What is a tree?

Though easily recognizable by their general impression, size, and shape (GISS), have you ever thought about what it IS to be a tree? Trees can be described by their physical characteristics, categorized by their taxonomic place in the Kingdom of plants, and understood by their function and value in natural and human communities.

Physical Characteristics: Mature trees have a single woody stem, are taller than 15 feet, and have a trunk greater than 3 inches in diameter. Together, these three features set trees apart from shrubs (multiple woody stems, between 3-15 feet high, each trunk less than 3 inches) and other plants (wildflowers, ferns and fern allies, grass-like plants, and mosses). Young trees that spring up in dense groves might be confused with shrubs, and some trees barely exceed the height requirement even at maturity. Together though, these three criteria will almost always help distinguish trees from shrubs.

Taxonomy: Trees, like other living organisms, can be categorized and given a unique scientific name. At the top of the taxonomic hierarchy are four Kingdoms: plants, animals, fungi, and protists. Plants get divided into Divisions with trees falling into either Coniferophyta (cone-bearing) or Magnoliophyta (flower-bearing) groups. The groups continue to be divided into Class, Order, Family, and finally the Genus and Species.

The Genus and Species are the scientific (technical, botanical, Latin) name. For example, the common names sugar maple, rock maple, or hard maple are all the same tree with one unique scientific name: Acer saccharum. You'll notice that the scientific name is italicized - it can also be underlined (Acer saccharum) to alert the reader to its binomial nomenclature (two names). Acer is a genus in the Division of Magnoliophyta.

Magnoliophyta are deciduous, and the seeds develop inside an ovary to produce some type of fruit rather than a cone. There are 16 families of Magnoliophyta in Maine: Sapindacea (maple and horsechestnut), Oleacee (ash), Cornaceae (dogwood), Adoxaceae (viburnum), Betulaceae (birch, hornbean, hophornbeam, and alder), Salicaceae (willow and aspen), Hamamelidaceae (witch-hazel), Ulmaceae (elm), Malvaceae (tilia), Fagaceae (oak, beech, and chestnut), Juglandaceae (hickory and walnut), Platanaceae (sycamore), Lauraceae (sassafras), Rosaceae (cherry, hawthorn, shadbush, and mountain-ash), Fabacee (locust and honeylocust), and Ericaceae (laurel and rhododendron).

Coniferophyta are evergreens with seeds developing on the scales of cones - "naked" seeds since they don't have a fleshy exterior. In Maine, Coniferophyta are represented in two Families: Pinaceae (pine, fir, hemlock, spruce, and larch) and Cuppressaceae (white cedar, redcedar, and juniper).

Functions and Values: In a landscape, trees provide vertical structure and habitat for wildlife. Contributions to wildlife habitat include shelter and food, and large tracts of forest provide the large range needed for bigger animals. Trees filter light to influence ground cover and intercept rain to minimize erosion. Their extensive root structure holds soil in place, further minimizing erosion. For humans trees provide building material, pulp for paper, sap that boils down to maple syrup, turpentine from pine trees, and the astringent witch-hazel - to name just a few important products.

What is a tree to you?