|What is a Tree?|
While most people have a clear image when they use the word "tree", botanists have yet to find a biologically meaningful distinction between "tree" and "shrub".
Definitions of trees differ from source to source in the details but they all center on a few common characteristics.
There are several more distinctions that have been made and every tweak of the definition usually eliminates something that someone, somewhere considers to be a tree.
Claiming that the trunk must be a solid eliminates banana trees whose "trunks" are actually multiple leaf stems pressed together. Requiring the trunk to be woody eliminates palms. The singular stem and minimum height requirements eliminate shrubs, which seems obvious to the layperson and completely unfounded to the biologist.
Making the distinction between small trees and large shrubs becomes even more problematic. Generally speaking the distinction focuses on the raised canopy and singular trunk of the tree versus the overall branching and multistemed growing pattern of the shrub. The problem with this definition is that it doesn't have any biological or taxonomic basis.
Many species of large plants are quite capable of growing as either a tree or a shrub. Members of the genus Rhododendron, for example, are seen most commonly in the west as large shrubs and only occasionally as small trees. Some of the same species in their native habitat, the base of the Himalayas, can reach hieghts of up to 80 feet and are most definately tall trees.
Taxonomically speaking, calling something a tree says nothing about how the species is related to others. Trees are found in almost every plant family on earth. They are found in the very different plant groups of gynosperms (e.g. ginkos and conifers) and angiosperms (flowering plants.) This wide distribution of tree species means that there are no common genetic ancestor that all trees have in common that non-trees do not.
We can sum up what a tree is not. It is not a taxonomical definition. Knowing that a plant is a tree tells us nothing regarding what other plants it is most closely related to. "Tree" is not a biological definition as it doesn't distinguish between trees and shrubs. It is also not a good term to describe a species as members of the same species might have different growing habits depending on particular growing conditions.
The standard definition of a tree does give us some important information about the role the plant plays in its local ecosystem. And this information is true whether the plant is a palm, a banana tree or a redwood.
All plants that reach a certain height and have a raised canopy bring certain universal characteristics to a local ecosystem. A raised canopy, for instance, often provides a shaded area underneath in which selection goes to shade tolerant plants. A raised canopy provides a hard to reach enironment that offers shelter, camophlage and food for many animals. And while there are no discernable differences in structure and function, simply being so large allows trees a greater impact on their local environment than shrubs in the form of oxygen production, temperature control, compost creation and water usage.
Therefore, the definition of "tree" is not a taxonomic definition, or even for the most part, a biological one. It is instead, a definition describing the form and function of a particular plant within its environment. A tree is a tall plant that creates a canopy with empty shaded space beneath.