Distribution and Evolution of Mycorrhizal Types and Other Specialised Roots in Australia

Abstract

A continental-scale review of the distribution of different mycorrhizal types and nonmycorrhizal (NM) plants with specialised nutrient-acquisition strategies, as well as nitrogen-fixing associations, is presented here. The importance of mycorrhizas and other mineral nutrient uptake mechanism in roots of major families of Australian plants is summarised along with the age of lineages where root types have changed. Maps showing the relative diversity and dominance of plants with different root types are also presented. In Australia as elsewhere, there is strong phylogenetic consistency for mycorrhizal association within plant families. However, there are also exceptions in two of the most diverse Australian families, the Fabaceae and the Myrtaceae. These families have very complex root strategies which seem to have evolved very rapidly, resulting in many species that have switched from arbuscular mycorrhizas (AM) to ectomycorrhizas (EcM), and these trends are unique to Australia. Some lineages of NM plants can be traced back to Gondwana, but there are others that evolved more recently in Australia, especially where AM was succeeded by EcM or NM cluster roots. These are examples of explosive and adaptive radiations in Novel and Complex Root (NCR) clades and occur in some families with a high degree of species richness in Australia. Most NCR lineages show increasing speciation rates in the past 30 Ma that coincide with continental aridification after Australia separated from Antarctica. These NCR plant lineages are substantially more diverse in the ancient landscapes and highly unfertile soils in the Southwest Australian Floristic Region than elsewhere in Australia or anywhere else on earth. Australia is a global diversity hotspot for root evolution, with about one third of all species of EcM plants; many NM plants, especially those with cluster roots; as well as one fourth of all carnivorous plants.