This 26-square-mile section of the Rock creek watershed in Maryland, now a heavily populated suburb of Washington, was rural in 1913, with many small tributaries fed by springs and seeps. Ensuing development, carried out in ignorance of natural processes, covered most of the old aquifer recharge areas with pavements and rooftops, so that more precipitation ran rapidly off the land instead of soaking in and flowing out gradually into streams. Flooding during storms and loss of flow at other times caused most of the tributaries to be covered over as storm sewers; of 64 miles of natural flowing stream channels that existed in 1913 in this section, only 27 miles could be found above ground in 1966.

Since the functions of business, commerce, manufacturing, and processing are largely independent of local biological and vegetational factors, major cities have been built in the United States in every vegetational and climatic region: Los Angeles, in the Mediterranean-like climate of southern California with its chaparral vegetation; Phoenix and Albuquerque, in the desert shrub; Denver, in the short-grass prairie of the high plains; and Boston, Hartford, New York, Philadelphia, Pittsburgh, Washington, Baltimore, all in the deciduous forest of Eastern North America. Duluth is in the hemlock-hardwood forest of the Great Lakes States, as are Green Bay, Milwaukee, Buffalo, and Detroit; Savannah was built in the broad-leaved evergreen ("magnolia") forest of the southeastern Atlantic Coastal Plain. The clue to the climate of these cities can be found in the vegetation type in which they are located.

Nashville and Knoxville are near the center of distribution of the deciduous forest of Eastern North America, and the diversity and size of forest species that grow in the surrounding forests are ample evidence of it. On the other hand, Minneapolis-St. Paul is at the edge of the oak-hickory forest (the dry phase of the deciduous forest), of the tall-grass prairie, and of the hemlock-hardwood forest of the lakes, and is far enough north that snow persists all winter long. Phoenix and Tucson are at the northern edge of the Sonoran Desert, a region with 15 or less inches of rainfall per year; Spokane is in the Palouse, the grassland of eastern Washington; and Seattle and Portland are located in the Sitka spruce-western hemlock forests of the Pacific Northwest, where rainfall may exceed 150 inches and 200 feet of snow may fall in the mountains.

Seattle and Portland, since they are on the western shore of a continent, have mild marine climates. International Falls, Minn., at the same latitude as Seattle, has the climate of the continental interior: very hot in the summer and very cold in the winter—a climate exactly like that of much of the steppes of central Asia. New York, at the latitude of Rome, has remarkably colder weather than Rome. Halifax, Nova Scotia, is much colder than London, which is at approximately the same latitude.

The cities on the eastern shores of continents, receiving their weather from the interior of the continent, are colder in winter and hotter in summer than cities at the same latitudes with marine climates. A city as far inland as Moscow, U.S.S.R., enjoys a modified marine climate, because there are no mountain barriers between it and the sea, which is some 2,000 miles to the west. Minneapolis-St. Paul has colder winters than Moscow, which is 500 miles farther north. Miami and Key West are the only subtropical cities in the United States (Honolulu is oceanic and tropical). Juneau, Anchorage, and Sitka are among the farthest north of our cities, but each enjoys a marine climate that considerably ameliorates the effect of the high latitude. Settlements across the Alaska Range on into the Yukon (Dawson) are in a climate that is considerably more harsh than the cities on Alaska's western shore.

Inasmuch as climate is the major factor controlling the development of the vegetation of the regional ecosystem, it follows that the vegetation of cities imbedded in those regional ecosystems will be similarly affected.

Comprehensive planning for environmental enhancement should take into account the adaptation of vegetation to the climate; the vegetation of the city should reflect the suitability of plant species for the area, as indicated by the presence of them or their ecological equivalents in the natural vegetation.

Generally these species are ideal for reintroduction into urban areas. If it is intended to have an area returned to a nearly natural state of vegetation, native species, when transferred to suitable drainage and exposure situations, are the most economical to utilize and the least likely to conflict with existing plants. Reestablishment of naturally occurring vegetation also serves to provide for animals that use the plant species for food and cover. Birds and mammals that once may have been common near an urban area might return if berries and preferred shrubs are reintroduced.

But while a program for enhancement of native species may be desirable, and perfectly suited to the overall climate and geographic location, it can be hampered by conditions brought about by man. Plant growth and the suitability of habitat for all living things are significantly affected by the technology of man, particularly in cities.