Guidelines for Determining Site Potential Tree Height from Field Measurements

The WDFW riparian ecosystem science synthesis¹ and management recommendations² describe how full riparian function can be achieved by delineating a riparian management zone (RMZ) with a width of the Site Potential Tree Height at age 200 (SPTH₂₀₀). The Site Potential Tree Height Mapping Tool lets a user look up SPTH₂₀₀ values for their location, but the current version has gaps in coverage. In other cases, site-level verification is warranted for an available SPTH₂₀₀ value. For both reasons, we (WDFW) are providing this procedure for forestry professionals (Appendix A) to determine a site index and thereby SPTH₂₀₀ from field-based measurements for land use planning under the Growth and Shoreline Management Acts.

A site index value characterizes the quality of growing conditions (higher = better). This procedure combines tree age and height field measures (from trees that are not 200 years old) with established growth curves to arrive at a site index value. This site index then allows estimation of SPTH₂₀₀ using our “SPTH₂₀₀ Calculator.”

To determine site quality using tree height as the indicator, appropriate site trees of each species are selected in a stand. The site trees’ heights and ages are measured in the field, and then plotted or indexed on species-specific growth curves or tables (see Figure 1). These tree height-to-age relationship curves are derived from growth and yield field data and show how the best trees from a variety of sites have grown over time.

1 Quinn, T., G.F. Wilhere, and K.L. Krueger, technical editors. 2020. Riparian Ecosystems, Volume 1: Science Synthesis and Management Implications. Habitat Program, Washington Department of Fish and Wildlife, Olympia.

2 Rentz, R., A. Windrope, K. Folkerts, and J. Azerrad. 2020. Riparian Ecosystems, Volume 2: Management Recommendations. Habitat Program, Washington Department of Fish and Wildlife, Olympia.