What is a Tree?

The line of distinction between trees and shrubs is not altogether clear-cut and therefore varies with different authors. For our purpose a tree may be defined as a woody plant with a single stem or trunk and a more or less definite crown, and attaining a height of at least 12 feet.

Omitting the intricacies of strict botanical classification, the tree species of the National Park System may be divided, in a broad sense, into three categories. In the first two listed below, growth in diameter is due to the addition of new layers of wood inside the cambium, which lies beneath the bark. This growth produces annual rings.

Conifers, or Cone-bearing Trees.—These trees are also known as evergreens and softwoods. Common examples of this group are pines, firs, spruces, hemlocks, and junipers.

Broadleaf, or Deciduous, Trees.—Also termed hardwoods. Common examples are oaks, maples, sycamores, and elms.

There are exceptions to these distinguishing characteristics in both groups. For example, among the conifers the larch and baldcypress are deciduous, shedding all their needles in the fall; and some of the southern pines, which are conifers, have wood which is harder than that of some of the so-called hardwoods. Furthermore, the yew, an evergreen with leaves similar to those of conifers, has a fruit which is a berry rather than a cone. In the broadleaf group are some species which are evergreen, such as hollies, live oaks, and some magnolias.

Palms, Yuccas, and Cacti.—Palms have their bundles of water- and food-conducting cells with strong fibers scattered through the softer tissue, usually much closer together toward the outer edge than in the center. The wood, especially in the outer part of the trunk, may be very dense and hard. Palm trunks have no true cambium, do not continue diameter growth from year to year by the addition of new layers of cells, and are mostly unbranched. There is but one main growing point—the terminal bud.

The trunks of the Joshua-tree (a yucca) and the saguaro (a giant cactus) become branched. Tree yuccas, with light and fibrous wood, also have separate conducting bundles but these are arranged in more or less concentric layers, new growth being laid down by a special kind of cambium.

In the saguaro and other tree cacti, the wood consists of elongated, somewhat latticelike, cylinders of dense fibers occurring in circles, and imbedded in more or less fleshy pulp. Tree cacti trunks enlarge their diameter yearly by laying down additional layers of pulp and woody tissue. After the death of the plant, the pulp decomposes, leaving a skeleton of woody cylinders.

These three types of trees are of limited occurrence in the National Park System, but are of unique interest. The Florida royalpalm and cabbage palmetto are found in Everglades National Park in southern Florida; California washingtonia, or California-palm, and Joshua-tree, in Joshua Tree National Monument in southern California; and the saguaro, in Saguaro and Organ Pipe Cactus National Monuments in Arizona.