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Vegetation Associations of North America
The landscape is a product of forces older than all the generations of our families. The shape of the land today is the result of ancient geological forces and more recent glaciers that retreated about 10,000 years ago. Glaciation gave form to the land, and the erosive forces of wind and rain continue to reshape it. Even though the glaciers penetrated only part way into the United States, the impact was felt all the way to the Gulf of Mexico. Beyond the reach of the glacial ice, plants from the North survived in pockets and intermingled with the southern flora. As the glaciers retreated, plants responded once again to the changes in climate and topography, and the distributions of some species were altered forever.
The first trees to return to the ravaged land released from the glaciers were cold-tolerant conifers that had sought refuge in the south. They proliferated in the virtually inert environment of sand and gravel. As the glaciers retreated farther still, the air and soil warmed, and deciduous forests replaced the conifers. By 4 to 6 thousand years ago, the forests, grasslands, deserts, and tundra that the first European settlers saw when they arrived in the New World covered the land. This landscape was made up of regional floras comprising what are referred to today as native plants.
Natives are those species that were found growing on a particular site under a given set of environmental conditions, before plants were massively displaced or introduced by European immigrants to this continent. Native plants give an area its regional identity. Their distribution is determined by physical and biological factors that influence or limit reproduction, growth, or dispersal. The native flora of an area includes trees, shrubs, herbaceous flowering plants, ferns, and mosses. For example, two or three hundred years ago, a blanket of prairie covered much of the Midwest. This sea of grass stretched from western Ohio to the rain shadow of the Rocky Mountains. Boreal conifer forests stretched from the Arctic Circle to the prairie, around the Great Lakes, and into northern New England. Between the prairie and the boreal forests, south and east to the pine savannahs and lush bottomland forests of the Coastal Plain, were deciduous forests.
In The Natural Geography of Plants, botanists Henry Gleason and Arthur Cronquist divide the United States and Canada into ten floristic provinces. The species in these distinct vegetation regions are determined by such regional factors as the high and low temperatures in summer and winter, total annual precipitation, timing and nature of precipitation, elevation, and soil type. Within any floristic province, these same variables are at work on a finer scale as well. The Eastern Deciduous Forest province, for example, stretches from Minnesota south to Texas, and east to Nova Scotia and Georgia. Within this one province, rainfall varies from 50 inches in the Southeast to 27 inches in Minnesota. A garden in Minneapolis is near the border of USDA Hardiness Zone 3, with winter temperatures dipping to -30 degrees Fahrenheit; Georgia gardeners in Zone 8 seldom see temperatures much below freezing. All these factors combine, at the sub-regional scale, to favor certain associations of plant species with a fairly predictable mix of species. These vegetation associations are sometimes called habitat types.
Understanding the habitat types or vegetation associations in your area is the key to being a successful gardener in that region. The habitat type or types are your guide to what sorts of plants will grow best in your garden. Native plants are the most logical choices. However, the major plant associations here have analogous associations in Europe and Asia, the source of many American garden plants, as well as in other parts of the world. If you understand the nature of, for example, a deciduous forest, then you will know how best to grow a woodland plant, regardless of its country of origin. You will also understand how it relates to other species in the woodland community .