The introduction and naturalization of Schinus molle (pepper tree) in Riverside, California

Abstract

In the late nineteenth century, an exotic tree from Peru, Schinus molle, was introduced for use in street planting at Riverside, California. Trees have since become naturalized in wildlands surrounding Riverside in areas with persistent moisture and bare soil, including riparian habitats, washes and springs, sites downslope from pavement which amplifies run-off, and irrigated citrus groves. The establishment of S. molle including seed viability and germination, seedling establishment, and phenology was evaluated from historical sources, field observations, and experimental tests in a lath house and the Agricultural Operations plot at the University of California, Riverside. Recruitment and migration of S. molle beyond the urban fringe has been slow, but cumulative because rare establishment is compensated by enduring survival. Phenotypic plasticity enables S. molle to adjust its phenology to a Mediterranean climate. Flowers and fruit develop continuously. Seeds are widely dispersed by birds, mammals, and water. Germination time varies inversely with temperature. Once well established seedling mortality was rare. Growth is greatest in the warm season until soil moisture is depleted. With continuous seed production and reliable dispersal the major bottleneck to establishment appears to be germination and seedling survival. While warm weather germination restricts establishment to occasional late spring and summer rains, it allows seedlings to flourish without competition from winter-active shrubs and grasses. S. molle maintains a continuous seed supply for germination whenever conditions are favorable. Autonomous development, free from interspecific interference, enables more efficient site occupation, allowing S. molle to grow as a tree in a shrubland.