PATTERNS AND REGULATION OF MYCORRHIZAL PLANT AND FUNGAL DIVERSITY

The diversity of mycorrhizal fungi does not follow patterns of plant d iversity, and the type of mycorrhiza may regulate plant species divers ity. For instance, coniferous forests of northern latitudes may have m ore than 1000 species of ectomycorrhizal (EM) fungi where only a few e ctomycorrhizal plant species dominate, but there are fewer than 25 spe cies of arbuscular mycorrhizal (AM) fungi in tropical deciduous forest in Mexico with 1000 plant species. AM and EM fungi are distributed ac cording to biome, with AM fungi predominant in arid and semiarid biome s, and EM fungi predominant in mesic biomes. In addition, AM fungi ten d to be more abundant in soils of low organic matter, perhaps explaini ng their predominance in moist tropical forest, and EM fungi generally occur in sails with higher surface organic matter. EM fungi are relat ively selective of host plant species, while AM tend to be generalists . Similar morphotypes of AM fungi collected from different sites confe r different physiological benefits to the same plant species. While th e EM fungi have taxonomic diversity, the AM fungi must have physiologi cal diversity for individual species to be so widespread, as supported by existing studies. The environmental adaptations of mycorrhizal fun gi are often thought to be determined by their host plant, but we sugg est that the physiology and genetics of the fungi themselves, along wi th their responses to the plant and the environment, regulates their d iversity. We observed that one AM plant species, Artemisia tridentata, was associated with different fungal species across its range, indica ting that the fungi can respond to the environment directly and must n ot do so indirectly via the host. Different species of fungi were also active during different times of the growing season on the same host, again suggesting a direct response to the environment. These patterns suggest that even within a single ''functional group'' of microorgani sms, mycorrhizal fungi, considerable diversity exists. A number of res earchers have expressed the concept of functional redundancy within fu nctional groups of microorganisms, implying that the loss of a few spe cies would not be detectable in ecosystem functioning. However, there may be high functional diversity of AM fungi within and across habitat s, and high species diversity as well for EM fungi. If one species of mycorrhizal fungus becomes extinct in a habitat, field experimental da ta on AM fungi suggest there may be significant shifts in how plants a cquire resources and grow in that habitat.