Ectomycorrhizal Associations Function to Maintain Tropical Monodominance

Abstract

Tropical rain forests are the epicenters of tree diversity. Nonetheless, tropical monodominance should be defined as >60%, rather than >50% of the tree species, co-occur in matrices of high-diversity, mixed rain forest. Several alternative mechanisms could produce this pattern, but one frequently cited observation is that most tropical monodominant trees form ectomycorrhizal (ECM) associations. The majority of other trees in mixed rain forest form arbuscular mycorrhizal (AM) associations, suggesting that ECM associations provide advantages to their monodominant trees; however, the mechanisms underlying this hypothesis have not been fully explored. This chapter will explore recent research in the tropical forests that has revealed evidence for positive feedbacks between ECM fungi, ECM monodominant trees and con-specific ECM seedlings. These positive feedbacks provide advantages to the ECM hosts that are not observed with AM and non-mycorrhizal trees. These advantages include linkages of seedlings to common ECM networks and interactions between ECM fungi and other saprotrophic microorganisms in forest soil that provide the ECM host with preferential access to limiting soil nutrients. These positive-feedback mechanisms may explain the local monodominance of an ECM tree species within the matrix of a typical high-diversity, predominantly AM rain forest community. Since tropical rain forests are currently threatened by human activities such as logging, development and industrial agriculture, understanding how mycorrhizal fungi function in maintaining tree diversity patterns is critical for managing and restoring these valuable ecosystems.