Growth, decay, and turnover rates of fine roots of basket willows Rose-Marie Rytter and Lars Rytter Abstract: The aim of the present study was to calculate fine-root turnover rates in stands of basket willow (Salix viminalis L.). Fine-root number was recorded in minirhizotrons in two adjacent short-rotation forest stands. Stand A was a regularly spaced plantation on clay soil. Stand B contained lysimeters, which were inserted in the soil and filled with either clay soil or washed sand. Both stands were irrigated and fertilized daily, to provide near-optimum conditions with respect to water and nutrient availability. The calculations were based on morphological studies and observations in minirhizotrons. Mean fine-root ages of growth and decay phases were calculated from third-order polynomials, and by summing up those phases and adding a short stationary phase, turnover time was obtained. Calculated fine-root turnover rates were 4.9–5.8 year –1 in the plantation and 4.8–8.1 year –1 in the lysimeters. No significant difference in turnover rates was detected between clay and sand substrates. Soil temperature had a significant effect on the decay phase, and in the calculations the data were weighted by soil temperature intervals. The importance of observing fine roots throughout the year is stressed. Résumé : Le but de cette étude consistait à calculer le taux de remplacement des racines fines dans les peuplements de saule osier (Salix viminalis L.). Le nombre de racines fines a été noté dans des minirhizotrons installés dans deux peuplements adjacents à courte révolution. Le peuplement A était une plantation avec un espacement régulier sur un sol argileux. Le peuplement B contenait des lysimètres qui avaient été insérés dans le sol et remplis avec du sol argileux ou du sable délavé. Les deux peuplements ont été irrigués et fertilisés quotidiennement pour créer des conditions quasi optimales de disponibilité en eau et en nutriments. Les calculs ont été basés sur des études morphologiques et des observations dans les minirhizotrons. L’âge moyen des racines fines en phase de croissance et de décomposition a été calculé à l’aide d’équations polynomiales du troisième degré. Le temps de remplacement a été obtenu en additionnant ces phases et en ajoutant une courte phase stationnaire. Le taux calculé de remplacement des racines fines allait de 4,9 à 5,8 par an dans la plantation et de 4,8 à 8,1 par an dans les lysimètres. Aucune différence significative dans le taux de remplacement n’a été observée entre les substrats d’argile et de sable. Étant donné que la température du sol avait un effet significatif sur la phase de décomposition, les données ont été pondérées pour des intervalles de température du sol. Les auteurs font ressortir l’importance d’observer les racines fines à longueur d’année. [Traduit par la Rédaction] Rytter and Rytter 902 In short-rotation forestry the amount of stem biomass pro- duced is of utmost significance for the economic outcome (Christersson et al. 1993; Johansson et al. 1994). One breed- ing strategy is, therefore, to select for increased carbon allo- cation to the stem. To make such selection work possible, the production of each plant fraction has to be estimated. A major obstacle, in this context, is the difficulty in estimating production of belowground parts and especially fine roots (Santantonio 1989; Dickmann and Pregitzer 1992). Esti- mates of fine-root production of trees are sparse (Hendrick and Pregitzer 1992), and their accuracy must so far be con- sidered low. Information on fine-root turnover rates must be included to obtain reliable estimates of fine-root production. If they are not, production will be severely underestimated in systems where fine-root turnover is high and production and mortality overlap (Santantonio 1989; Nadelhoffer and Raich 1992; Publicover and Vogt 1993). Reported fine-root turnover times vary from a few weeks to more than a grow- ing season (Nadelhoffer et al. 1985; Nadelhoffer and Raich 1992; Schoettle and Fahey 1994). This variability may well be the case since the differences between, for example, spe- cies, clones, stand ages, soil conditions, and management methods, have not yet been analysed. Variations in root lon- gevity within a growing season may further complicate these analyses (Hendrick and Pregitzer 1992). This behaviour is likely attributable to variation in soil temperature, with fine-root turnover increasing with temperature (Marshall and Waring 1985; Burke and Raynal 1994). Major efforts must be made in future studies of the dynamics of fine-root growth and decay to obtain reliable estimates of root production. Our need for knowledge concerning fine-root growth and decay dynamics cannot be met by destructive root sampling alone. The need to study continuously the same roots in an undisturbed environment presents a problem, but this has Can. J. For. Res. 28: 893–902 (1998) © 1998 NRC Canada 893 Received October 23, 1997. Accepted March 25, 1998. R.-M. Rytter. Swedish University of Agricultural Sciences, Department of Short Rotation Forestry, P.O. Box 7016, S-750 07 Uppsala, Sweden. e-mail: rose-marie.rytter@lto.slu.se L. Rytter. The Forestry Research Institute of Sweden, Ekebo, S-268 90 Svalöv, Sweden. e-mail: lars.rytter@skogforsk.se