Interspecific mycorrhizal networks and non-networking hosts: exploring the ecology of the host genus Alnus

While the dominant ectomycorrhizal (ECM) fungi in most temperate and tropical forests have low host specificity , a commonly cited exception to this pattern is the ECM fungal community associated with the host genus Alnus. In this chapter, we discuss multiple hypotheses that have been put forth to explain the specificity of the Alnus ECM symbiosis and consider their strengths and weaknesses in light of current research on the topic. In addition to reviewing the range of suggested explanations, we also propose and discuss a new alternative explanation of Alnus ECM specificity involving three-way interactions among Alnus plants, ECM fungi, and Frankia bacteria. With specific regard to common mycorrhizal networks (CMNs), we believe they may play an important role in the specificity observed in the Alnus ECM system. To understand that role in the larger context of research on Alnus ECM fungal communities, we begin our chapter with a synopsis of the studies documenting the unique specificity pattern. From there, we discuss why it appears to be advantageous for Alnus plants not to participate in interspecific CMNs. Finally, we elaborate on how specificity may be established and maintained in the Alnus ECM system and suggest what we consider to be promising future research directions.