abney level. A hand surveying instrument designed to measure angles of elevation or depression, expressed in degrees or percent. Similar to clinometer.
          acre. A unit of land measurement, 43,560 sq ft or 10 square chains, or a square 208.7 ft on each side.
          advance growth. Young trees that have become established naturally in a forest before cutting or regeneration begin. Syn. advance reproduction.
          afforestation. Establishment of a forest on an area not previously forested.
          age clan. One of the intervals into which the range of ages of trees in a stand are divided into for classification and use.
          agro-forestry. The practice of raising trees, forage, and livestock on the same ground, at the same time. Common associations are cattle and trees or sheep and trees.
          all-aged. Applies to a stand that contains trees of all ages. See also even-aged and uneven-aged.
          all-aged management. A system of growing forest trees in groups where the individual trees are not the same age (theoretically, an all-aged forest has trees scattered throughout that range in age from I year to the oldest tree, whatever its age may be).
          allowable cut. The amount of wood that can be removed from a landowner's property during a given period, without exceeding the net growth during that period on the property.
          animal, unit (AU). A measure of livestock numbers by which different kinds, classes, sizes, and ages of animals are converted to an equivalent common standard in relation to feed and forage needed by a mature cow (approximately 1,000 lb, five weight). In the western range territory, one animal unit is equal to one head of cattle, one horse, one mule, five sheep, five swine, or five goats.
          annual ring. The growth layer of I year, as viewed on the cross section of a stem, branch, or root. One year's growth consists of a layer of lighter-colored wood (springwood) and a layer of darker-colored wood (summerwood).
          arch. A trailer or structure in the shape of an inverted V or U, which is used in logging and is towed behind (or attached to the back of) the skidding machine, lifting one end of the logs off of the ground during the yarding operation.
          aspect. The direction toward which a slope faces. Syn. exposure.
          available water. The amount of moisture in the soil that plants can "tract, usually not including water that drains readily and water beyond the "wilting point".

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