Topography affects tree growth largely because of its influence on soil depth, moisture availability, and sunlight exposure.

Figure 3-3: Landscape positionBecause gravity pulls soil particles and water downhill, soil depth, nutrient supply, and water supply usually are greater on lowlands, hillside benches, and coves than on steep hills. For similar reasons concave slopes are better for tree growth than convex slopes.

Aspect is the compass direction a slope faces when you are standing on it looking downhill. Aspect influences the intensity of sunlight reaching the ground, which in turn affects the moisture evaporation rate. Slopes that face north and east tend to be cooler and moister than slopes facing south and west. In the northern hemisphere the sun?fs rays shine more directly on south slopes, warming and evaporating moisture from them. West slopes are drier than east slopes because the sun shines on west slopes during the hottest part of the day, increasing water use by trees and evaporation from the soil. These effects become exaggerated as the slope becomes steeper or the hills become higher.

Definitions of Basic Soil Texture Classes
Sand is loose and single-grained. Individual grains can readily be seen or felt. If you squeeze a handful of dry sand, it will fall apart when you release the pressure. If you squeeze a moist handful, it will form a clump that will crumble when touched. Sandy loam contains a great deal of sand, but also has enough silt and clay to make it stick together when wet. Individual sand grains can readily be seen and felt. Squeezing a dry handful of sandy loam will form a clump that readily falls apart. If you squeeze a moist handful, a clump will form that will bear careful handling without breaking. Loam has a relatively even mixture of different grades of sand, silt, and clay. It has a somewhat gritty feel, yet is fairly smooth and slightly plastic. If you squeeze a dry handful, it will form a clump that will bear careful handling. A clump formed by squeezing moist soil can be handled quite freely without breaking it.
Silt loam has a moderate amount of fine grades of sand and only a small amount of clay. More than half of the particles are silt. When dry, silt loam may appear lumpy, but the lumps can be readily broken, and when pulverized it feels soft and floury. When wet, the soil readily runs together and puddles. Whether dry or moist it will form clumps that can be freely handled without breaking, but when moistened and squeezed between thumb and finger to form a ribbon, it will break apart. Clay loam is a fine-textured soil that usually breaks into hard lumps when dry. When moist clay loam is pinched between a thumb and finger, it will form a thin ribbon that will break readily, barely sustaining its own weight. Moist clay loam soil is plastic and will form a clump that will bear much handling. When kneaded in the hand, it does not crumble readily, but works into a compact mass. Clay is a fine-textured soil that usually forms very hard lumps when dry and is quite plastic and usually sticky when wet.