mycelium are an example of convergent evolution. Differences in enzymatic activity, and hence the ability to degrade organic compounds dictate fungal nutrient access, with wide-ranging ecological implications. Phosphorus Because diverse ectomycorrhizal fungal taxa differ greatly in their metabolic activity they also often differ in their capacity to trade nutrients with their hosts. Phosphorus acquisition by mycorrhizal fungi, and the subsequent transfer to plant hosts, is thought to be one of the main functions of ectomycorrhizal symbiosis. Extramatrical mycelium is the site of collection for phosphorus within the soil system. This relationship is so strong that starving host plants of phosphorus is known to increase the growth of extramatrical mycelium tenfold. Nitrogen Recently, Isotopic studies have been used to investigate relative trading between ectomycorrhizal fungi and plant hosts and to assess the relative importance of exploration type on nutrient trading ability. 15N values are elevated in ectomycorrhizal Fungi and depleted in fungal hosts, as a result of nutrient trading. Fungal species with exploration types producing greater amounts of extraradical mycelium are known to accumulate greater amounts 15N in both root tips and fruit bodies, a phenomenon partially attributed to higher levels of N cycling within these species. Assessment methods Determining the longevity of extramatrical mycelium is difficult, and estimates range from just a few months to several years. Turnover rates are assessed in a variety of ways including direct observation and 14C dating. Such estimates are an important variable in calculating the contribution that ectomycorrhizal fungi have to carbon sequestration. In order to differentiate ectomycorrhizal mycelium from the mycelium of saprotrophic fungi, biomass estimates are often done by ergosterol or phospholipid fatty acid analysis, or by using sand-filled bags, which are likely to be avoided by saprotrophic fungi because they lack organic matter. Detailed morphological characterization of exploration type has been defined for over 550 different ectomycorrhizal species and are compiled in www. deemy. de. Due to the difficulty in classifying exploration types of intermediate forms, other metrics have been proposed to qualify and quantify the growth of extramatrical mycelium, including Specific Actual/Potential Mycelium Space Occupation and Specific Extramatrical Mycelial Length which attempt to account for the contributions of mycelial density, surface area and total biomass. References Fungal morphology and anatomy Fungus ecology Mycology