Using long-term datasets to examine effects of disturbance and climate change on ecosystem function in forested headwater streams

Background/Question/Methods

Climate change scenarios predict altered precipitation (particularly storm intensity and frequency) and temperature regimes, which may alter hydrologic, nutrient, and carbon export patterns in streams, riparian vegetation composition, and susceptibility of ecosystems to pests and disease.  Climatic variations may also interact with other anthropomorphic disturbances, resulting in unexpected changes in ecosystem structure and function.  For 20+ years we collected biological, chemical, and physical data using consistent methods to examine ecosystem responses in disturbed and undisturbed headwater streams at Coweeta Hydrologic Laboratory in western North Carolina, USA.  These datasets each designed to address unique questions regarding aquatic/terrestrial linkages, trophic interactions, and nutrient cycling in detrital-based streams have provided a wealth of information that can be used to examine potential effects of climatic variability as well as interactions of disturbance and climate on small stream processes in forested watersheds.  We used our long-term datasets to examine relationships between organic matter standing crop and export, sediment export, invertebrate functional feeding group abundance, biomass, and annual production, and physical/climatic variables such as total degree days/max water temperature, mean/max discharge, number of winter and summer storms, and the Palmer Drought Severity Index.

Results/Conclusions

Our long-term datasets (1985-2006) for three streams included the wettest and driest years at Coweeta based on yearly departures from mean annual precipitation (63-year record).  Fine particulate organic and inorganic matter export was positively related to annual precipitation and maximum discharge in undisturbed streams and streams without detrital inputs demonstrating that increases in storm frequency and intensity will lead to increased transport of sediments to downstream reaches, particularly those reaches without adequate riparian buffers. Functional feeding group invertebrate production over this time period was more strongly related to in-stream standing crops of coarse particulate organic matter (basal food resources) than any of the climatic variables.  Invertebrate production of most functional feeding groups was weakly related (-) to the number of summer storms each year.  Additionally, large inputs of dead wood in the form of American chestnut and black locust trees killed by outbreaks of disease and pests between 1934 and the 1970s likely have altered stream processes over relatively short periods of time.  Our long-term data suggest that forested headwater streams may be more resistant to climatic changes such as altered hydrology and stream temperatures than changes in the quality and quantity of organic matter inputs from riparian habitats.