Anthropogenic influences on the distribution of the Casuarina-Frankia symbiosis

Abstract

Casuarina species are evergreen angiosperm trees native to Australia, South East Asia and the Pacific archipelagos. The genus comprises 14 confirmed species that are able to grow on a wide range of soils. Casuarina spp. are able to enter into an actinorhizal root nodule symbiosis with Frankia casuarinae, nitrogen-fixing soil actinobacteria; the symbiosis renders them independent of soil nitrogen sources. Casuarina spp. can also form symbiotic associations with ectomycorrhizal fungi (EMF) and/or arbuscular mycorrhizal fungi (AMF) for better phosphate supply. Under severe deficiency of phosphate and iron, several Casuarina spp. can form cluster roots. These trees are highly tolerant to salinity, drought, flooding and heavy metal pollution and are extensively used for the rehabilitation of degraded sites. Casuarina spp. plantations also play a significant role in prevention of soil erosion, coastal reclamation, dune stabilization, and as windbreaks in agricultural lands. Due to their multipurpose applications in agriculture and forestry, they have been widely introduced outside their native habitat especially in tropical, arid and semiarid countries across the globe. Therefore, the global distribution of Casuarina spp. has been significantly affected by anthropogenic influences. While in most of these countries, Casuarina spp. have become well-integrated into the regional farming systems, they have become invasive in other regions.