Establishing Understory Plants into Restored Riparian Forest on the Middle Sacramento River
ABSTRACT ESTABLISHING UNDERSTORY PLANTS INTO RESTORED RIPARIAN FOREST ON THE MIDDLE SACRAMENTO RIVER by Prairie L. Johnston Master of Science in Botany California State University, Chico Spring 2009 Restorationists commonly plant overstory and understory species simultaneously at the outset of restoration, although a mature forest canopy may be necessary to facilitate survival of native understory species, either directly (providing a shady microhabitat) or indirectly (shading out nonnative competitors). I conducted a manipulative study at six riparian forest restoration sites along the Sacramento River to answer the following questions: 1. When during the restoration process, at the beginning or when the canopy has matured, is introduction of understory species most successful? 2. Does canopy cover directly or indirectly facilitate the survival of native understory species? 3. Is seeding or planting understory species most effective? I set up a two-way factorial experiment with canopy cover (present or removed) and grass herbicide (control or herbicide) at three sites restored ≥7 yr previously (‘old’ sites). The grass herbicide treatment was also replicated at three new restoration sites without a canopy. I transplanted seedlings and sowed seed of seven native understory species in these treatments. In addition, to quantify the direct effect of light, three species were planted in the field under four thicknesses of neutral shade cloth (decreasing photosynthetically active radiation by 0, 30, 60, and 80%). In the primary experiment, the herbicide treatment only slightly reduced the overall cover of nonnative species (herbicide treatment: 45.2 ± 4.5%, control treatment: 58.7 ± 4.6%), as nonnative forbs increased in plots where nonnative grass cover was reduced. Canopy cover, however, strongly reduced the nonnative understory cover (open plots: 68.8 ± 2.1%, canopy plots: 19.8 ± 2.9%). Survival of four planted species was higher in canopy plots, whereas three species showed no difference in survival between the canopy and open plots. Seed germination and survival were generally low and highly variable among species and years. In the shade cloth experiment, Clematis ligusticifolia had higher survival in lower light conditions; Artemisia douglasiana showed no difference among treatments, and Vitis californica had minimal survival in all treatments. I recommend planting seedlings of later successional species into restored forests after the canopy has closed and the light environment is more favorable. My results also suggest that establishing canopy cover is the most effective method to reduce nonnative understory cover.