As used in this chapter: (a) “Basal area per acre” means the sum of the cross-sectional areas at breast height of the tree stems of commercial species per acre. (b) “Countable tree” means a tree that can be used in calculating the degree of stocking under the following criteria:

(1) The tree must be in place at least two growing seasons.

(2) The tree must be live and healthy.

(3) The tree must have at least one-third of its length in live crown, except in pure stands of Douglas fir the tree must have at least one-fourth of its length in live crown.

(4) The tree must be a commercial species from a local seed source or a seed source which the registered professional forester determines will produce commercial trees physiologically suited for the area involved. (c) “Stocking” means a measure of the degree to which space is occupied by well-distributed countable trees. (d) “Site classification” means the classification of productive potential of timberland into one of five classes by board regulation, consistent with normally accepted forestry practices.

Site I shall denote sites of highest productivity, site II and site III shall denote sites of intermediate productivity potential, and site IV and site V shall denote sites of lowest productivity potential. (e) “Fuel break” means a strip of modified fuel to provide a line from which to work in the control of fire. (f) “Stream” means a natural watercourse as designated by a solid line or dash and three dots symbol shown on the largest scale United States Geological Survey map most recently published.