How Trees Grow

Back to Top

Life Cycle of a Tree

As with all living things, trees have a life cycle – from conception (seed), to birth (sprout), to infancy (seedling), to juvenile (sapling), to adult (mature), to elderly (decline), and finally to death (snag/rotting log). Because trees are renewable, the cycle begins again either artificially through planting or naturally with regeneration of new seeds.

For the life cycle to run full circle, external and internal conditions must be favorable for the tree. This includes adequate space, water, nutrients, and sunlight for the individual species. The tree’s chances of growth and survival through a complete cycle greatly improve with these conditions. However, even with optimum conditions, various stresses such as insects, diseases, injuries, competition from other trees, weather, and time itself weaken the tree and can cause it to die. Although a cycle may be stopped at any time for many different reasons, a new cycle can begin again through varying regeneration methods (seed bank, new seed, root or stump sprout, transplanting, etc.). It may be the same tree, a new tree, or another tree of a different species but all trees originate from seeds.

Seed

Think about which came first - the tree, or the seed? Regardless of what you believe, the more you learn about them, trees are simply amazing. Seeds come in a wide variety of shapes, weights, colors, and sizes, depending on the species. All seeds develop from male and female parts of the trees producing fruits but not all of them are easily recognizable or edible. Some seeds are contained in a protective nut like an acorn, pecan, or hickory. Other seeds are found in fleshy fruits, like the black cherry, mulberry, or persimmon. The fruit of a pine is a cone and the seed is winged and resembles a miniature helicopter when falling in the wind from an open pinecone.

Wind, water, animals, and people disperse seeds to a wide range of landscapes including the forest floor, open fields, yards, rocky slopes, and roadsides. Anywhere the conditions are favorable for germination, seeds will sprout and grow.

Sprout

An embryo is within each seed, but not all seeds will germinate. Favorable environmental conditions enable the embryo to grow, expand, and break through the seed coat using the stored food supply of the seed for the necessary energy to grow. The root grows downward to the soil to anchor the sprout and search for water and nutrients, while the sprout emerges from the ground seeking sunlight. Ideally, the sprout will find light and then the leaves, needles, or scales will develop further to allow the tree to make its own food through photosynthesis.

Seedling

The sprout persistently grows and begins to develop woody characteristics. The soft green stem begins to harden, change color, and develop a thin protective bark. Leaves or needles develop and continue to search out light. The root grows and branches down and out resembling an upside down underground tree with a flattened top. The majority of the tree’s roots are in the upper portions of soil to absorb available water and nutrients but also to breathe. Like us, tree roots need oxygen or they will die. The seedling must compete with other trees and plants for its share of nutrients, water, sunlight, and space. Other threats include fire, flood, drought, ice and snow, disease, insect attacks, and the threat of being consumed by animals. At this stage, the tree is most susceptible to being killed. If it can survive these early years without harm, the seedling is well on its way to the next phase in the cycle.

Sapling

A sapling is a small tree usually between 1 and 4 inches in diameter at 4.5 feet. This is the standard height where a tree’s diameter is measured – diameter at breast height (DBH). Typically, a sapling is the size of a tree that is growing in a commercial nursery for transplanting to your yard. In this juvenile state, the tree is not mature enough to reproduce. However, it is growing rapidly. The sapling encounters similar types of competition and threats to that of a seedling.

Mature

With favorable conditions, a sapling will continue to develop into a mature tree. During this stage in the cycle, each tree will grow as much as its species and site conditions will permit. In addition, flowers develop, reproduction ensues, fruits form, and seed dispersal can now occur.

The optimum time to harvest trees for forest products beneficial to people is during this stage in the lifecycle. In Texas, the majority of products are made from southern yellow pine (loblolly, longleaf, shortleaf, and slash). Hardwoods such as ash, hickory, mesquite, pecan, and oak are also used but southern yellow pine is the main commercially grown species to meet public demand for wood products such as those listed below. A few of the products made from Texas trees include: paper, dimensional lumber, cabinets, molding, doors, pallets, boxes, crates, trusses, fireplace mantels, furniture, oriented strand board (OSB), flooring, crossties, joists, decking, log homes, posts, poles, cooking wood, fencing, fuel, interior finishing, pilings, bridges, animal bedding, mulch, heavy construction timbers, charcoal, shingles, and many more. If a tree is never harvested, over the course of time it continues to provide many other benefits, but eventually will begin to decline.

Decline

At this point, the tree’s survival is determined more by external stresses rather than the tree’s vigor. These stresses take a toll on the tree making it more susceptible to insects and diseases, and it eventually succumbs to a causal agent or the pressures of competition from other more vigorously growing plants adjacent to the tree. The end result is no surprise.

Snag

The life span of a tree is as wide-ranging as the number of tree varieties, yet death is inevitable. Usually it is a combination of factors that finally overcome a tree and cause it to die. Injury, drought stress, followed by disease, rot, root dieback, coupled with a lightning strike and insect infestation is just one of many scenarios. However, sometimes it can be just one factor serious enough to cause mortality. Yet, the cycle does not end here. A standing dead tree, also called a snag, still plays a vital role in the life cycle.

Decomposition takes time. A snag slowly breaks down and returns nutrients to the soil as small limbs, bark, and branches fall to the ground. The snag also provides habitat, cover, and food for wildlife and insects. In turn, animals, insects, and fungi help break down the tree. Eventually, the snag will fall to the ground and gradually return nutrients to the soil where they are taken up again by other trees by providing for their growth.

And, the cycle begins anew.

Educator Extensions

Project Learning Tree (PLT) is an award-winning environmental education program designed for teachers and educators of students in pre-kindergarten through grade 12. The PLT Activity Guides are available from participating in a one day workshop. Find more information about Texas PLT and workshops near you at http://plttexas.org/. If you can’t find a workshop in your area, contact us to schedule one at your facility.

The following activities relate to How Trees Grow from the PLT PreK-8 Activity Guide:

Children’s books that help depict a tree’s lifecycle include:

• The Grandpa Tree by Mike Donahue;
• The Lifecycle of an Oak Tree by Angela Royston;
• The Lifecycle of An Oak Tree by Linda Tagliaferro;
• From Acorn to Oak Tree by Jan Koffke;
• The Pecan Tree by Barbara Langham