Proposed Site Indices for Engelmann Spruce on the College Forest of USU


Ernst Pflugbeil

Document Type

Full Issue

Publication Date



Classifying forest land an the basis of productivity is of prime concern to forest management. As MoLintock and Bickford (1957) pointed out, levels of intensity in forest management are largely controlled by four factors: markets, labor supply, accessibility and site. The least favorable factor dictates the level of management. Markets, labor supply and accessibility are bound to change in the future with increased demand for wood and wood products through increase in population. This will be especially true for the Intermountain States as pictured by Herrington (1959). Site, on the contrary, at least for practical purposes, will remain the same. The concept of site is complex, but the term site quality has a very definite and well established meaning among foresters. It expresses the capacity of an area to support tree growth. The question, however, remains to be solved of how good or poor sites can be recognized in the field. Spurn (1952), after emphasizing the importance of site quality determination as the key factor for growth determinations and predictions in forest management, suggests two basic approaches to its evaluation. First, to determine and measure the one or two environmental factors best associated with tree growth such as soil characteristics and topographic features. Second, to isolate and measure one or two characteristics of the vegetation itself which best expresses the sum total of the environmental factors. He suggests the volume of the forest stand, indicator plants and the height of the forest stand. The average forester needs a simple but sensitive and reliable method for determining site classes. This is probably one of the main reasons why the height of the forest stand is such a widely used index of site qualities. In this paper the height of trees in relation to diameter was adopted as a measure of the quality of a site. The review of literature has two objectives: first, to explain why height growth is a reliable index of site quality: and second, to show why he height over diameter at breast height (d.b.h.) approach was used instead of alternative methods such as height over age, height over age at breast height, height over main stand age or height at maturity. For wider application of the new site indices, it has to be kept in mind that the data were gathered from a relatively small area of the range of the Engelmann spruce-alpine fir type and they will therefore only fit the encountered variation in this area. Another disadvantage of this approach is that the site classes are based on height measurements of Engelmann spruce trees and are therefore only valid for this species. Realizing this weakness in the approach outlined above, a first step towards an undoubtedly more fundamental but also more time-consuming solution to the problem of site classification was made. Site quality is the result of many biological as well as abiological factors interacting at a given site. Soil properties and topographic features are among them. Under the supervision of Professor R. W. Miller, Agronomy Department, USU, the author dug several soil pits and helped to collect soil and humus samples. With that a multiple correlation study of these factors to site index is outlined for the future.


This item is a thesis published by a student who attended Utah State University. Abstract can be accessed through the remote link. Fulltext not available online.

This document is currently not available here.