Germination, Establishment, and Demography of Coastal Bush Lupine (Lupinus Arboreus) at Bodega Head, California

Abstract

Lupinus arboreus is a large, fast—growing, short—lived shrub dominant in the northern coastal scrub of California. The age structure of its stands and the factors which contribute to that age structure were examined at Bodega Head, a coastal peninsula in Sonoma County, 73 km north of San Francisco. No plants older than 7 yr were found. Major causes of mortality of established plants were herbivory by two species of insect larvae and summer drought. Most plants flower and fruit after the age of 2 yr. Nearly all seeds dropped under lupine canopies are taken by mice, and the absence of establishment there is not caused by allelopathic factors or competition for light or moisture. Establishment in surrounding grassland is limited by competition with grasses for light and moisture and, again, allelopathy is not a factor. Germination was enhanced by seed burial and exposure to alternate heat and cold, wetting and drying, and salt spray. Following seedling establishment, increase in biomass is approximately exponential for the first 4 yr, then growth rate decline. The mosaic of scrub and grassland at Bodega Head appears to be approaching a steady state, following release from grazing by cattle and protection from other disturbance since 1961.