Site Index expresses site quality using which measurement?

Site Index is a measurement used to evaluate the potential productivity of a specific area of forest land based on the growth of trees, particularly the dominant or co-dominant trees in that area. The primary measurement that Site Index uses is the height of trees, typically at a specified age. This relationship allows foresters and land managers to assess the quality of the site for timber production by understanding how well trees can grow in that environment.

Using tree height as a basis for Site Index reflects the site's capacity to support growth; taller trees usually indicate better site conditions, such as adequate sunlight, moisture, and nutrients. By specifying tree height at certain ages, Site Index provides a consistent framework for comparing the growth potential of different sites, making it a crucial tool in forest management and planning.

The other options, while related to trees, do not directly measure site quality in the same way that tree height does. For instance, diameter can inform about tree growth but not specifically about the site's potential productivity in terms of height and age relationship as clearly as the height measurement used in the Site Index.