Plants generally fall under three categories: trees, shrubs and herbs. It is easy to distinguish herbs as those plants which are generally small and without any firm support or woody stem. On the other hand, shrubs have a woody stem, are much bigger than herbs (with exceptions, for example, Banana – Musa paradisiaca, is a herb and much taller than most shrubs) and have a branching stem system with a wide bushy foliage, spreading out horizontally, with leaves even close to the ground. The last category would leave us with trees.
According to the American Forests’ definition of trees, "Trees are woody plants with one erect perennial stem or trunk at least 9 ½ inches in circumference (3 inches in diameter) at 4 ½ feet above the ground (breast height), a definitely formed crown of foliage, and a height of at least 13 feet. In contrast, shrubs are small woody plants, usually with several perennial stems branching at the base."
Once a distinction has been made between plants to categorize them as 'trees', further differentiation can be made on the basis of the leaf to identify a tree in more detail.
* Trees with needles: In many plants as in the Gymnosperms, the trees do not possess the usual broad, flat leaves, but rather needle like leaves. These needles can be in clusters or bundles as seen in Larches and Pines. If the needles stand alone or less than three together when attached to a twig, they can be Hemlocks, Spruces, Cypress, Firs, etcetera.
* Trees with scale-like leaves: The peels of an onion are an example of scale leaves. When trees have scale-like leaves, which are flattened and broad in arrangement, they can be Cedars. Check for small cones or small pink flowers attached to the twigs of scale leaves. When trees have scale like leaves arranged in a cylindrical manner and with small bluish cones, they can be identified as Junipers.
* Trees with leaves: Trees that have leaves consisting of a mid rib, vein and leaf stalk, fall under this category. Leaves can be simple or compound.
Simple leaves, that is, one blade attached to a petiole / stalk, can be lobed or unlobed. Lobed can be leaves with serrations, teeth or an appearance of many leaves fused together. Unlobed leaves are those which are generally smooth around the margins and do not have serrations , wedges or cuts in the leaf with many main veins.
Compound leaves are leaf stalks with multiple leaves on it, or many leaflets together making up the primary leaf. When leaflets grow from the end of the leaf stalk they are called palmately compound leaves. When leaflets are spaced out along the leaf stalk they are called pinnately compound leaves.
Using the leaf key for identification, many trees can be identified. Similar goes with studying trees of a particular group and why they have their specific leaves.