Ecosystematic functions of mycorrhizae

The great majority of vascular plants haveevolved to a dependence on mycorrhizae as the mostmetabolically active parts of their root systems.Most woody plants require mycorrhizae to survive, andmost herbaceous plants need them to thrive. Despitetheir relatively small biomass, the mycorrhizal fungi(mycobionts) are vital for uptake and accumulation ofions from soil and translocation to hosts because oftheir high metabolic rate and strategically diffusedistribution in the upper soil layers. The mycobiontsproduce enzymes, auxins, vitamins, cytokinens, andother compounds that increase rootlet size and longev-ity. They commonly protect rootlets from pathogens.They absorb and translocate water to the host.
Most mycobionts, in turn, depend on their hostsfor carbon products. Except for orchid mycobionts,few are capable of decomposing organic matter,although their respiration contributes significantlyto evolution of CO2 from soil. The fungal myceliumand sporocarps are sources of accumulated nutrientsand energy for decomposers and consumers. Nutrientsand carbon can be translocated from one vascularplant to another by a shared mycorrhizal mycelium.
The several thousand species of fungi believed toform mycorrhizae encompass great physiological diver-sity. They differ in numerous ways, including degreeof host specificity, resistance to environmentalextremes, selectivity in ion uptake, and productionof biologically active products. Net effects of onemycobiont on a host can differ from those of another,although overall functions are shared by most.
As key links in belowground nutrient and energycycling, mycorrhizae and their mycobionts can beignored only at substantial peril of reaching unrealconclusions about ecosystem processes.