Modeling the environmental drivers of distribution for a river ecosystem engineer, Carex nudata

Background/Question/Methods

Carex nudata, commonly known as torrent sedge, occurs in rivers throughout the Pacific Northwest and northern California and appears to function as an ecosystem engineer capable of altering channel morphology.  Established plants are found along the edges of the low flow summer channel and in emergent islands.  Carex nudatamay be capable of establishing in shallow riffles mid-stream and facilitating the emergence of islands.  Observers have speculated that the species may be causing changes in river pattern and morphology, enhancing channel complexity, a goal of river restoration. Previous studies (Levine 1999) have also found that islands formed by the species may indirectly facilitate the presence of other plant species.  While the species appears to play an intriguing role in riparian ecosystems, relatively little is known about the species’ distribution and the environmental drivers of that distribution. Using archival data from herbariums, GIS spatial analyses and a species distribution model, Maxent, I have constructed a regional distribution model of the species.  In addition to climate variables, the model incorporates river variables such as discharge, gradient, stream power and sediment size derived from digital elevation models (DEMs) and other sources using the NetTrace river network model.

Results/Conclusions

Physical fluvial characteristics are critical to predicting the distribution of C. nudata. The range of rivers occupied by the species is smaller than would be predicted simply by climatic and other terrestrial variables.   Stream power sets upper and lower limits on the distribution of the sedge.  At high stream power, the sedge is likely unable to maintain its position in the river, and low stream powers are typically associated with small sediment size, another limiting factor. In addition to stream power, modeled distribution of the sedge is strongly associated with particular sediment sizes of the channel bed.  The sedge typically occurs in gravel bed and bedrock channels but not in channels with finer sediment sizes.  Finally, an interaction between valley constraint and vegetation type is apparent.  Constrained valleys coupled with dense forest types limit distribution, likely due to an intolerance of shade but tolerance of partial shade.  The regional distribution model is the first step in generating hypotheses and designing a field survey to test hypotheses about both large scale and small scale distribution patterns of C. nudata and the interactions between this species and physical river processes.