Print PageCarex nudata (torrent sedge) appears to exhibit a distinct within-river distribution pattern in and along rivers of central/northern California, Oregon and southern Washington. Established plants can be found both in lines along the edges of streams and in emergent islands suggesting that Carex nudata may 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, and previous studies (Levine 1999) have 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, Garp, I have constructed a large-scale distribution model of the species. In addition to climate variables, the model incorporates river variables such as discharge, gradient and sediment size derived from digital elevation models (DEMs) and other sources using the NetTrace river network model.
Results/Conclusions
The range of rivers occupied by the species is smaller than would be predicted simply by climatic variables. Within river networks, the model predicts species occurrence in streams of intermediate order with lower and upper limits on discharge tolerated. Modeled distribution is also strongly associated with particular sediment sizes of the channel bed, typically occurring in gravel bed and bedrock channels but not in channels with finer sediment sizes. The large scale distribution model is the first step in a broader study that will construct a model of the species within-river distribution at a smaller scale and examine the interaction between the species and physical river processes and development.
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