Forest age, channel morphology, and biogeochemical processes in mountain rivers

A hierarchy of valley morphometry and forest type regulates stream channel morphology and subsequent ecosystem function. Unconfined channels have high or low quantities of wood, but confined channels store little wood biomass. Within this geomorphic context, forest stand age is also critical; streams draining old growth forests (>350 y) have much higher wood loads and log jam densities than either younger, unmanaged forests (>120 y) or intensively managed forests. Reaches with extensive log jams have anastomosing channels, large pool volumes, and high amounts of organic carbon (OC) storage both as wood and as sediment. We hypothesized that these reaches would be much more biogeochemically active with higher rates of primary production, ecosystem respiration, and nitrate removal. Nitrate removal was undetectable in most single-thread channels, and was much higher in reaches with extensive log jams. Ecosystem respiration was also much higher in reaches with log jams. The mechanism for greater nitrate removal and higher ecosystem respiration is increased organic matter storage. Jams stored 10 times more sediment OC than unjammed reaches. Bioassays showed that this OC had high rates of denitrification which may have stimulated NO3 uptake in these reaches. Forest control of channel morphology dramatically altered biogeochemical cycling with higher rates of OC retention cycling and increased net N removal.