Effect of grass competition and overstory cover on restoring understory riparian forest plants along the Sacramento River

Most riparian forest restoration projects have focused on restoring overstory species, and few studies have experimentally assessed factors limiting recruitment at multiple spatial scales.  The goal of this research is determine which factors most strongly affect seedling recruitment of native understory plants along the Sacramento River in order to inform restoration efforts.  In fall 2005, we established experimental plots at six restoration sites of different ages to test the effects of grass competition, overstory cover, and distance to the forest edge on native understory establishment.  We broadcast seeded and planted seven species (Aristolochia californica, Artemisia douglasiana, Carex barbarae, Clematis ligusticifolia, Euthamia occidentalis, Rubus ursinus, and Vitis californica).  During winter-spring 2006 most plots flooded three times.  By September 2006, seedlings showed relatively high overall survival given that they were not irrigated (range: Carex - 68% to Euthamia - 23%).  Survival of most species was higher in older restoration sites and in plots with overstory cover.  Treatments to reduce grass competition had minimal effect on survival and growth, which is likely due to extensive growth of broadleaved weeds in herbicided plots.  Establishment from seeds was low (<5 seedlings/m²) for all species.  We are also monitoring natural recruitment of native understory species in all treatments at two distances from remnant forest edge.  To date, too few plants have recruited to draw conclusions about spatial effects.  We replanted some species and reseeded all species in December 2006 and will continue monitoring natural recruitment, seed germination, and seedling growth and survival through fall 2008.