1. What makes a Tree?
“Wherever they grow, trees dominate the landscape and attract people with wide ranging interests from natural history to gardening. However, defining exactly what constitutes a tree can be rather difficult since examples come from almost all plant families; the group characteristically being woody and large in size. Trees may grow in solitary or magnificent splendour or en masse, blanketing the terrain [in a group, as a whole]1.However, in whichever situation they are found they are seen as a unique and fascinating group” (Sterry and Press 1995).
The science of trees is called dendrology which literally means “the study of trees” (usually dendrology also includes information on tree habits and ranges). Through common usage it has come to signify the taxonomy of woody plants, including trees, shrubs and vines. Taxonomy is concerned with classification, nomenclature and identification of natural objects.
Among all living plants, trees are the tallest, and they are highly visible parts of the natural world. Everyone expects to recognize a tree when they see one. This is mostly true, yet one can find interesting differences when studying definitions of trees.
According to a conventional definition: a tree is a woody perennial, that at maturity is 6 meters or more in height, usually with a single trunk unbranched for several meters above the ground with a more or less definite crown (Harlow et al. 1979).2
Woody plants differ from herbaceous plants because of their long lasting structure. In the case of trees, this is largely due to something called lignin in cell walls. The lignification process influences the property of wood products and how they can be used in construction, for instance, by determining how strong or how light the wood is. Another characteristic of trees is that they grow in diameters by producing outwards oriented (mostly periodic) growth rings, which are produced by the cambium layer below the bark. This growth mode separates trees from tall grasses, such as bamboos and palms, which typically have endogenous or inwards oriented growth.
The basic parts of a tree are the roots, trunk, branches, smaller twigs and the leaves composing the crown. The roots absorb water and nutrients from the soil but the majority of a tree's biomass comes from carbon dioxide that is absorbed from the atmosphere via the leaves as a result of photosynthesis.
The wood of the trunk consists of xylem cells, where lignin works to make the cells firm. Bark is made of phloem and other tissues external to the vascular cambium. The main role of the trunk is to give height to the crown or branches bearing leaves, aiding in the competition with other plants for sunlight. Straight trunks are effective in this endeavour and are representative of a tree's so-called “growth economics”.
Returning to the topic of tree height, we should recognize that height is not an exclusive characteristic of trees. For example, the bamboo tree is also tall and can be used for many of the same purposes as trees, (i.e., furniture construction or as a fibre for papermaking). It has even been called “poor man’s lumber.” However, biologically it is a grass, even if it is tall when compared to other grasses. Much of these same characteristics’ apply to palm trees.
The “minimum” size of a tree varies in different contexts: ranging from 2 – 3 meters up to 5 or 7 meters. The FAO’s (Food and Agriculture Organisation of the United Nations) Global Forest Resources Assessment adopted a 5 meter benchmark. Moreover, the height of some single trees from the same species at maturity may vary from between 0.5 meter to 50 meters or more, depending on the climate and soil conditions.
Shrubs, in contrast, are smaller and usually exhibit several erect, spreading, or prostrate stems and an overall bushy appearance. They are generally more than 0.5 m and less than 5 m height, and often without a definite stem and crown (TBFRA 2000). The line of demarcation between shrubs and trees is by no means clear-cut. For instance, sometimes trees may have several stems, either naturally or as the result of a coppice.
A given tree species may be shrubby near the extreme locations of its range, or at timberline, and still attain large proportions elsewhere. For example, the Alaska cedar (Chamaecyparis nootkatensis) is ordinarily a moderately large tree, but at timberline is often reduced to a dwarfed or even heathlike shrub. Similarly, the distinction between shrubs and vines is often poorly defined since several native3 species are first vinelike and then become shrubby. Lianas are climbing vines; plants of this sort are extremely numerous in the rain-drenched forests of the tropics (Harlow et al. 1979).
Related to the point above, it should be noted that neither in nature nor in international statistics are "forests" composed of only trees. For example, the FAO Global Forest Resource Assessment includes bamboos, palms and other woody plants into its forest definition.
What is dendrochronolgy?
A long life span is one key characteristic of trees and, as mentioned above, the age of a tree is determined by its growth rings. An accurate determination of age is only possible for trees which produce growth rings, and these generally grow in seasonal climates. Trees that grow in non-seasonal tropical climates do so continuously and do not have distinct growth rings. By using a specific tool, it is possible to estimate the age of a piece of wood taken from a tree (which has been maintained in building or in unaerobic conditions in the bottom of a lake). This practice is known as the science of dendrochronolgy.
2. All plants are beautiful in their typical ways and deserve our appreciation. Since trees have crowns, they apparently belong to the ”royal“ family of plants.
3. The textbook by Harlow et. al. (1979) considers trees in the USA and Canada and all measurements are in feet and inches requiring metrification (one foot is 12 inches or 30.38 cm).