What controls DOC flux in small watersheds? The roles of hydrology, forest management, and climate

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Dissolved organic carbon (DOC) is a critical component of the carbon (C) cycle of both terrestrial and aquatic systems.  For small headwater allochthonous streams, terrestrial C delivery fuels the metabolism of receiving waters and significantly influences biotic diversity and function.  While nutrient fluxes in streams have long been used as indicators of terrestrial ecosystem processes, less attention has been given to terrestrial controls on DOC export.  We used the long-term stream chemistry records from the H.J. Andrews Forest LTER to examine forest management, climatic, and hydrologic controls on both seasonal and annual DOC fluxes.  Within a watershed, annual DOC flux was highly related to annual discharge (Q), although considerable variability in higher discharge years suggested a role for indices of storminess, especially early in the water year.  Among watersheds, younger, previously harvested watersheds generally had significantly lower DOC fluxes for a given Q than old-growth watersheds, even 4+ decades after harvest.  The exception to this pattern was a harvested watershed that had significant downed wood retained on site, and had densities of coarse woody debris (CWD) close to that of the old-growth watersheds even though live tree biomass was similar to the other harvested watersheds.  Other climatic factors did not appear to have significant roles in predicting either seasonal or annual fluxes of DOC.  This is in sharp contrast to fluxes of nitrate at our site, which appears to be related most significantly to the presence of alder within the watershed.  Taken together, our data suggest a persistent and cascading role for CWD in old-growth forest ecosystems.