With the objective of examining seasonal fine-root growth in a short-rotation forest plantation of Salix viminalis L., standing crop, growth and decay dynamics and depth distribution of fine roots were investigated with the minirhizotron technique. Fine-root number was counted during two growing seasons, 1988 and 1989. Soil cores were sampled during the same period for estimation of standing crop and depth distribution of fine-root mass and fine-root length. The experimental site is situated at Uppsala, central Sweden, on former agricultural land, where the soil consists of a heavy clay. The plantation was irrigated and fertilized daily, through a computer controlled system, from late May until mid-August. The intention was to create near-optimum conditions for growth with respect to water and nutrients. Fine-root growth started in May and fine-root number increased throughout summer and early autumn. A decrease followed from September and continued during autumn, although root growth was still occurring in October. Growth and decay of fine roots, measured as positive and negative net changes in fine-root number between observations, occurred simultaneously throughout the growing seasons. Most of the fine roots were found in the upper 40-45 cm of the soil profile. A dry period in the summer of 1989 resulted in a lower fine-root number in the upper soil layers and in a deeper mean root depth, as compared to 1988. A comparison of relative depth distribution of fine-root number, obtained from minirhizotrons, and fine-root mass, achieved from soil coring, revealed a close agreement between methods. The relative depth distribution remained fairly constant during the growing seasons. The seasonal variation of fine-root number, observed in minirhizotrons, and fine-root mass and live fine-root length, measured by soil coring, showed no significant differences between the methods or between mass and length. Copyright (C) 1996 Elsevier Science Ltd.