Airborne Digital Camera Image Semivariance for Evaluation of Forest Structural Damage at an Acid Mine Site

Abstract

A forest downstream of a heavy metal acid tailings area at the KamKotia mine site near Timmins, Ontario shows visible signs of damage which include varied leaf size, leaf discoloration, dead branches, and increased individual tree crown and forest canopy openness. High resolution remote sensing has potential for providing means to spatially and temporally evaluate such damage. In particular, image texture can be used in modeling forest and individual tree structural variations which may result from stress. In this study, an airborne multispectral digital camera system was used to acquire imagery with ground pixel spacing of 0.25 m, 0.5 m, and 1.0 m. Relations of image semivariance measures with field forest structure and health measures were determined. Semivariograms were derived using two sampling techniques: transects in two perpendicular directions, and omnidirectional sampling within pixel matrices. Sampling was conducted over the forest canopy as well as within individual tree crowns. The principal objective of the study was to determine the types of forest canopy and individual tree crown structure and health information captured by semivariograms at the three spatial resolutions. In canopy scale sampling, the 1 m pixel semivariograms were best related to forest canopy closure, stem density, and a visually derived tree stress index. The 0.5 m pixel semivariograms related better to tree crown size and tree height. The transect technique was more sensitive to tree height and the matrix technique produced stronger relations with tree stress. In individual tree crown samples, the 0.25 m pixel semivariograms were well related to tree crown closure. This suggests that semivariograms extracted from the 1 m and 0.5 m pixel images are suitable for mapping structural and textural information related to forest damage at the canopy level while the semivariograms extracted from the 0.25 m pixel images depict information at the tree crown level.