The changing composition of forests: a competition for resources
Many landowners don’t realize that the forest they have lived or worked in all of their lives is constantly changing. Some mistakenly believe that if they do nothing to their forest it will remain in its current state indefinitely. Unfortunately for them, the eternal battle for light, water, nutrients and growing space is constantly shaping and reshaping the composition of their forest.
Learn more about How Trees Grow in this publication.
The role of sunlight in tree growth
Some species of trees can grow well under the shade of other trees, while other species are adapted to growing in full sunlight; this is referred to as shade tolerance. The first trees to colonize an open area are the fast-growing, sun-loving (shade intolerant), short-lived trees. Examples of these types are aspen and white birch. While these trees are growing, more shade tolerant species are growing in their understory, waiting for the day when the aspen and birch die to create an opening of sunlight and space. The next trees are likely to be red maple, red pine, and white ash. As the canopy gets thicker, and the forest floor darker, the more shade tolerant trees take over. These trees include sugar maple, hemlock and balsam fir.
Learn more about the shade tolerance of various trees in the publication Tolerance of Tree Species.
The role of soil in tree growth
A site's soil also plays a role in the type and quality of trees that will grow in your woodland. In general, soil type is classified by its texture (or size of soil particles), porosity and fertility. These soil characteristics will determine the quality and quantity of water and nutrients that are available to help trees grow. Soil types span a range from sand to loam to clay, and every combination in between. Sandy soils drain easily, and so tend to be dry and low in nutrients. Clay soils tend to be higher in nutrients, but wet because they drain poorly. Loam soils are a combination of sand, silt, and clay. Their nutrient and water holding capacity varies, depending on the degree of clay, sand, and organic matter present. Similar to sunlight conditions, some trees have adapted better than others to withstand less than ideal soil conditions, which means they have a competitive advantage there. For example, tamarack can grow in wet soils, and jack pine and oak can grow in sandy soils.
The competition for sunlight, water, nutrients and space will affect how or whether a tree continues to grow. You can look at the growth rings of trees and see when resources may have been plentiful or restricted. A dry season or an extended spell hidden in the shade of another tree will reduce a tree’s growth, while an opening of sunlight and space will increase its growth.
Changes can take place slowly over time, as tree species replace one another in a process called succession. The driving force behind succession is that competition over resources, and trees have adapted to be the best competitor under specific conditions. Changes can also occur rapidly, when wind, fire, insects, or humans disturb the way the forest is growing and cause new growth patterns to begin.
When a forest first begins, the pioneer species take advantage of an open space created by some event. This vast amount of space and sunlight allows them to outcompete other trees. Eventually, these trees reach their mature age or can no longer compete with the trees around it. One by one, they die and open up a small space in the woods, where the next generation of trees awaits their chance to take over. Over time, the sun-loving pioneer species are replaced by more shade tolerant species. What that species may be depends on what the site supports based on its climate, topography and soil.
Learn more about Forest Succession with this publication.
Understanding which trees grow best under which conditions helps to ensure a successful management plan. As foresters prepare a plan they take stock of not only the trees, but also the soil types and the plants growing beneath the trees. This information gives them the clues they need to be able to match the ecological trends for each forest with the landowner’s goals for their forest.
How Forest Trees Grow G3277
Forest Soils of Wisconsin: An Overview G3452
Maintaining Soil Quality in Woodlands: A Lake States Field Guide FR-409
Forest Succession Forestry Fact 78
Tolerance of Tree Species Forestry Fact 79
Hear from another woodland owner
Zdanovec discussing matching trees to the site
Thompson and promoting natural regeneration