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 can be ecologically important in infertile habitats by virtue of their ability to fix and contribute significant amounts of ecosystem N. Buffaloberry is commonly found as a dominant understory species in lodgepole pine (Pinus contorta Douglas ex Louden) communities in the Rocky Mountains. However, there is a lack of information regarding buffaloberry’s ability to fix atmospheric N2 and the amount of N that it contributes to these forest ecosystems. Many of these forests are currently undergoing dramatic changes due to mountain pine beetle (Dendroctonus ponderosae) infestations that result in increases in understory buffaloberry. The aim of this study was to determine the proportion of N that buffaloberry derives from symbiotic N2-fixation via Frankia. We used a 15N natural abundance method in a survey of buffaloberry in conifer forests of north central Colorado to estimate the percent of foliar N derived from fixation (%Ndfa). A total of 59 plots (0.1-ha) were sampled in July 2009.
Results/Conclusions
Buffaloberry had average foliar 15N abundance (−0.63‰) closer to the atmospheric standard and higher N content (3.48%) compared to non-N2-fixing reference shrubs (15N: −1.29 - −4.81‰, N: 1.11 - 3.20%), indicating biological N2-fixation. Based on these findings we estimate a probable range of foliar %N derived from biological fixation as 60-100%. Based on buffaloberry cover and production measures in these forests we estimate a pool of 3.8 (±0.6) kg ha-1 of biologically fixed N in buffaloberry biomass. Given the increased canopy opening of these forests in the presence of mountain pine beetle infestations, our results suggest that buffaloberry has the potential to make significant contributions to ecosystem N budgets of Rocky Mountain conifer forests.