Stocking Requirements for Forestland

This is for Clackamas County but a good simple explanation of the  requirements with the exception of Site Class which is applicable only to Forestland in Western Oregon.  Eastern Oregon is all one class, one assessment value per acre applicable to all Designated Forestland in Eastern Oregon.
http://www.clackamas.us/at/forestland.html

Stocking Requirements for Forestland
ORS 321.257 to 321.390

Definition

A tax deferral for lands used predominantly to grow and harvest trees of a marketable species.

Requirements for qualification

1. There must be at least two contiguous acres of forestland to qualify.
2. The land must meet one of the following stocking requirements:
   1. 
      

      Site productivity (Clackamas Co.)

      Cubic Foot Site Class I, II, and III (Douglas fir 100- year site index 124 and higher)
      

      Seedlings (less than 1-inch DBH*)

      200 per acre
      

      Saplings and [oles (1 to 10-inches DBH)

      120 trees per acre
      

      Trees 11-inches DBH and larger

      80 square feet of basal area** per acre
   2. If the land does not meet the requirements in (1) above, the owner must submit to the assessor a written management plan for establishing trees to meet the minimum stocking requirements. (OAR 150-321.358)
   3. Qualifying marketable species include Ash, White Oak, Big Leaf Maple, Red Alder, Cottonwood and all conifers except Monterey Cypress.
* "DBH" means the tree diameter, including bark, at 4.5 feet above the ground (breast height).
** Basal area means the area of a cross section of a tree stem at breast height, expressed in square feet.