An investigation of nitrogen fixation by Russet buffaloberry in Colorado conifer forests

Abstract

Russet buffaloberry (Shepherdia Canadensis (L.) Nutt.) is an actinorhizal shrub capable of forming a symbiotic relationship with the N2-fixing soil actinomycetes Frankia. Actinorhizal shrubs are important species as they are able to fix an ecologically significant amount of N and can inhabit disturbed sites with infertile soils. Buffaloberry is commonly found as a dominant understory species in lodgepole pine (Pinus contorta Douglas ex Louden) communities and is a common post-fire disturbance species. There is a lack of information regarding buffaloberry's ability to fix atmospheric N2 in Colorado forests. This study used the 15N natural abundance method in a survey of buffaloberry in north central Colorado to determine the percent of foliar N that buffaloberry derives from fixation (%Ndfa) and how fixation may be affected by local environmental factors. The mountain pine beetle (Dendroctonus ponderosae) epidemic is currently responsible for large losses in lodgepole pine forests. As the overstory canopies of lodge pole pine communities die off, there is an increase in available light in the understory. I investigated buffaloberry's response to light availability because with more photosynthetic activity, buffaloberry could potentially have more energy to expend in the energy intensive N2-fixation process. 59 plots (0.1-ha) were sampled in July 2009 and were distributed among Larimer, Jackson, and Grand counties in Colorado. Buffaloberry (15N: ‒0.63 /, N: 3.48%) had a 15N abundance closer to the atmospheric standard with high foliar %N content as compared to non-N2-fixing reference species (15N: ‒.29- ‒4.81 / N: 1.11-3.20%), indicating biological N2-fixation. I estimate a probable range of foliar %N derived from biological fixation as 60-100%. Buffaloberry (2.65%) also had higher % foliar N as compared to the reference species (1.50%) in the autumn, just before leaf abscission. There were no significant correlations between light availability and N2-fixation by buffaloberry suggesting that N2-fixation in buffaloberry may not benefit from an increase in light availability.