(a) Management plans for national forest timber resources shall be prepared and revised, as needed, for working circles or other practicable units of national forest. Such plans shall:

(1) Be designed to aid in providing a continuous supply of national forest timber for the use and necessities of the citizens of the United States.

(2) Be based on the principle of sustained yield, with due consideration to the condition of the area and the timber stands covered by the plan.

(3) Provide, so far as feasible, an even flow of national forest timber in order to facilitate the stabilization of communities and of opportunities for employment.

(4) Provide for coordination of timber production and harvesting with other uses of national forest land in accordance with the principles of multiple use management.

(5) Establish the allowable cutting rate which is the maximum amount of timber which may be cut from the national forest lands within the unit by years or other periods.

(6) Be approved by the Chief, Forest Service, unless authority for such approval shall be delegated to subordinates by the Chief.

(b) When necessary to promote better utilization of national forest timber or to facilitate protection and management of the national forests, a management plan may include provisions for requirements of purchasers for processing the timber to at least a stated degree within the working circle, or within a stated area, and, when appropriate, by machinery of a stated type; and agreements for cutting in accordance with the plan may so require.