Trees grow taller and expand their crowns through a process known as primary growth. This is also how tree roots grow longer. On the other hand, trees increase in diameter via secondary growth. Both types of growth occur in areas of specialized cells known as meristems. Understanding how trees grow – and the difference between primary and secondary growth – helps arborists and foresters do a better job of caring for trees. In the first of this two-part series, we will focus on tree height. In part two, we will focus on the diameter growth of stems, branches, and roots.
Trees have apical (or terminal) buds at the ends of branches and shoots. In the spring, when buds open, the new shoots begin actively elongating and the small leaves – that overwintered inside the bud – unfurl and grow into full-sized leaves. To “prove” that trees grow from branch tips and not from the base, imagine that you ax blaze a boundary tree in your woods. When you return years later to repaint the blaze, how much will it have moved? The answer, of course, is that it will be found at the exact same height as the day it was first made.
In contrast, grass grows from the base of plants, pushing new growth up from the bottom. This difference means that grass can tolerate removal of the top of the plant via mowing or grazing. Trees, on the other hand, can be severely damaged by improper removal of branch ends via a pruning method known as “topping.” Topping is strongly discouraged by the International Society of Arboriculture and other professional tree care organizations. If a tree has outgrown its space, a professional arborist may be able to use proper crown reduction methods to decrease tree size. However, a much better choice is selecting a tree that is matched to the growing space available in the first place.
Tree height is determined by genetics, climate, and soil. Although soil fertility can be improved in managed landscapes through addition of amendments like fertilizer, altering the soil in a forest to increase tree height is not practical. Instead, a forester’s primary objective when “thinning” stands of trees is to improve diameter (secondary) growth of the remaining trees by increasing the availability of sunlight, water, and nutrients and will be discussed further in Part II of this series.
If you have questions about your trees, check out UNH Extension’s Forests & Trees website or contact your County Forester for more information.
Have a question about your woods? Contact your Extension County Forester today!
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