EXPORT OF FINE PARTICULATE ORGANIC CARBON FROM REDWOOD-DOMINATED WATERSHEDS

In recent years, researchers have recognized the significant role of small mountain river systems in the transport of carbon from the terrestrial environment to the ocean. However, the magnitude of carbon export from rivers draining old-growth redwood forests has not been evaluated to date. Old-growth redwood stands support some of the largest quantities of biomass in the world, up to 3,500 Mg of stem biomass ha^-1, and soil organic carbon averages 200 Mg ha^-1. Suspended sediment samples were collected at five gaging stations on streams draining old-growth redwood forests in Redwood National and State Parks, California, over a range of discharges for five years. Carbon content was determined through loss-on-ignition tests. Organic content was strongly correlated to turbidity, and continuous turbidity records from the gaging stations were used to estimate annual organic export. The concentration of fine particulate organic carbon (<0.063 mm) was highest in autumn at the beginning of the rainy season, but most carbon was transported during a few days of winter high flows. In moderate flow years (return period of five years) 80% of the export occurred in 5% of the time. Annual yield of organic carbon ranged from 3 to 5 t/km², which represented about 20% of the annual suspended sediment yield in these basins. The biogeochemical significance of this type of carbon transport may have changed in this region during the last century. The current extent of old-growth redwood stands is less than 10% of its pre-settlement distribution, but before timber harvesting, the rivers draining these forested basins probably exported much more particulate organic carbon.