As well all know from direct experience, vegetation is strongly influenced by landforms and soil. Vegetation on an upland, that is, on relatively high ground of moderate surface slope and thick soil, is quite different from that on an adjacent valley floor where water lies near the surface much of the time. Vegetation is also strikingly different in form on rocky ridges and on steep cliffs where water drains away rapidly and soil is thin or largely absent.
Thus, each region which we assign to a given formation class is actually a mosaic of smaller units reflecting the inequalities in conditions of slope, elevation, and soil type. Such subdivisions of the plant environment are described as habitats. In the example shown, the Canadian needleleaf forest (a formation class) actually comprises at least six habitats: upland, bog, bottom land, ridge, cliff, and active dune. Just where each habitat is located and how large an area it occupies depends largely upon the geologic history of the region, e.g., what process of erosion and deposition have acted in the past to shape the landforms. The patterns of landforms from such processes are in turn influenced by the ancient geologic history, which brought about varied combinations of arrangements of weak and resistant rocks.
Closely tied in with the effect of landform upon habitat is the distribution of water in the soil and rock. Finally, each habitat has characteristic soil properties, which are determined not only by the conditions of slope and water, but in part by the plants themselves.
Adjacent habitats may possess strikingly different vegetation structures so that the question arises as to which habitat shall be selected as that for which the formation class is named. For this purpose the upland habitat, with its well-drained surface, moderate surface slopes, and well-developed soil, is usually taken as the reference standard. The vegetation structure found in the upland habitat is thus what we have in mind when naming and describing a formation class.
Within a given habitat will be found a smaller unit of ecosystem, the plant community, composed of rather definite proportions of organisms which are more or less interdependent and which use the resources of their habitat in such a way as to either maintain the habitat or modify it. No particular dimensions and boundary limits can be set for plant communities; these properties will depend on the changing habitat patterns. Within a given plant community can be recognized a structure of vegetation, which is distinctive, and a certain floristic composition, which may seem more or less fixed, or which may be gradually changing with time.
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