The diversity of microorganisms associated with trees and their different functional capabilities is thought to be a consequence of variation in carbon compounds in the rhizosphere. We used the Biolog(R) system (Biolog Inc., Hayward, CA), a redox-based test, to construct sole carbon source utilization profiles (metabolic fingerprints) of microbial communities from the rhizospheres and rhizoplanes of hybrid larch (Larix eurolepis A. Henry) and Sitka spruce (Picea sitchensis Bong. Carr.) taken from a farm woodland site and two second-rotation plantation forest sites. Canonical variate analysis (CVA) of carbon utilization data differentiated among the microbial communities from the three forest sites, with the greatest discrimination between the farm woodland and the two second-rotation forest sites. Carbohydrates and carboxylic acids were the substrates responsible for this discrimination. Carbon profiles of the microbial communities from the rhizospheres of the two tree species also clustered when evaluated by CVA, as a result of differences in utilization of carboxylic acids and amino acids, suggesting that these tree species differ in the exudates they produce. Isolation and enumeration of organisms confirmed that there were qualitative and quantitative differences in the culturable populations of microorganisms at the different sites and between tree species. We conclude that Biolog is a useful technique for evaluating the functional diversity of microbial communities; however, to interpret the results accurately, they must be assessed in conjunction with the actual carbon substrates available in the particular ecosystem under study.