Actinorhizal Plants in Rangelands of the Western United States

Abstract

Actinorhizal plants are a diverse group of trees and shrubs that have the ability to form a dinitrogen-fixing symbiosis with Frankia bacteria. Actinorhizal plants are found throughout the world and are a significant component of rangelands in the western United States. Many actinorhizal species play important ecological roles in the habitats where they occur. Actinorhizal shrubs such as bitterbrush (Purshia tridentata [Pursh DC.]), mountain mahogany (Cercocarpus spp), and Ceanothus spp. are among the most important wildlife browse species in the western U.S. Other actinorhizal taxa such as alders (Alnus) and dryads (Dryas) play important roles in soil development and community succession following disturbance. Despite their importance, the biology of these plants in rangeland ecosystems is poorly understood. Particularly lacking is understanding of the dinitrogen-fixing ability of these plants and how symbioses with Frankia bacteria affects the ecology of these plants in western U.S. rangelands. Difficulty in isolating and culturing Frankia bacteria and in measuring inputs of fixed N from actinorhizal plants has contributed to slow progress in this field. In spite of these shortcomings, the actinorhizal plants of western U.S. rangelands represent a valuable resource for expanded utilization. This review is a summary of current knowledge of actinorhizal range plants and their Frankia symbionts. It is intended to provide a scientific basis for the study and utilization of this symbiosis for those involved in rangeland research and management.