Estimating the mass density of fine roots of trees for minirhizotron-based estimates of productivity

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Allocation of carbon for the production of fine roots is a significant component of the carbon budget within trees. Transformation of fine-root volumes or lengths as seen with minirhizotrons into fine-root mass per unit of horizontal area requires an estimate of the mass density or specific root length of fine roots for the species of interest. We obtained values of mass density of fine roots using three different sampling strategies on temperate and boreal forested sites. The strategies examined were (1) the use of bulk root samples from soil cores, (2) the use of individual roots from seedlings, and (3) the use of individual roots from soil cores. Our results show that the mass density of fine roots taken from seedlings is strongly dependent on root diameter, as shown by the strong drop in mass density with a decrease in diameter in all species examined. However, the dependency of mass density of individual fine roots extracted from soil cores on root diameter varies with the species mixture. Examination of thin cross-sections of roots using microscopy reveals that the proportion of xylem cell walls as a percentage of total cell walls also decreases strongly as root diameter diminishes for seedling fine roots, but that this relationship is not as clear in fine roots obtained from soil cores. We conclude that using the mass density from core fine roots may yield the best estimate of fine-root productivity when deriving such a value from the analysis of minirhizotron images. We also discuss some of the problems associated with the use of specific root length.