Trees are plants with an erect perennial stem at least 4 meters (13 feet) tall, a diameter measured at breast height (1.3 meters or 4.5 feet) of at least 7.5 centimeters (3 inches), and a distinct crown of leaves or leafy branches. This definition is widely used by foresters and forest ecologists to divide woody plants into trees, shrubs (smaller plants, often with clustered stems), and vines (plants not self-supporting and usually without a distinct crown). However, plant species are not constrained to fit definitions, thus plants of the same species may grow as trees in some areas and as shrubs in others, especially at the edge of the species' range where growing conditions are harsh. Thus lodgepole pine, valued by Native Americans for its tall, straight "lodge poles," becomes a sprawling shrub near timberline in the Rocky Mountains and on sandy beach dunes of the Pacific Coast.

Trees are usually woody; that is...