What determines tree height?

Steve Roark - Tri-State Outside

No doubt when you were a kid your mom encouraged you to eat all your food so you would grow up big and tall. She meant, of course, that by eating a balanced meal you would have all of the vitamins and proteins you would need for a healthy body. That same logic holds true for trees, as their height growth is closely related to how much water and minerals they are able to get from the soil.

Trees have two dimensions important to look at: Diameter of the trunk and total height. The stem diameter of a stand of trees is directly related to how many trees there are in a given area, say an acre. A young forest may have hundreds of trees per acre, with diameters averaging around 3-5 inches. A 1-year-old hardwood forest regenerating after a timber harvest can have many thousands of trees per acre with diameters of only a fraction of an inch.

As this tiny forest grows the trees begin to compete for sunlight, as there’s only so much solar energy delivered to that acre. Competition leads to slower diameter growth and ultimately some trees lose the sun battle and die out, reducing the number of trees per acre. When the trees reach 12 inches in diameter, that acre of forest may only be able to support 150 trees. When the trees reach 20 inches in diameter, there may only be 50 to 75 trees per acre.

What is interesting about all this is that while tree diameter slows through competition with other trees, tree height growth remains relatively unaffected. An acre of trees 20 years old may have 200-300 trees per acre with diameters averaging say 6 inches, with an average height of 25 feet. If that same acre had been thinned out during its growth so that at age 20 there were only 100 trees per acre, the average diameter would be much larger, say 10 inches, but the trees would be no taller than the un-thinned stand. In a nutshell, height growth is not related to sunlight, diameter growth is.

What determines tree height growth are the environmental conditions of that acre of ground the forest is growing on, things like soil and slope conditions. Deep, moist soils with plenty of organic matter (found on north and east facing slopes and lower slopes) will grow taller trees over a given time than a dry or shallow soil found on south and west facing slopes and upper ridges. Foresters use this knowledge to evaluate land for its tree growing potential. By measuring the total height of the trees and comparing that to their age, foresters can determine which sites will grow the best trees for a given species, and even figure out how much wood those sites could potentially produce. In forester-speak it’s called “site index,” referring to the height a tree of a given species should reach on a site at age 50. For instance, a site index of 70 means the tree should be 70 feet tall by age 50. 70 is kind of a medium quality site. Great sites would have a 90 plus site index, while a sites below 60 are considered poor.

For more information on tree botany and forest management, contact your local state forestry office.

Steve Roark is the area forester in Tazewell, Tennessee, for the Tennessee Department of Agriculture, Forestry Division.

Steve Roark

Tri-State Outside

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