Study Site Location: Mountains of Northern Colorado.
The objective this study is to provide unique and crucial information regarding the basic ecology of actinorhizal shrubs in forest and rangeland habitats in Colorado. Specifically, REL staff is currently working on providing the first estimates of the contributions of biologically-fixed nitrogen by actinorhizal shrubs to a variety of forest and rangeland habitats in Colorado.
Actinorhizal shrubs and trees are a group of species capable of forming a symbiosis with N2- fixing soil actinomycetes of the genus Frankia. Many actinorhizal species are capable of fixing ecologically significant amounts of N2 in perennial woody root nodules. This ability allows them to flourish in soils where low levels of available N 
may preclude the growth of other species. With their Frankia symbionts, many actinorhizal shrubs play crucial roles in the successional development and restoration of disturbed ecosystems. In Colorado, actinorhizal species of Ceanothus and buffaloberry are important early colonizers in post-fire forest habitats. Similarly, species of bitterbrush and mountain mahogany play critical roles in Colorado rangelands. Once actinorhizal plants are established on a site, they can fix considerable amounts of N and add it to the soil as either leaf or root detritus. In so doing, actinorhizal plants build the soil organic matter pool and create a more favorable habitat for other plants and soil organisms. However, much of what is known about actinorhizal taxa is based upon studies conducted in the Pacific Northwest and the ecological role of similar actinorhizal species in the Rocky Mountains remains largely unknown.
Actinorhizal shrubs are keystone plant species in Colorado forests and rangelands. Despite this recognition, we know surprisingly little about the symbiosis between these shrubs and their Frankia symbionts. Current studies by REL staff are the first attempt to estimate N inputs by actinorhizal shrubs to forests and rangelands of Colorado.
Zoe Miller MS Thesis 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 dominate 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 symbiotically 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 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 light availability 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. Buffaloberry had a 15N abundance closer to the atmospheric standard with high foliar %N content as compared to non-N2-fixing reference species, indicating biological N2-fixation. A probable range of estimated %Ndfa is 60-100%. There were no significant correlations between light availability and N2-fixation by buffaloberry suggesting that N2-fixation in buffaloberry may not be positively impacted by an increase in light availability.
