How
Trees Grow
When planning for timber harvest, it is helpful
to understand how forest trees grow to predict yields. Understanding the
growing cycles of trees will also help you decide when to prune, plant
or thin trees for optimal growing conditions.
Several factors affect tree growth, including site characteristics such as soil depth, texture, fertility and moisture, sunlight, and topography. On the whole, deep soils are better for tree growth than shallow soils because they are generally more fertile (nutrient rich) and hold water better. Rooting depth may be restricted by bedrock, coarse gravel or excess soil moisture. Researchers have found tree roots, including the main tap root, absorb the most nutrients and water in the top two feet of soil. Topography also affects tree growth because of its influence on soil depth and moisture. Lowlands hold more water and have a greater nutrient supply than uplands, while heavily sloped lands are more prone to erosion, thus less top soil and lower moisture levels.
Trees vary widely in growth, longevity and branching habits. Trees do not grow as much in heavy seed years due to the energy needed for reproduction. As trees grow older, the crowns grow together, sometimes closing off available sunlight and reducing moisture levels. Pruning and thinning becomes a necessary management practice at this stage. Thinning generally accelerates diameter growth by creating more growing room and allowing more sunlight in, which stimulates more growth in crowns and stem bases. How much growth is stimulated depends on how suppressed the trees were from overcrowding. Pruning has just the opposite effect of thinning. Removing lower branches tends to slow diameter growth at the stem base and reduce stem taper.
For more information read "How forest trees grow", Wisconsin Woodlands G3277.
The changing composition of forests
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, and growing space is constantly shaping and reshaping the composition
of their forest. Changes can take place slowly over time, as tree species
replace one another in a process called succession. 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. The driving force behind
succession is competition over resources and trees have adapted to be
the best competitor under specific conditions. For example, aspen is the
gold medal winner when it comes to growing in full sunlight while sugar
maple is king in the deep shade. Here is a simple, yet common, successional
trend for forests in many parts of Wisconsin:
A large section of forest, maybe 10 to 20 acres in size, is knocked down by a windstorm or an ice storm (or by timber harvesting) creating an opening where full sunlight reaches the ground. Within a year, aspen sprouts and white birch seedlings are abundant and dense. Over the next 30 years they will race to the sky as each tree tries to get the most sunlight it can. The best competitors will survive while the inferior aspen and birch become too shaded and die off. Between 30 and 60 years, the trees will mature and begin to succumb to old age and disease. During this time, seedlings of trees that can tolerate some shade will begin to flourish and take over the ground cover. These shade tolerant trees like maple, ash, and balsam fir will become the dominant trees in the forest over the next 50 years or so. The forest canopy is much denser under these trees and only those species that have adapted to very low light levels will survive. Sugar maple and hemlock seedlings will continue to develop under these conditions and will occupy gaps in the canopy created by the death or harvesting of a tree or trees. Stands may continue in this state for decades and even centuries, but will eventually succumb to a disturbance of some kind that opens up large portions of the canopy like a windstorm or an ice storm or by a timber harvest and then the whole cycle repeats itself.
Understanding which trees grow best under which conditions helps to ensure
a successful management regime. As foresters prepare a management 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 manage the successional trends for each forest
to match the landowner’s goals for their forest.
Stand
Management
Forests can be divided into stands of various timber types based on tree
species and age. Developing a management plan for a forest entails creating
plans for each of the different stands within the forest. The pages linked
below describe stand-management concepts for timber types widespread
in Wisconsin, and follows some common landowner objectives. While these
introductions are not fully comprehensive, this section can serve as a
model for stand-management guidelines that should be addressed during
the woodland management planning process.
Follow the links here for more information on the varying management
practices based on tree species: Red
pine, Red oak, Aspen
and White Birch, Lowland
Conifers, Floodplain Hardwoods,
Swamp Hardwoods,
Mixed Hardwoods in Southern
Wisconsin, Mixed Hardwoods
in Northern Wisconsin.
The image to the left depicts an area managed using even-aged management
for red pine. The prescription is designed to mimic historic natural disturbance
patterns typical of this ecosystem.
The image to the right depicts an example of a stand managed in an uneven-aged manner. Notice the presence of areas with both young and old trees. In this example, the silvicultural system utilized would be classified as a group selection since groups of trees are harvested to regenerate a new age class.
Stand Age Structure
As stated in “The changing composition of forests” section of this page, some species can grow well under the shade of other trees while different species are adapted to growing in full sunlight. Those species that are adapted to full sunlight tend to grow as a cohort following a disturbance (e.g. wildfire, wind or ice storm, clearcut) that creates a large opening in the forest. These trees are essentially the same age and these stands are identified as being even aged. Uneven aged stands are made up of trees of many different age classes from very old trees to seedlings and saplings. In these stands, mature trees die off or are harvested creating a gap that allows seedlings and saplings to grow and mature.
Trends
The Wisconsin Department of Natural Resources has identified several trends
within our state's forests that have potential social, economic, and ecological
implications for all who enjoy our wonderful woodlands. Wisconsin's forests
were mostly shaped by initial settlement, wide-scale conversion to agricultural
use, and the early 20th-century logging practices and resulting fires
in the northwoods. Today our forests are slowly changing.
Five examples of these trends will be briefly mentioned here.
Trend #1: Wisconsin's Forests are aging and succession is occurring. The composition of Wisconsin's forests is changing to include less early successional species like aspen and birch and more late successional species like maple and basswood. This is important in Wisconsin because of the paper industry's reliance upon early succession species like aspen and birch as well as habitat for many important game species.
Trend #2: Some species are declining. Species like American elm, butternut, and jack pine are slowly disappearing from Wisconsin's forests due to exotic tree diseases and the disruption of natural disturbance patterns like wildfires. This can impact the biodiversity of wildlife habitats found in Wisconsin.
Trend#3: There is limited oak regeneration in southern Wisconsin. Oak forests in southern Wisconsin are being converted to elm-ash-soft maple as a result of harvesting and difficulty in regenerating oak. Oak is a very important timber and wildlife tree in southern Wisconsin.
Trend #4: Invasive exotic species are an increasing threat. Exotic (or non-native) species like gypsy moth, Asian long-horned beetle, and garlic mustard all pose a threat to Wisconsin's forests. These species have no native competitors or predators and thus can alter the structure and diversity of our woodlands.
Trend #5: Some ecosystems are becoming rare. Savannahs, barrens, and advanced successional forests are all becoming more rare in Wisconsin as a result of man-made and natural forces. This trend has reduced the suitable habitat for rare and endangered species and also has decreased the human benefits we receive from ecosystems. A good example is riparian forests that protect our favorite lake or river's water quality.
Questions to think about: Are any of these trends visible in your own woodland or in your corner of Wisconsin? Is there anything you can do on your property to address any of these issues?
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