Geomorphic, or landform, factors influencing plant forms include such elements as slope steepness slope aspect (the orientation of a sloping ground surface with respect to geographic north), and relief. In a much broader sense, the geomorphic factor includes the entire sculpturing of the landforms of a region by processes of erosion, transportation and deposition by streams, waves, wind, and ice, and by forces of vulcanism and mountain building. The unique landform assemblage found in any one region is understood in terms of geomorphic process. An infinite variety of plant habitats can be ascribed to the geomorphic processes and their individual landforms.
Slope steepness acts indirectly by its influence upon the rate at which precipitation is drained from the surface. On steep slopes surface runoff is rapid and the water does not long remain available to plants. On gentle slopes much of the precipitation can penetrate the soil and become available for prolonged plant use. More rapid erosion on steep slopes may result in thin soil, whereas that on gender slopes is thicker. Slope aspect has a direct influence upon plants by increasing or decreasing the exposure to sunlight and to prevailing winds. Slopes facing the sun have a warmer, drier environment than slopes facing away from the sun and therefore lying in shade for much longer periods of the day. In middle latitudes these slope-aspect contrasts may be so strong as to produce quite different plant formations on north facing and south-facing slopes.
Geomorphic factors are in part responsible for the dryness or wetness of the plant habitat within a region having essentially the same overall climate. Each plant community has its own microclimate. Upon divides, peaks, and ridge crests the soil tends to dryness because of the surface are more exposed to sunlight and to drying winds. By contrast, the valley floors tend to wetness because surface runoff over the ground and into streams causes water to converge there. In humid climates the ground water table may lie close to the ground in the valley floors or may actually coincide with the ground surface to produce marshes and swamps. Water table ponds and bogs may also occur. Hygrophytic plants thus form distinctive plant communities in the valley floors in humid climates while at the same time mesophytic or xerophytic communities occupy the intervalley surface.
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