Description of forest stand structure is of great interest in modern silviculture, because uneven-aged stands with great complexity show beneficial features. In this article, a new, simple index, derived from dissimilarities within pairs of neighboring trees, is proposed and studied by simulation. The index, named the "dissimilarity coefficient," is the ratio between the difference and the sum of sizes within the pairs. It ranges from 0 to 1 and it has the value .5 when sizes are drawn independently from an exponential distribution. The coefficient is used for measuring spatial variations in the distribution of sizes. In a practical example, it is shown that a stand which exhibits a general great variation in diameter may have very limited overall local variation. In such cases this coefficient can be used to reveal spatial distribution of irregularities and aggregation. The coefficient might also be of value for studies of ecosystems other than the forest ecosystem.

Key Words
Clustering; Dissimilarity; Exponential distribution; Index; Natural; Stand structure; Uniformity; Variation.

Mats Hagner is Professor, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden (E-mail: Mats.Hagner@ssko.slu.se). Hans Nyquist is Professor, University of Umeå, 901 87 Umeå, Sweden (E-mail: Hans.Nyquist@stat.umu.se ).