Assessment of age bias in site index equations

Abstract

Abstract: The most widely accepted method of evaluating site productivity is site index. In spite of some important restrictions it is still a useful concept in both forest research and management. One of the most important challenges when using site index is an age trend manifested by a negative correlation between site index and stand age. Age trend may result from the inappropriateness of site index models. In this paper we develop a new approach for assessing age bias in site index models. Field data collected from 311 sample plots established in Norway spruce stands in the Polish region of the Carpathians formed the basis of this study. In the proposed approach the appropriateness of site index models is assessed by analyzing the existence of age trends in residuals of geocentric site index prediction models. Using the developed approach we demonstrated that when significant correlations exist between residuals of site index prediction models and stand age, it likely indicates the existence of an age trend and thus the inappropriateness of site index model. To remedy this situation, we demonstrated that the observed age trend can be quantified and utilized in new, non-biased, site index models.