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    Bitterbrush (Purshia tridentata) and mahala mat (Ceanothus prostratus) are common N-fixing shrubs in interior forests of the western United States, yet their contribution to ecosystem N pools is poorly characterized. We compared N fixation and soil N accretion by these shrubs in old-growth ponderosa pine (Pinus ponderosa) -- Jeffrey pine (Pinus jeffreyi) stands versus stands that had been harvested 50 years earlier. No differences (Alpha = 0.10) in cover, biomass, or percent N derived from fixation by bitterbrush or mahala mat were found between harvested and uncut stands. Approximately 46% of bitterbrush N was derived from symbiotic N fixation as measured by the 15N natural abundance method. No accurate measure of percent N derived from fixation was attained for mahala mat using this technique due to the absence of a well-matched reference plant. Estimates of total N fixation rates in both stand types were 0.2 kg ha-1 year-1 for bitterbrush and 0.3 kg ha-1 year-1 or less for mahala mat. No appreciable soil N accretion resulted due to the presence of bitterbrush or mahala mat in either stand type. Nitrogen addition by these shrubs, although small, accounts for 10?60% of annual N input in these dry forest ecosystems.

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    Busse, Matt D.; Jurgensen, Martin F.; Page-Dumroese, Deborah S.; Powers, Robert F. 2007. Contribution of actinorhizal shrubs to site fertility in a northern California mixed pine forest. Forest Ecology and Management. 244: 68-75.


    symbiotic nitrogen fixation, bitterbrush, mahala mat, Purshia tridentata, Ceanothus prostratus, natural 15N abundance, old-growth, timber harvesting

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