Characterizing hydrochory along the Middle Sacramento River, California

Most major rivers have dams, which has enormous impacts on the timing and amount of flow. This alteration influences the relationship between the timing of peak flows and seed dispersal of riparian species. Nonetheless, there has been little study of the timing of high flows on plant regeneration. Seed dispersal by flooding (hydrochory) is important for regeneration of riparian plant communities. We measured the species composition and abundance of viable seeds deposited during flood events along the Sacramento River, the largest river in California.  Specifically, we compared hydrochory in January when most high flow events occur under the regulated system and in April when historic high flows would have occurred due to snow melt to test the hypothesis that species composition and abundance differs between winter and spring flood events.  Sediment traps were placed on floodplains in five restoration sites and five remnant forest sites in the one-year flood-frequency interval. During germination trials we identified a total of 48 native and 48 non-native species. The vast majority of seeds in both winter and spring floods were herbaceous species (86%). Results showed a greater percentage of seeds deposited in winter compared to spring, but that the relative percentage of native individuals deposited in the spring was higher than the winter. These results indicate that there may be a difference in species composition with timing of high flow-events along large rivers. Our results suggest that managing the timing of high flows may influence the seed composition deposited in sediments along large rivers.