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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Invasive Species and Pollinator Health » Research » Publications at this Location » Publication #167886
Title: IMPACTS OF NITROGEN-FIXING SHRUBS IN WASHINGTON AND COASTAL CALIFORNIA

Author
item Haubensak, Karen
item D Antonio, Carla
item ALEXANDER, JANICE - UC DAVIS, EXTENSION

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2004
Publication Date: N/A
Citation: N/A

Interpretive Summary: Invasive species can sometimes have effects on soil properties that may be long-lasting and will interfere with restoration potential of a site. Two shrub species that have been aggressively invading grasslands in California, Oregon and Washington are now the target of removal efforts by many state and federal agencies. Because both of these species are know to fix nitrogen, we were interested in the possibility that they enhance soil nitrogen and that higher levels of soil nitrogen could interfere with revegetation efforts because many other weeds are known to respond positively to high soil nitrogen. We used a stable isotope technique to evaluate the extent to which broom populations were fixing nitrogen across several sites and we then evaluated how some control techniques might affect soil nitrogen. We found evidence that indeed N fixation was high at many sites and that fire, the most commonly used method of control in National Parks in coastal California, led to a decline in total soil N. Nonetheless most burned sites were still dominated by undesirable species. In hand-cleared sites we evaluated the potential for sawdust to bind up available soil nitrogen and thereby reduce the competitive pressure provided by other weeds. We did not however, find sawdust to be effective at altering the relationships between desirable and undesirable species in the post broom environment.

Technical Abstract: Open grasslands in California and Washington are being invaded by two closely related European shrubs, French broom and Scotch broom, that are considered among the most invasive and damaging of wildland species in those habitats. Here we present evidence of their impacts on soil nitrogen and the implications for restoration. Using natural abundance 15N signatures of leaves, we show that nitrogen-fixation by brooms varies across sites and may depend on a suite of site-specific factors. Nonetheless, in sites in both California and Washington, we observe up to a two-fold increase in soil nitrogen availability as assayed in the laboratory. Across a range of sites we determined that burning decreases total soil nitrogen by nearly 40%. We found burning to have the simultaneous effect of decreasing the broom seedbank by 68% after one burn. In a separate experiment, we removed broom and added sawdust to the soil in order to test whether a nitrogen-immobilization effect would help slower-growing native perennial grasses in competition with European annual grasses. We found that while sawdust effectively decreased nitrogen availability after a two-year application period, we could not effectively target which group of species would benefit most.