After the first year, trees grow by increasing the thickness of the older buds.  Increase in height and density of crown cover is due to the development of the younger twigs.  New growth on the tree is spread evenly between the wood and bark over the entire body of the plant.  This process of wood production resembles a factory enterprise in which three layers of material are engaged.  In the first two of these delicate tissues the wood is actually made.  The inner side of the middle layer produces new wood while the outer side grows bark.  The third layer is responsible for the production of the tough, outer bark.  Year after year new layers of wood are formed around the first layers.  This first layer finally develops into heartwood, which, so far as growth is concerned, is dead material.  Its cells are blocked up and prevent the flow of sap.  It aids in supporting the tree.  The living sapwood surrounds the heartwood.  Each year one ring of this sapwood develops.  This process of growth may continue until the annual layers amount to 50 or 100, or more, according to the life of the tree.

One can tell the age of a tree by counting the number of annual rings.  Sometimes, because of the interruption of normal growth, two false rings may be produced instead of a single true ring.  However, such blemishes are easy for the trained eye to recognize.  Heartwood does not occur in all varieties of trees.  In some cases, where both heartwood and sapwood appear, it is difficult to distinguish between them as their colors are so nearly alike.  Because it takes up so much moisture and plant food, sapwood rots much more quickly than heartwood.  The sapwood really acts as a pipe line to carry water from the roots to the top of the tree.  In some of our largest trees the moisture is raised as high as 300 feet or more through the sapwood.

Strange though it may seem, trees fight with each other for a place in the sunlight.  Sprightly trees that shoot skyward at a swift pace are the ones that develop into the monarchs of the forest.  They excel their mates in growth because at all times they are exposed to plenty of light.  The less fortunate trees, that are more stocky and sturdy, and less speedy in their climb toward the sky, are killed out in large numbers each year.  The weaker, spindly trees of the forest, which are slow growers, often are smothered out by the more vigorous trees.

Some trees are able to grow in the shade.  They develop near or under the large trees of the forest.  When the giants of the woodland die, these smaller trees, which previously were shaded, develop rapidly as a result of their freedom from suppression.  In many cases they grow almost as large and high as the huge trees that they replace.  In our eastern forests the hemlock often follows the white pine in this way.  Spruce trees may live for many years in dense shade.  Then finally, when they have access to plenty of light they may develop into sturdy trees.  A tree that is a pigmy in one locality may rank as a giant in another region due to different conditions of growth and climate.  For example, the canoe birch at its northern limit is a runt.  It never grows higher than a few feet above the ground.  Under the most favorable conditions in Florida, where this species thrives, such trees often tower to a height of 125 feet.