type also fails to take into account other aspects of hyphal morphology, such as the extent to which hyphae cross into deeper soil horizons. Field studies have shown that extramatrical mycelium is more likely to proliferate in mineral soils than in organic material, and may be particularly absent in fresh leaf litter. However, the presence of different ectomycorrhizal groups in different soil horizons suggest that different groups have evolved specialized niche separation, possibly attributable to exploration type. Ecological implications Community assembly Because ectomycorrhizas are small and possess a limited contact area with the surrounding soil, the presence of extramatrical hyphae significantly increases the surface area in contact with the surrounding environment. Increased surface area means greater access to necessary nutrient sources. Additionally, the presence of rhizomorphs or mycelial cords, can act comparably to xylem tissue in plants, where hollow tubes of vessel hyphae shuttle water and solubilized nutrients over long distances. The abundance and spatial distribution of host root tips in the rhizosphere is an important factor mediating ectomycorrhizal community assembly. Root density may select for the exploration types best suited for a given root spacing. In Pine systems, Fungi that display short-distance exploration types are less able to colonize new roots spaced far from the mycorrhiza, and long-distance types dominate areas of low root density. Conversely, short-distance exploration types tend to dominate areas of high root density where decreased carbon expenditure makes them more competitive than long-distance species. The growth of extramatrical mycelium has a direct effect on the mutualistic nutrient trading between ectomycorrhizal fungi and their hosts. Increased hyphal occupation of the soil allows the fungus to take greater advantage of water and nutrients otherwise inaccessible to plant roots and to more efficiently transport these resources back to the plant. Conversely, the increased costs in carbon allocation associated with supporting a fungal partner with an extensive mycelial system presents a number of questions related to the costs and benefits of ectomycorrhizal mutualism. Carbon cycling Although there is evidence that certain species of mycorrhizal fungi may obtain at least a portion of their carbon via saprotrophic nutrition, the bulk of mycorrhizal carbon acquisition happens by way of trading for host-derived photosynthetic products. Mycorrhizal systems represent a major carbon sink. Laboratory studies predict that around 23% of plant-derived carbon is allocated to extrametrical mycelium, although an estimated 15-30% of this is lost to fungal respiration. Carbon allocation is distributed unequally