How woodland trees grow

April 18, 2007 at 10:38 am Leave a comment

This post includes information about how site, climate, tree characteristics, and shade tolerance affect the growth and vigor of trees.

Effect of site characteristics
Site characteristics that affect tree growth include soil depth, texture, moisture, and fertility, along with topography.

On the whole, deep soils are better for tree growth than shallow soils because they potentially have a greater nutrient supply and water-holding capacity.

Soil texture refers to the size of soil particles.  Particles are classified by size (from smallest to largest) as clay, silt, and sand. Different soils have different proportions of each particle size.  Sandy soils absorb water quickly, but also lose it quickly to drainage.  Clay soils have a large water-holding capacity but absorb water slowly and hold it so tightly that much absorbed water is not available for plant use.

Soil fertility is based largely on the type of parent material from which the soil originated.  On the whole, fine-textured (clay) and medium-textured (silt) soils have a greater nutrient supply than coarse-textured (sandy) soils. Read more about Soils and Landscapes of Minnesota.

Forested slope near Lanesboro MN. Click for a better view.

Flickr photo by esagor. Click for original.

Topography affects tree growth largely because of its influence on soil depth and moisture availability.  Because gravity pulls soil particles and water downhill, soil depth, nutrient supply, and water supply usually are greater on bottomlands, lower slopes and benches than on steep slopes and ridge tops.

Aspect also influences the amount of sunlight and soil moisture available to trees.  In Minnesota, slopes that face north and east tend to be cooler and moister than slopes that face south and west.  These effects become exaggerated as the steepness of the slope increases.

Effect of climate
Length of the frost-free growing season, cold temperature extremes, precipitation amount, and duration of droughts are some of the elements of the climate that influence tree growth.

Native trees have evolved in our climate and are adapted to it. When we attempt to import trees from the south, they often cannot survive the winters, are damaged by late spring or early fall frosts, or find the growing season too short to consistently produce viable seed.

In prairie regions, rainfall is not evenly distributed over the growing season and prolonged summer droughts combined with factors such as wildfire limit tree survival and growth.  Some tree species will grow in dry, wet, or other tough conditions, but may need supplemental watering, mulching, or weed control, especially when young.

Effect of tree characteristics
A tree’s crown size, genetics, and ability to tolerate shade and competition from other plants all influence how well it will grow in different environments.

Large, healthy red oak crown.  Click for a better view.
Columnar crown in red oak. Click for a better view.
Large, healthy crown (left) and small, columnar crown (right) in Minnesota red oak. Flickr photos by esagor. Click for originals.

The most important aspect of crown form is the live crown ratio.  The live crown ratio of a tree is the percentage of the total tree height that has live branches on it.  The optimal live crown ratio varies by species and tree age.   If trees are too crowded and the crown is too small, the tree will grow slowly.  If the crown is too large, the amount of usable wood in the main stem will be reduced (knots tend to reduce the value of harvested logs).

Live crown ratios apply primarily to coniferous trees.  For hardwoods, the more important measures are crown position and spread.   The image below illustrates the different crown positions.  The dominant tree in the center has the optimal crown position (above its competitors) as well as a wide spread, allowing it to capture a lot of sunlight for growth.  The two codominant trees, on the other hand, have narrower crowns.  The intermediate and suppressed trees are under heavy competition and have inferior crown positions.

An illustration of different crown classes. Source: Woodland Stewardship: A guide for Midwestern Landowners.

Note: One widespread misconception is that small, stunted trees will bounce back and thrive after partial harvests. In many cases this is not true. Trees that have been stunted and suppressed, with poor form, often will continue to grow slowly and will not improve appreciably. During partial harvests, it’s crucial that you leave some large, dominant trees in the stand.

Shade tolerance
Shade tolerance refers to a tree species’ ability to thrive under low light conditions.  Tree species differ with respect to their tolerance for shade and competition.  Trees that are very shade tolerant will reproduce and grow beneath a dense canopy.  Trees that are very intolerant will survive only in openings that receive direct sunlight.  Shade tolerance can change with tree age.  The table below shows the shade tolerance classification for selected Minnesota tree species.


Very Tolerant balsam fir, ironwood, sugar maple

Tolerant American basswood, black spruce, northern white cedar, white spruce

Intermediate American elm, bitternut hickory, eastern white pine, green ash, northern red oak, red maple, shagbark hickory, sycamore, white ash, white oak, yellow birch

Intolerant black ash, black cherry, black walnut, butternut, paper birch, red pine, silver maple

Very Intolerant black willow, eastern cottonwood, eastern red cedar, jack pine, quaking aspen, tamarack

(Table adapted from Silvics of North America)

Why is all of this important? Forest ecologists and land managers depend heavily on knowledge of site characteristics and which tree species will perform best on a site. For more information about Minnesota’s native plant communities, visit the MN DNR’s Ecological Classification Systems page.

Advertisements

Entry filed under: silviculture. Tags: , , , , , , , , , , , , , , , , .

How forests grow Minnesota’s eastern white pine forest type

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Subscribe to the comments via RSS Feed


About this blog

News and information of interest to Minnesota woodland owners. Sister site to MyMinnesotaWoods.org.

In the news...


%d bloggers like this: