What is a Tree?

By Kelly Vowels

Bernheim Forest is dominated by hardwood forests, which contain over 70 species of trees. Everyone knows that trees are made of wood and have trunks and branches, but what is a tree?  

By simple definition, a tree is a perennial plant with elongated stems with many branches that support leaves.  They have a root system underground that anchors the plant, and they are usually long-lived.  Trees can live anywhere from less than a hundred years (willow) to more than a few thousand years (bristlecone pine) depending on the species. All trees are unique from other plant forms, except some vines, in that the wood stem growth occurs from a thin layer of cells called the cambium that is located just under the bark. Every year the cambium produces a new layer of growth on the inside and outside. The outside layer is called the phloem and transports sugars from the leaves to the rest of the plant. The inner layer is the xylem and transports water to the plant.  

Most people think of gymnosperms and angiosperms when thinking about trees since these groups are mainly dominated by tree forms, although pteridophytes also contain trees as well. Gymnosperms are seed-producing plants that do not produce fruit. Their seeds are exposed and can be found on the end of the leaf structures or in cones. Pines, hemlocks, spruces, and firs are gymnosperms. Angiosperms are different from gymnosperms in that their seeds are produced in the ovaries of flowers and are surrounded by protective fruit. Angiosperms dominate the world with over 300,000 species of plants, while gymnosperms are represented by about 1000 species. Most of the tree species found in Bernheim Forest are angiosperms and include oaks, elms, maples, hickories, and many more. Pteridophytes are seedless vascular plants that include tree ferns.  

Tree ferns have been included as trees, but there is disagreement based on the definition of a tree. If a tree is classified as any plant that produces woody stems, they are not considered a tree, since they do not produce woody structures. They are considered a tree when a tree is defined as any structure that produces a trunk and elevates the reproductive structure above the ground. Tree ferns resemble palm trees and date back over 300 million years ago– they predate dinosaurs and are found throughout the world in subtropical and tropical forests, as well as temperate rainforests.  

So what is a tree? By the simplest definition, a tree is any plant that has elongated stems that support branches and leaves, which includes everything from tree ferns to oaks. A more complicated definition is a perennial plant with an elongated woody stem that supports branches and leaves, and this definition excludes tree ferns. So as with most scientific questions, there is no easy answer, just more explanations to ponder.  


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