Variables That Affect Tree Size

By: Stephen Jordan (email)

There are many factors or variables that affect size of a tree. They range from the amount of space available for growth to insects and diseases defoliating the tree. Can you name any other factors or variables that would influence growth of trees? Hopefully you thought of a few. Common factors are discussed below.

The species of tree is a very large player in how large a tree can become. A dogwood tree is not going to be as large as a white oak tree. The flowering dogwood is a small stature tree where as a white oak tree is a much larger stature tree. Every species of tree grows in its own unique way. Detailed information on the biology of many tree species found in the United States can be located at the United States Department of Agriculture Forest Service website.

Another factor that influences how large a tree can become is the space available for it to grow roots. Below are two pictures. One is a tree in a parking lot and the other is growing out in an open area. The tree in the field will have ample space to grow. The tree in the parking lot only has a 4 foot by 4 foot growing area. If these two trees were planted at the same time and given the exact same amount of water, fertilizer, and sunlight the tree in the field would still grow to be larger because of the space available for its roots to expand.

A tree growing in a parking lot in approximately a 4 foot by 4 foot growing area.

A tree growing in an open area that has ample room to grow.

Tree growth is also influenced by how closely trees are planted to each other. For example in the two photos below one tree is open grown and the other trees are in a forest. The open grown tree doesn’t have competition from other trees for sunlight. There fore it is more likely to grow outwards than upwards. In the forested picture the trees have very few low branches and compete to be the tallest. The forest trees are shaded by each other. This encourages them drop to their lower branches and concentrate growth at the top so that the tree doesn’t become suppressed by the other trees around it. Notice how the trees in the pictures below differ from growing in an open area to growing in the forest.

A tree that is open grown.

Trees that have grown in a forest close together.

Other factors that affect tree growth are water, available nutrients, and pH of the soil. Water is needed for tree growth. If water is limited the tree will have little growth. This is also true if nutrients in the soil are limited. The major nutrients that trees and all plants use are Nitrogen (N), Phosphorus (P), and Potassium (K). There are also micronutrients that trees and plants need as well. 

The pH of the soil is also critical in tree growth. pH is the measure of acidity. There are tree species that prefer to grow in an acidic environment and grow poorly in alkaline environments. This is also true in reverse. Trees such as red maple can grow in a broad range of pH. However, pin oak a very popular urban tree requires an acidic pH and is often sickly and chlorotic (yellow) in soils that are not acid enough – A condition know as iron chlorosis.

The factors mentioned here are the major factors that influence the size of a tree. Please visit the websites above or http://www.cnr.vt.edu/dendro/dendrology/factsheets.cfm for detailed information on each species.

Questions:

1. What are the main factors influencing the size of a tree?

2. What are the three main nutrients that trees need?

3. What influence does pH have on trees?