The impact of changing flow regimes on riparian vegetation and the riparian species Mimulus guttatus

Bret D. Elderd, University of California, Santa Cruz


Alteration of riparian stream flow through the damming of rivers and streams impacts not only river morphology but also the vegetation communities that exist within the confines of a river's banks. To examine changes in vegetation community composition and structure resulting from human control of water flow, I conducted a series of surveys on dammed and undammed streams in the Eastern Sierra Nevada Mountains of California. These surveys documented that areas below dams contain increased leaf litter and grass thatch deposition, increased grass species coverage, and an altered community of existing forbs as compared to sites with naturally flowing streams. There was also an increased woody species canopy coverage as distance from the stream increased. To examine the proximal causes of damming on herbaceous plants, I set up a factorial field experiment examining the impact of grass thatch, shading, and herbivory on a common riparian species, Mimulus guttatus, the common monkeyflower. M. guttatus germination was decreased in the thatch augmentation plots and under artificial shading. For seedlings, shading increased final plant height and thatch increased herbivory. In plots where herbivory was not controlled by insecticide, M. guttatus plants grew smaller and had lower reproduction. I also repeated this experiment in a more controlled greenhouse environment using two seed sources and two levels of shading coverage. Results from this experiment largely confirm patterns seen in the field, but also revealed a strong interaction between shading and thatch treatments for both germination and growth. Overall, my results suggest that altering riparian stream flow may impact plant performance through a cascading set of biotic interactions, and that controlled releases from dams that mimic bankfull or greater discharge events may be required to restore these important and diverse communities by resetting disturbance rates.