Picture a world without trees—no tree climbing or tree forts for the kids, no Christmas tree to decorate, no squirrels, chipmunks or birds living and playing in the branches, no gently filtered light creating ideal shade on your lawn, no lumber, paper, or forests. It is clear that trees are important for our existence and our enjoyment. This enjoyment increases with the more knowledge you have about the trees that surround you. Trees in a forest determine the other flora and fauna that can inhabit that ecosystem. They determine the structure of the canopy and understory and the composition of the forest floor in terms of leaf litter, pH (acidity), and how much light will penetrate. Ultimately this determines what flora and fauna can occupy a particular forest.
Trees are woody plants what are at least 12’ in height. They have a single trunk and a distinct crown. A person who studies trees is a dendrologist. Many field guides show silhouettes of trees so that the crown shape can be used as a tool for identification. Two important divisions of plants are the gymnosperms, which have naked seeds and angiosperms that flower. Coniferous trees that bear cones are part of the gymnosperm group and most conifers have needles and are evergreen, however the bald cypress and tamarack are exceptions and lose their leaves in the fall. It is a misconception that evergreens don’t lose their leaves, as the forest floor of a spruce-pine forest is covered in needles. They don’t lose all their needles at once like deciduous trees do. Some conifers don’t have woody cones. Yew have what looks like a berry, but is really a single seed surrounded by a fleshy layer derived from a single cone scale called an aril. These “berries”, though attractive in color, are poisonous. In the lumber industry, conifers are also generally called softwoods, although there are some exceptions where a conifer may have harder wood than a broadleaf tree (douglass fir is harder than basswood.)
Angiosperms are typically deciduous and lose their leaves, but there are some evergreen exceptions such as the American Holly. All angiosperms are all flowering broadleaf trees and most are considered hardwood, making them ideal for flooring and furniture. Beech, maple, oak, and birch trees are part of this group.
Forests are named after the dominant tree type: beech-maple, oak-pine, and spruce-fir. Other forest species inhabit zones parallel to the forest floor created by these trees. The canopy is the highest zone made of the crowns of trees. Some trees will have leafy crowns and crowns so close together that the canopy will be termed a closed canopy – one in which little light penetrates to the forest floor. An open canopy is the opposite; trees are widely spaced and light can penetrate. The top side of the canopy can be very inhospitable as it is subjected to harsh winds and precipitation, with little insulation from very cold or hot temperatures.
The understory is the next zone down and is home to trees that don’t grow as high or younger trees of the same type as the dominant canopy species. Some species grow well under their parent species (spruce and fir), while some do not (aspen). If a young tree cannot grow under a parent tree, this can lead to succession, which is a change in the dominant species over time. Every landscape is in a stage of succession from the pioneer species like the lichen that first colonize rock, to old growth forests. Unfortunately, we only have pockets of old growth forests that have been preserved, as most mature stands have been cut over long ago and are at an earlier stage of succession
The shrub layer is made up of woody plants that are shorter than 12’ and have multiple stems. You’ll find more shrub species in an open canopy broadleaf forest. The herb layer is home to herbaceous plants that lack woody tissue. This is home to many important wildflower species. At the bottom of a forest, you find the forest floor. Life does not stop there. There are many small organisms that burrow in the earth and fungi species that may have hyphae that stretch for miles underground.
All of these vertical zones in a forest are exposed to varying amounts of sunlight, wind, precipitation, and temperature. Have a look at the species pages for trees on myspecieslist.com, and please add your own unique observations about these species in the comment section. Learning more about the trees, will teach you much about the forests that you visit.