Carex nudata (torrent sedge) occurs in river basins throughout Oregon and California. Where prominent, it appears to play a key role in channel evolution. Furthermore, in eastern Oregon, it appears to be expanding following passive restoration (reductions in cattle grazing). While the species appears to play a key role in river ecosystems, little is known about the drivers of its distribution. C. nudata occurs across different climate types but exhibits a patchy distribution within basins. The primary objective of this research is to test the hypothesis that distribution is driven by hydrological and geomorphological variables that vary systematically within basins. I began by constructing a range-wide distribution model using MAXENT and herbaria data. This model served as the basis for a stratified random sampling design for basin-wide field surveys in two representative basins, the John Day and Santiam. At 30 study sites, I measured C. nudata abundance, channel metrics and other environmental variables. I used generalized linear models (glms) to test the hypothesis that hydrological & geomorphological variables drive C. nudata abundance. As a secondary objective, we resampled green line transects first surveyed in the 1990s throughout a focus subbasin to describe changes in species abundance following passive restoration.
Results/Conclusions
The range-wide distribution model demonstrated that hydrological variables dwarfed climate variables in importance in explaining C. nudata. Data from the basin-wide field surveys further elaborated on this model. Consistent with its disturbance-adapted traits, C. nudata displayed a threshold response relative to stream power (a measure of stream energy and disturbance capacity) with C. nudata mostly absent in streams with low stream power and varying in abundance in medium to high stream power reaches. C. nudata was also associated with coarser bed sediment sizes, but sediment size covaried with stream power. More importantly, while tolerant of partial shade, C. nudata abundance was inversely related with canopy cover. Given that both stream power and light availability vary systematically within basins (light via stream canopy gap and valley constraint), C. nudata exhibits distinct patterning within basins along a headwater-to-mouth continuum. Furthermore, the systematic patterning of these driving variables interacts with the underlying geology and climate of each basin, leading to differing, but predictable patterning of C. nudata within each. Finally, within the focus subbasin, reaches within the range of C. nudata occurrence showed distinct shifts in species composition toward C. nudata following passive restoration.