Effects of forest disturbance on particulate organic matter budgets of small streams

Organic matter dynamics were studied in five streams at Coweeta Hydrologic Laboratory. Three of these streams drained logged watersheds, and two drained reference deciduous forest watersheds. Litter inputs to the streams draining disturbed watersheds were significantly lower than to reference streams. Additionally, while undisturbed litterfall consisted primarily of relatively refractory leaf species, litterfall in the disturbed watersheds was composed of more labile leaf material. Non-woody benthic organic material was generally lower in disturbed streams than in reference streams, and woody benthic material was substantially lower in disturbed streams. Particulate organic matter transport was measured intensively during storms. These data were used in a computer model to estimate annual particulate organic matter transport. The model was driven by empirical equations relating particulate concentration to the rate of increase in flow during storms, time since peak storm discharge, and average baseflow concentation. Results showed that disturbed streams exported significantly more particulate organic matter and that most of this transport occurred-during storms. In order to place our results in perspective, the model was also used to estimate transport over a 47-year period. Transport during our study was not significantly different from the long-term average. Organic matter budgets were calculated from input, standing crop, and export data. This synthesis showed that forest disturbance has increased export, has accelerated turnover of benthic particulate organic matter, and is depleting benthic material. These changes are related primarily to the decline of woody debris dams in the disturbed streams.