The contribution of coarse woody debris to carbon, nitrogen, and phosphorus cycles in three Rocky Mountain coniferous forests Raija Laiho and Cindy E. Prescott Abstract: The contribution of coarse woody debris to C, N, and P cycles was assessed in forests of lodgepole pine (Pinus contorta Dougl. ex Loud.), white spruce (Picea glauca (Moench Voss), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) – Engelmann spruce (Picea engelmannii Parry ex Engelm.) in southwestern Alberta. Mass loss and changes in C, N, and P concentrations in decomposing log segments were measured for 14 years. Litter input was measured during 10 years for coarse woody debris, 1 year for ground vegetation, and 5 years for other aboveground litter types. Release of C, N and P from decomposing litter were simulated for a period of 40 years. After 14 years, log segments of pine, spruce, and fir had lost on average 71, 38, and 40%, respectively, of their dry mass. The N content of the pine logs increased, spruce changed little, and fir lost N. Phosphorus accumulated in all logs. The greatest imports of N and P occurred at the pine sites and fir sites, respectively, where these nutrients were the least available, indicating that wood decay organisms may compete with vegetation for limiting nutrients in these forests. Coarse woody debris comprised 3–24% of aboveground litter and contributed less than 5% of the N and P released. Coarse woody debris does not appear to make a significant contribution to N and P cycling in these forests. Résumé : La contribution des débris ligneux grossiers aux cycles de C, N et P a été évaluée dans des forêts de pin lodgepole (Pinus contorta Dougl. ex Loud.), d’épinette blanche (Picea glauca (Moench) Voss) et de sapin subalpin (Abies lasiocarpa (Hook.) Nutt.) – épinette d’Engelmann (Picea engelmannii Parry ex Engelm.) dans le sud-ouest de l’Alberta. La perte de masse et les changements dans les concentrations de C, N et P dans des segments de billes au sol en décomposition ont été mesurés pendant 14 ans. L’apport de litière a été mesuré pendant 10 ans dans le cas des débris ligneux grossiers, 1 an dans le cas de la végétation au sol et 5 ans dans le cas des autres types de litière épigée. La libération de C, N et P dans la litière en décomposition a été simulée pour une période de 40 ans. Après 14 ans, les segments de billes au sol de pin, d’épinette et de sapin avaient perdu en moyenne respectivement 71, 38 et 40% de leur masse sèche. Le contenu en N des billes de pin a augmenté, il a peu changé chez l’épinette et a diminué chez le sapin. Le contenu en phosphore a augmenté dans toutes les billes. Les plus fortes importations de N et de P sont survenues respectivement dans les sites avec pin et dans les sites avec sapin, là où ces nutriments étaient les moins disponibles, ce qui indique que les organismes responsables de la décomposition du bois sont probablement en compétition avec la végétation pour les nutriments qui sont déficients dans ces forêts. Les débris ligneux grossiers représentaient 3 à 24% de la litière épigée et contribuaient moins de 5% de N et de P rendus disponibles. Les débris ligneux grossiers ne semblent pas apporter une contribution significative au cyclage de N et P dans ces forêts. [Traduit par la Rédaction] Laiho and Prescott 1603 Introduction With the current trend towards more natural management of forests, the role of coarse woody debris (CWD) in forest ecosystems has received increasing attention. An obvious difference between natural and managed forests is the rela- tive scarceness of CWD in managed forests (e.g., Andersson and Hytteborn 1991). The role of CWD as a critical habitat element for many species of animals, plants, and fungi is well recognized (Freedman et al. 1996). Less is known about the role of CWD in element cycling and productivity of for- est ecosystems (Harmon et al. 1986). Studies in the coastal forests of the Pacific Northwest, where CWD is particularly abundant and large (Harmon et al. 1986; Sollins et al. 1987; Keenan et al. 1993), have suggested that CWD is an integral component of forest ecosystems, acting as a long-term stabi- lizing storage pool for nutrients. In other coniferous forests, the role of CWD in nutrient cycling has been found to be small (Busse 1994). There is some controversy as to how critical CWD is to forest productivity (Fahey 1983; Harmon et al. 1986; Spies et al. 1988). Assessing the role of CWD in element cycling requires measurements of the rates of input and decomposition of CWD relative to other litter types. The long time span and considerable temporal variation makes it difficult to directly measure CWD input and decomposition. Most previous studies have relied on indirect methods for estimating rates of decomposition and have usually involved chronosequence techniques. Decomposition estimates have usually been based Can. J. For. Res. 29: 1592–1603 (1999) © 1999 NRC Canada 1592 Received December 21, 1998. Accepted May 14, 1999. R. Laiho. Department of Forest Ecology, University of Helsinki, P.O. Box 24, FIN-00014 Helsinki, Finland. C.E. Prescott. 1 Department of Forest Sciences, University of British Columbia, 3037-2424 Main Mall, Vancouver, BC V6T 1Z4, Canada. 1 Corresponding author. e-mail: cpres@unixg.ubc.ca