How Do Trees and Forests Grow?

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Tree in the Sun  (photo: Matt Duvall)

This month we are going to get our hands dirty by digging into how trees grow and how trees interact in a forest.  My quest is to help you understand how it ties to the trees you see in your woods, and why the types of trees changes over time.  Understanding tree biology is also important if you want to plant trees (i.e. what trees grow best in the soil types and sun conditions you are working with?).

Like any other plant, trees need sun, water and nutrients for survival.  There are three main parts to a tree: the roots, the trunk and the crown (branches and leaves), and each plays a part in helping the tree use the sun, water and nutrients.  The roots absorb water and nutrients from the soil.  The crown uses the water, nutrients and sunlight to produce sugar for tree growth.  Most of the trunk of a tree is dead wood, but the outmost part directly under the rough bark is where all the action happens.  There are cells that act like straws taking the water and nutrients from the roots to the crown, and there are a second set of cells taking the sugars from the crown to the roots. 

Here’s the kicker, there is only so much sun, water and nutrients available in any given space, and the trees present in that area have to compete for those resources.  Those trees that can compete in or are adapted to those conditions will flourish.  Otherwise they tend to die. 

Trees can adjust to the conditions they are in by changing the way they grow.  For example, think of that big oak tree growing in an open farm field.  That oak has all the sun it wants, to itself.  The result is long, outstretched branches with lots of leaves to take advantage of the sun.  However, if that same oak were grown in a more crowded woods, it wouldn’t spend valuable energy growing long branches and lots of leaves that would only be shaded by surrounding trees.  Instead, it puts its energy towards growing tall and skinny to get up into the sun faster. 

Though some trees have adapted to grow in more unfavorable soil and light conditions, most trees only grow well in relatively specific circumstances.  Soils vary in the amount of nutrients available, so some trees have adapted to living in sandier soils that are less nutrient rich.  That doesn’t mean that those sand adapted trees won’t grow in more nutrient rich soils, but it means that they can now outcompete the ones that can’t handle the sand. Some trees need a lot of sun in order to grow and survive, while others are adapted to grow under shadier conditions.  Again, those trees that are adapted to shadier conditions can grow in full sun.  However, full sun adapted trees won’t survive or grow in the shade.  This concept is important to you as a landowner, particularly if you are interested in specific trees.  For example, if you want to produce acorns for turkey, you need to understand the soil and sun preferences for various oak species. 

How trees replace each other is the next part of this equation. A tree (or trees) dies from old age or poor health or a natural event, and other little trees are waiting in the wings to take advantage of the resources newly made available.   Let us explore what this means for specific trees in a forest.  Based on where your woodlands are located, foresters can fairly well predict the path your trees will take.  For example, in the northeast part of WI, it is common to find soils that are considered mesic (a moderate amount of water/ moisture) and are rich with nutrients.  If you had an open field (full sun), the first trees to start growing would be aspen and white birch.  Over time, you would have a forest with these aspen and white birch, but since they need lots of sun to grow, they can’t produce little aspen and white birch under them.  Instead, sugar maple, basswood, white ash, hickory and elms are moving in to take advantage of the shadier growing conditions.  As the aspen and birch die, the next stage of the forest will be these trees, unless something happens (fire, wind storm or something else) that knocks down almost all the trees and opens that are to full sun again where the aspen and birch flourish.  This process is called succession.

It gets more complicated, but I’ll spare you the gore.  Foresters are well versed in the requirements for various trees and forest processes.  They can help you understand which trees will grow best in your woods, and how that may change over time.  If you want to learn more about any of these concepts, check out these three publications:

How Forest Trees Grow

Forest Succession

Forest Trees of Wisconsin (describes some characteristics of each tree).