Pinaceae (the pine family) are trees or shrubs, including many of the well-known conifers of commercial importance such as cedars, firs, hemlocks, larches, pines and spruces. The family is included in the order Pinales, formerly known as Coniferales. Pinaceae are supported as monophyletic by its protein-type sieve cell plastids, pattern of proembryogeny, and lack of bioflavonoids. They are the largest extant conifer family in species diversity, with between 220-250 species (depending on taxonomic opinion) in 11 genera, and the second-largest (after Cupressaceae) in geographical range, found in most of the Northern Hemisphere with the majority of the species in temperate climates but ranging from sub arctic to tropical. The family often forms the dominant component of boreal, coastal and montane forests. One species just crosses the equator in southeast Asia. Major centres of diversity are found in the mountains of southwest China, Mexico, central Japan and California.
They are trees (rarely shrubs) growing from 2 to 100 m tall, mostly evergreen (except Larix and Pseudolarix, deciduous), resinous, monoecious, with subopposite or whorled branches, and spirally arranged, linear (needle-like) leaves. The female cones are large and usually woody, 2-60 cm long, with numerous spirally arranged scales, and two winged seeds on each scale. The male cones are small, 0.5-6 cm long, and fall soon after pollination; pollen dispersal is by wind. Seed dispersal is mostly by wind, but some species have large seeds with reduced wings, and are dispersed by birds. Analysis of Pinaceae cones reveals how selective pressure has shaped the evolution of variable cone size and function throughout the family. Variation in cone size in the family has likely resulted from the variation of seed dispersal mechanisms available in the environment over time. All Pinaceae with seeds weighing less than 90mg, are seemingly adapted for wind dispersal. Pines having seeds larger than 100mg are more likely to have benefited from adaptations that promote animal dispersal, particularly by birds. Pinaceae that persist in areas where tree squirrels are abundant do not seem to have evolved adaptations for bird dispersal. The embryos of Pinaceae are multi-cotyledonous, with 3-24 cotyledons.
The native flora of the United States includes about 17,000 species of vascular plants, plus tens of thousands of additional species of other plants and plant-like organisms such as algae, lichens and other fungi, and mosses. About 3,800 additional non-native species of vascular plants are recorded as established outside of cultivation in the U.S., as well as a much smaller number of non-native non-vascular plants and plant relatives. The United States possesses one of the most diverse temperate floras in the world, comparable only to that of China.
Several biogeographic factors contribute to the richness and diversity of the U.S. flora. While most of the United States has a temperate climate, Alaska has vast arctic areas, the southernmost part of Florida is subtropical to tropical, Hawaii is fully tropical (including high mountains), and alpine summits are present on many western mountains, as well as a few in the Northeast. The U.S. coastline borders three oceans: The Atlantic (and Gulf of Mexico), the Arctic, and the Pacific. Finally, the U.S. shares long borders with Canada and Mexico, and is relatively close to the Bahamas, Cuba and other Caribbean islands, and easternmost Asia.