary growth, which occurs at their very tips. Specialized regions of rapidly dividing cells, known as apical meristems, are found at the ends of stems and branches, as well as at root tips. These meristems continuously produce new cells that elongate, causing the plant to extend in length. This growth ensures the tree can reach for sunlight and its roots can explore new soil for water and nutrients.
The existing parts of the trunk and branches do not stretch or move upwards once they have formed. Instead, new growth is added on top of the old, effectively pushing the tree’s height higher. This means that a branch that formed at a certain height on a young tree will remain at that same height as the tree ages.
How Trees Increase in Width
Trees also increase in girth through a distinct process known as secondary growth. This lateral expansion is managed by another type of growth tissue called the vascular cambium, which forms a thin, cylindrical layer just beneath the bark. The vascular cambium is responsible for producing new wood, or xylem, towards the inside of the tree, and new inner bark, or phloem, towards the outside.
The continuous production of these new layers by the vascular cambium causes the trunk and branches to thicken. Each year, a new layer of xylem is added, which leads to the formation of visible annual growth rings in the wood. These rings provide a record of the tree’s age and past growing conditions. This increase in width provides the structural support necessary for the tree to withstand its own weight and environmental forces like wind.
What Happens to Markings on a Tree
Markings on a tree’s trunk illustrate its growth. If you were to carve initials into a young tree, or if a branch were to form at a particular height, that marking or branch would remain at the exact same height as the tree grows taller.
As the tree continues to increase in width, the new layers of wood produced by the vascular cambium will grow around the marking. This means the carving will become embedded deeper within the trunk over time, and the bark will eventually grow over it. This demonstrates that a tree’s height increases only at its tips, while its trunk expands outwards.