Decay and Position Class Dynamics Of Large Woody Debris In Small Streams In The Alberta Foothills
Department of Forest Sciences, University of British Columbia, Vancouver
Large woody debris (LWD) plays an important role in stream function and can influence channel morphology, nutrient cycling, and habitat availability. Twenty-four small streams in the Foothills region of Alberta were surveyed in order to quantify time since death and wood density of LWD in different decay and position classes. Abundance of wood ranged from 5 to 70 logs per 50m reach with a mean abundance of 31.7 ± 2.63. In-stream volume of LWD varied from 0.007 to 0.052m3 m-2 of stream surface area with a mean in-stream volume of 0.018 ± 0.002m3m-2. Total volume of LWD ranged from 0.021 to 0.244m3 m-1 of stream reach with a mean total volume of 0.102 ± 0.012m3m-1. Dendrochronological methods were used to estimate time since death. Wood density was also measured and trends in these two variables were quantified across decay and position class gradients using 426 samples of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (P. Mill.) B.S.P.). Both pine (P<0.0001) and spruce (P<0.0001) showed a significant trend of increasing time since death along a decay class gradient from least decayed (decay class I) to most decayed (decay class IV). Time since death also increased significantly from bridged, to partially bridged, to buried, to loose LWD in both pine (P=0.0014) and spruce (P<0.0001). Wood density decreased significantly along the decay class gradient of pine (P=0.0011) and spruce (P<0.0001). There were also significant differences in wood density for pine (P<0.0001) and spruce (P<0.0001) among different position classes with bridges having the highest density and loose LWD having the lowest density. The high variability in times since death and wood density among specific decay and position classes highlights the non-linear nature of the decay process in streams. Further long-term research is necessary to better understand disturbance-related recruitment processes and in-stream decay processes.