Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as well as a small number of extinct orders, in the class Pinopsida of the division Pinophyta (Coniferophyta). All are woody plants, and they are often the predominant trees of forests worldwide, including pine, spruce, fir, hemlock, cedar, larch, juniper, cypress, yew, and redwood species. Because the ovules (young seeds) of these plants are exposed directly to the air at the time of pollination—that is, these plants produce seeds that are not enclosed in a fruit or ovary—conifers are classified as gymnosperms. (In contrast, the flowering plants, or angiosperms, have ovules enclosed in an ovary.) Today, conifers constitute the principal source of lumber and pulp for paper and wood products. See also: Forest and forestry; Forest timber resources; Lumber; Pinales; Pinophyta; Pinopsida; Tree; Wood products

A mixed conifer forest primarily composed of western white pine and mountain hemlock surrounding Cliff Lake, Lassen Volcanic National Park, California. (Photo by Amanda Sweeney, National Park Service)

Conifers first appeared in the fossil record during the Pennsylvanian Period, approximately 310 million years ago (MYA). Modern phylogenetic analyses have determined that the conifers evolved from the most primitive seed plants, that is, the seed ferns (pteridosperms). Their evolution involved a number of primary changes, including the reduction and streamlining of fernlike fronds to simple needle leaves, the aggregation of plant reproductive regions and structures into compound seed cones and simple pollen cones, and the establishment of mycorrhizal associations (mutualistic symbioses that are formed by soil-inhabiting fungi and terrestrial plants) in the roots. See also: Mycorrhizae; Paleobotany; Pennsylvanian; Plant evolution; Plant phylogeny; Plant reproduction; Pollen; Pteridosperms

Seed dormancy is another important feature that enabled the conifers to become an extensive and thriving plant group. Dormancy allows seeds to persist in the soil until conditions are favorable for germination and the successful establishment of a seedling. This innovation, as well as the other aforementioned adaptations, allowed the conifers to increase their diversity and geographical distribution rapidly. They became the first seed plants to colonize habitats outside the wetlands extensively, particularly during the Mesozoic Era (beginning approximately 245 MYA), when wetland habitats were diminished and dryland and water-stressed habitats increased dramatically. This success eventually led to the emergence of today's conifer families, with most being well established by the Jurassic Period (210–145 MYA). Conifers remained the preeminent plants in the world until the rise and dominance of the flowering plants, approximately 100–80 MYA. See also: Dormancy; Fossil seeds and fruits; Seed; Seed germination