Annual climate variability and surface flow of headwater streams in forests of the Coast Range and Western Cascade Range, OR, USA

Background/Question/Methods

Headwater streams and their associated riparian areas provide reproductive, foraging, and dispersal habitat to many forest-dependent species, especially amphibians.  Although previous studies have shown that the composition of these aquatic and riparian communities is sensitive to the characteristics of headwater reaches, the relationships between inter-annual climate variability and reach characteristics are not well known.  We characterized the habitats associated with 109 headwater reaches in 13 managed forest sites in the Coast Range and western Cascade Range,  Oregon, USA over a period of 16 years and retrospectively examined relationships among water flow characteristics and climate over space and time. 

Results/Conclusions

Of 78 flow characteristics examined, headwater reaches were distinguished from one another over space and time primarily by the proportional importance of cascades, pools, and riffle habitat units as well as the dry-to-wet reach length ratio in spatially discontinuous streams, average water depth, maximum depth, and the number of homogeneous units identified.  An increased importance of cascade and riffle habitats was associated with climate characteristics reflecting warmer and shorter winters, and negatively related to colder and longer winters.  Dry:wet ratio, average and maximum depth, and the proportional importance of pools vs. riffles, cascades and dry habitat units were not related to any of 87 climate metrics examined. Furthermore, these distinguishing characteristics were not related to geography, elevation, or drainage area.  Our results suggest that shorter, warmer winters associated with climate change scenarios may result in flashier, more homogeneous headwater reaches.  We expect this shift to result in increases in the importance of animals associated with this cascade habitats and decreases in the importance of animals negatively affected by cascades. Species-habitat unit associations suggest a subset of headwater amphibian taxa within these assemblages would be most affected. Forest management can affect stream surface hydrology, and warrants further investigation relative to implications of retaining headwater stream habitats for species of concern.