How Trees Grow

From the time of seed germination to the death of a tree, it never ceases to grow. The growth rate varies according to its stages of development: seedling, sapling, maturity and senescence. In nature, trees reproduce primarily by seed, but some species also propagate themselves clonally. The potential maximum age, height and width a tree may achieve is generally determined by its genetic makeup, but certain other factors play important roles, such as soil, drainage, water, fertility, light, exposure and so on. Annual tree growth is described as slow (<12 inches per year), medium (13-24) and fast (>25).

In this day and age, the Coast Redwood trees (Sequoia sempervirens) represent the world’s tallest single-stemmed plants. It is an evergreen coniferous monoecious softwood species, which begins coning at about 10-15 years of age and grows at the extraordinarily fast rate of 5-7 feet per year as a younger tree. The Coast Redwood is a prolific seed producer, bearing many cones about 3 inches in length, containing small seeds no larger than those of a tomato. The tallest tree found in California has been measured to be 379 feet tall. This champion tree begs the question of how do trees grow so tall and what limits their maximum stature.

The growth patterns of all dicot trees are similar, but they are different from other plants which are popularly called “trees,” such as palms which are monocots. Continuing with the Coastal Redwood example, trees grow by producing new cells in four specific locations on the plant. Branch tips (buds) result in trees increasing their height. Root tips grow to build a huge lateral root system anchored in soil and rock to support the massive tree as it grows taller. Male and female cones are produced by new cell development on branches. Trees increase the diameter of their trunk and branches by producing new cells in the cambium layer (xylem and phloem), located between the outer layer of bark and the inner sapwood. The sapwood contains vascular bundles which transport water and nutrients within the tree. The heartwood represents the core of the tree and is the densest wood. Contrary to some popular beliefs, tree trunks do not elongate as they grow, they only increase in diameter. Coast Redwoods reproduce by seed, but if a tree falls or is cut new sprouts develop from the stump or downed tree root system. This tree produces high-value lumber.

A question that has intrigued scientists is how and why is height limited in these tallest of plants, when trees never cease growth. Studies of the Coast Redwood have shown that leaves at the top of the tree are smaller than those lower down. This may the result of a hydrostatic limitation; at about 380 feet in height the tree cannot pump water and nutrients any higher and hence increased height ceases.

Like all trees, Coast Redwoods ultimately reach senescence, in their life span of 500-700 years or longer. Old trees lose vigor and are less able to recover from physical damage caused by lightning or wind, and have diminished resistance to pests and diseases. But it is only in protected forests where the trees die of old age, elsewhere chain saws end their life during maturity.

Besides the Coast Redwood, we should make reference to two very large multi-stemmed trees which propagate by seed as well as clonally. For example, the Banyan Tree (Ficus benghalensis), propagates itself by aerial roots dropping from branches to strike root to form new stems, such that a single tree can expand in this manner and occupy up to entire acre of land. Also, Aspen trees (Populus tremuloides) , which self-propagate from lateral roots just below the soil surface which produce new stems, are believed to represent the largest organisms on earth, with colonies of genetically-identical trees that can extend for as much as 5 miles. The largest identified clone in Utah is estimated to be 80,000 years old! In terms of stature, neither of these record-breaking trees are particularly tall.

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