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Thirteen-year growth response of ponderosa pine to various manipulations of understory vegetation was studied to determine if a threshold of understory cover can be established for plantation productivity and whether nitrogen-fixing Ceanothus species benefit plantation growth compared to non N-fixing Arctostaphylos species,given their ability to improve site fertility.Results showed that completely controlling competing vegetation increased plantation growth,as was universally expected,but the effects of partial shrub control on plantation performance was inconsistent.We did not find a benefit of nitrogenfixing shrubs on tree growth,as young plantation growth was slightly better in the non N-fixing plots compared to the N-fixing plots due to differences in shrub cover.An understory cover threshold for tree growth was not observed.Due to the overriding effect of shrub competition,controlling herbaceous species had little effect on plantation growth even though it benefited tree seedling survival.A long-term value of N-fixing shrubs at these sites,if there is one,must be balanced by successful plantation establishment and rapid early growth.