Site index uses which of the following to express site quality?

Site index is a critical tool in forestry that measures the potential growth of trees on a specific site, and it is expressed primarily through the height of trees over a defined period of time, often referred to as the "index age," typically around 25 or 50 years, depending on the species. The relationship between tree height and site quality indicates how well a site can support the growth of certain tree species. Higher tree heights at mature ages generally signify better site quality, allowing foresters to predict timber yield and make better management decisions.

Using tree age, soil type, or climate data alone does not provide a direct measure of site quality in the same way tree height does. While these factors contribute to overall site quality, they do not capture the specific aspect of growth potential as effectively as tree height can. Thus, expressing site quality through tree height allows for a clear and practical understanding of a site's capability to support healthy tree growth.