Identifying the leaves of assorted species of trees and shrubs first involves classifying the leaves and inspecting their many features. Putting all the data together can then permit you to identify the species to which the leaf belongs.
Leaves are simple or compound. Simple leaves have one blade, while compound leaves are more complicated. A compound leaf features one longer centralized axis upon which leaflets occur in rows. Those compound leaves with two rows of leaflets are pinnately compound leaves, while those with smaller "branches" of leaflets radiating off the main axis are bipinnately compound. Palmately compound leaves have leaflets radiating out like fingers from the end of a stem.
The shape, margins, tips, bases and the venation of a leaf all figure heavily into its identification, according to the book "Trees of North America" by C. Frank Brockman. For example, the leaf of a gray birch (Betula populifolia) will be triangular with toothed edges, an acutely pointed tip, a straight base and veins fanning out from its center.
Size and Color
Leaf size and color also factor into the recognition equation. Variances in color from the underside of the leaf to its top surface occur. Lengths and widths of leaves can often be deciding components, especially between leaves of different species that have striking similarities.