Impacts of Discharge on the Woody Plant Communities of the San Gabriel and Tujunga Rivers in the Angeles National Forest
Authors:Brian De La Torre, Devin Keogh
Mentor:Cheryl Swift, Professor Biology and Environmental Science, Whittier College
The role of factors such as minimum summer discharge (drought) and maximum winter discharge (disturbance) in determining the species composition and structure of riparian communities in Mediterranean type ecosystems is not well understood. We were interested in how the fluvial regime might influence plant community structure on reaches on the same river, and reaches on different rivers. We sampled three separate reaches and did three transects on each reach on the Tujunga and San Gabriel Rivers in the Angeles National Forest. On each transect we measured species density, age and water status in relation to the thalwag of the stream for five of the most common woody species. We also recorded sediment size distribution in order to get a qualitative measure of disturbance across a transect as well as between reaches. Finally, we measured stream velocity and calculated discharge for each reach we sampled. Our hypothesis was that reaches with similar minimum water availability would have more similar plant community structure and composition than reaches with similar disturbance regimes. Areas of higher discharge (east and north fork of the San Gabriel) had more individuals with larger stem diameters signifying older age. The water potentials, however, were similar in both the higher discharge San Gabriel River sites and the lower discharge Tujunga River sites. All of the sites among the lower discharge Tujunga River sites were more similar to each other in terms of density and age structure, but the community structure in the areas of higher discharge on the San Gabriel River varied from reach to reach. Our results suggest that increased intensity of disturbance removes individuals of species at random creating novel combinations of species for different reaches on the San Gabriel River, and that minimum summer water availability is less important in determining species composition.