Trees and other plants have often been called factories because they convert light energy into chemical energy. Through the process of photosynthesis, leaves capture solar energy by converting carbon, hydrogen, and oxygen into complex  sugars. One of these complex sugars is cellulose, the main ingredient of wood fiber. Although a complete knowledge of photosynthesis is not essential to understanding how trees grow, it is important to realize that, in theory, the faster and more efficiently a tree carries on photosynthesis, the faster it will grow. Availability of light and water are two factors that can be controlled by harvesting. 


When a portion of the growing stock is removed, the photosynthetic material is reallocated to the remaining trees. In a young stand, it is usually only a matter of 5 to 10 years before the crowns of residual trees grow into the spaces left by those taken out. Trees are principally composed of four main parts: roots, stem, branches, and leaves. Other specialized structures, like flowers and seeds, develop periodically for purposes of reproduction. However, virtually all of the important physiological processes in trees involve one or more of the four main parts.


Roots


About 20% of the mass of a forest-grown tree is devoted to roots. In addition to anchoring the tree, roots gather mineral nutrients, take up water, and store the products of photosynthesis. Forest tree roots are much more extensive than they appear. For example, the root system of a sugar maple may extend as much as 2 to 5 times beyond the spread of its crown. Most of these roots, known as fine or feeder roots, are within a few inches of the soil surface.  Though the fine roots may account for only 14% of the total root mass, 80% of the total root length is in fine roots. Consequently, roots are everywhere in the forest and the ones that are most sensitive to damage are also the most susceptible.


Stem


The main stem usually makes up about 60% of a tree's weight. It supports the branches and leaves and serves as the main plumbing system, with vessels to transport water and nutrients up to the leaves and with other cells to transport photosynthesis sugars to living tissue throughout the tree. The growing portion is only a thin layer of cells surrounding the main stem. Each year this thin sheath of cells puts down a new layer of sapwood. The rate at which it does this determines how fast a tree grows in girth.


Branches & Leaves


A tree's branches support leaves in a configuration that maximizes light availability or that protects them from excessive exposure on harsh sites. Branches also serve as the second-order plumbing of the main stem. The leaves carry on photosynthesis and exchange important gasses, such as oxygen and carbon dioxide, with the atmosphere. Combined, branches and leaves make up about 20% of the tree's total weight. Although all trees have roots, stems, branches, and leaves, the form of each of these components differ among species - and within a species in some cases.  Some characteristics, such as the size and shape of leaves from the top of the crown versus those on shaded lower branches, even differ on the same tree. For this reason, leaf size and shape are said to be plastic because they are characteristics that mold themselves to the circumstances. This is one of the reasons why learning trees solely by their leaves is so unreliable and frustrating.


Environmental Factors


Although many of the obvious differences among trees within a species are random, some are not. Important differences in a tree's form and function may be caused by environmental factors. For example, the root systems of most trees tend to be more extensive on drier sites. Another example: open-grown trees tend to have short stems and wide, deep crowns, while forest-grown trees of the same species, in their struggle to obtain crown space, tend to have long stems and short, irregular crowns that fit the available space in the canopy. Important genetic differences between species have evolved over millions of years.


Although not all structural differences are due to adaptations that make one species a better competitor than another on a given site, many of them are. For example, many conifers have adapted to become better competitors on dry sites than most hardwood species. Though most conifers will do well on better sites, their natural habitat is defined by the limits of tolerance of other species. White pine is a good example. It grows extremely well on moist, protected, "hardwood" sites, but it is nearly impossible to get a new stand of pine started using natural regeneration.  Hardwood species, such as sugar maple and yellow birch, are much better competitors in the understory. 


On a drier site, the reverse may be true - white pine can compete more effectively than most hardwoods. Each species has a range of environments in which it will grow. These extend from circumstances where it is a minor component, poorly formed and slow growing, to situations where the tree is able to take full advantage of a site and grow to its maximum biological potential. One of the most common silvicultural errors in forest management is trying to grow a species on a site where it can achieve only a fraction of its growth potential.


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[This information has been excerpted from the Introduction to Forest Ecology and Silviculture Thom J McEvoy, Extension Forester, published by the University of Vermont in 1995]