Yellow Bush Lupine

Lupinus 512

Yellow bush lupine (Lupinus arboreus) is perennial, bushy shrub up to 6 ft (2 m) tall, with bright yellow flowers. The species is native to central and southern California where it occurs as a component of a native dune scrub community. It occurs as an invasive species in northern California coastal dunes, where no other large, native shrubby lupines are found in the dunes.Yellow bush lupine hybridizes with the native seashore lupine (L. littoralis), which is smaller (<2 ft (60 cm)), and more prostrate in habit (Wear 1995). Like other members of Lupinus, L. arboreus has relatively large seeds with corresponding high seedling survivorship. Once a population becomes established, it spreads via short-distance seed dispersal by rodents or by seeds rolling from the parent plant down dune slopes. 

The introduction of yellow bush lupine to the Humboldt Bay dune system was traced by Miller (1985, 1988). In 1908 the operator of a fog signal station on the North Spit of Humboldt Bay gathered seeds of yellow bush lupine from the Presidio (where it had previously been introduced) and planted them around the station. In 1917 seeds from the new signal station population were collected and scattered beside railroad tracks along the spit. From these and later plantings, the extent of yellow bush lupine has increased from 244 ac (98 ha) in 1939 to over 1,000 ac (400 ha) in 1998 (Pickart and Sawyer 1998). 

The seeds of yellow bush lupine are long-lived and form a persistent seed bank, creating the need for repeated removal. However, more serious problems are caused by yellow bush lupine’s ability to cause ecosystem-level changes. Yellow bush lupine is a fast-growing, short lived shrub, maturing in 1-2 years, and generally living less than 7 years (Davidson and Barbour 1977). Periodic die-offs occur locally and population-wide due to pathogens (Pickart and Sawyer 1998). As a nitrogen-fixer, it readily colonizes the open, mat-like vegetation of northern California dunes (known as dune mat). Once the species has been present for more than a few years, it causes elevated nitrogen levels that facilitate invasion by native and non-native species (Pickart et al. 1998a). Eventually, desirable native species in invaded areas are almost entirely displaced by a combination of lupine shrubs, weedy grasses, and/or adventive natives, especially coyote brush (Baccharis pilularis). Eventually Yellow bush lupine may die out completely, leaving a scrub community dominated by coyote brush that, while native, is not normally common in our local dune systems. After these changes, plant cover is much higher than that of the native plant community, so the dune system becomes overstabilized. Although dunes are naturally subject to cyclic stabilization and rejuvenation in response to major tectonic events, exotic species such as yellow bush lupine can greatly accelerate stabilization and could conceivably replace dune mat altogether in the post-disturbance stage of the cycle. 



If yellow bush lupine has been present only a short time, and native vegetation is still intact beneath the shrubs, restoration of dune mat can be accomplished by cutting off mature lupines at the base of the trunk, and splitting the trunk to discourage re-sprouting. Alternatively, a Weed Wrench (a tool available from New Tribe of Grants Pass, Oregon) can be used to remove the root intact, but this tool may prove awkward on steep, sandy slopes. Small individuals can be hand-pulled. Removal should be carried out prior to seed set. Managers may find it easier to detect plants during flowering, but in a large population this could result in detrimental impacts to pollinators. Plants are usually piled on a nearby bare area and burned after a few weeks of drying. Treatment will need to be repeated for as many years as individuals emerge from the seedbank. To ensure that all plants have been removed, recheck the area during the flowering period, when smaller plants are readily seen. 

If bush lupine has been present long enough to alter soils, non-native grasses or other plants not normally found in dune mat will be present, and a duff layer will have accumulated. In this case, lupine can still be removed using the method described above, but this action will not in itself be sufficient to restore dunes. In addition to removing lupine, all associated non-native or adventive native plants must be removed and the duff layer should be scraped off to reveal the mineral soil (Pickart et al. 1998a). This treatment will need to be repeated for up to four years. The disturbance caused by this treatment actually stimulates germination from the seedbank, so bush lupine and other weeds in the seedbank will be depleted sooner than if bush lupine alone were being removed. Depending on the amount of remnant native vegetation present, native species may need to be re-introduced to the area. 

Dune areas invaded by yellow bush lupine have been restored using heavy equipment in small-scale experiments at Humboldt Bay dunes (Pickart et al. 1998b). Areas in which few native species remain, and which are relatively flat and accessible, are suited for this treatment. In the experiments, all vegetation was removed using a brush-rake, followed by scraping of the duff layer with a plough-blade. Weed-mat was then placed over the soil surface, fastened with staples, and left for two years. One year after removal, recolonization by lupine and other non-natives was very low. The weed-mat apparently caused the death of seeds in the seedbank. Revegetation with native species is essential for this treatment. 

Literature Cited


Davidson, E.D., and M.G. Barbour. 1977. Germination, establishment and demography of coastal bush lupine (Lupinus arboreus) at Bodega Head, California. Ecology 58:592-600. 

Miller, L.M. 1985. 

Miller, L.M. 1988. How yellow bush lupine came to Humboldt Bay. Fremontia 16(3):6-7. 

Pickart, A.J. and J.O. Sawyer. 1998. Ecology and restoration of northern California coastal dunes. California Native Plant Society, Sacramento, California. 

Pickart, A.J., L.M. Miller, and T.E. Duebendorfer. 1998a. Yellow bush lupine invasion in northern California coastal dunes: I. Ecology and manual restoration techniques. Restoration Ecology 6:59-68. 

Pickart, A.J., K.C. Thesis, H.B. Stauffer, and G.T. Olsen.1998b. Yellow bush lupine invasion in northern California coastal dunes: II. Mechanical restoration techniques. Restoration Ecology 6:69-74. 

Wear, K.S. 1995. Hybrid lupine (Lupinus arboreus x L. littoralis) on the Samoa Peninsula. Unpublished document. The Nature Conservancy, Arcata, California. 

Wear, thesis.