Forest Plan Implementation
Procedures and Tools
Size-Density Model
The classification and mapping of productive old-growth (POG) forests on the Tongass National Forest has recently been refined through the development of the Size-Density Model, or SDM. The SDM is based on tree size and density attributes of the POG in Southeast Alaska and provides a means of broadly separating old growth on the basis of forest stand structure (Caouette and DeGayner 2008).
The SDM better differentiates old-growth forest types based on structural features than previous POG mapping that was based on timber volume. Previous mapping differentiated POG on the basis of timber volume classes (4, 5, 6, and 7) or timber volume strata (high, medium, and low). The SDM uses timber volume class mapping, in addition to aspect and soils (hydric vs. non-hydric).
The final size-density model includes seven mapping classes: 4H, 4N, 4S, 5H, 5N, 5S, and 67. The class labels help identify those spatial data attributes used to create them (Caouette and DeGayner, 2008). These seven types are briefly summarized below and described in more detail, with illustrations of most probable forest type, in the SDM POG diagram.
- SD4H: Volume class 4 on hydric soils
- SD4N: Volume class 4 on non-hydric soils, north aspect or flat
- SD4S: Volume class 4 on non-hydric soils, not north aspect or flat
- SD5H: Volume class 5 on hydric soils
- SD5N: Volume class 5 on non-hydric soils, north aspect or flat
- SD5S: Volume class 5 on non-hydric soils, not north aspect or flat
- SD67: Volume classes 6 and 7
This classification can also be converted directly into the volume-strata approach used in the 1997 Plan Revision. The seven mapping classes are converted to volume strata as follows:
- High volume = SD67 + SD5N + SD5S
- Medium volume = SD5H + SD4N + SD4S
- Low volume = SD4H
Although new vegetation map products and analytical tools using plot data are in development by the Forest and the Region, the SDM is the best map product currently available on the Tongass for describing and evaluating wildlife habitat, biodiversity, and timber. Use of the SDM for Forest and project planning applications is described in greater detail in the white paper developed by Krosse and O'Connor (2009). The current version of SDM is maintained in the Tongass GIS library as a coverage and is named "size_density". Eventually, SDM may become a feature class within the Vegetation feature dataset that now contains the CoverType feature class.