Saprotrophic cord-forming fungi: meeting the challenge of heterogeneous environments

Many saprotrophic fungi, especially basidiomycetes, which are able to grow out of the substratum that they are colonizing in search of new resources (i.e., nonresource-unit-restricted fungi) form mycelial cords, which are aggregations of predominantly parallel, longitudinally aligned hyphae. These cords often form extensive, long-lived systems which interconnect discrete nutrient resources, e.g. woody litter components, on the floor of boreal, temperate and tropical forests, and some can form networks in tropical canopies. They exhibit foraging strategies which vary among species, and which alter depending on resource quantity and quality, the presence of other organisms, and microclimatic environment. The morphological patterns they form are well described by fractal geometry. Their mycelia show remarkable patterns of reallocation of biomass and mineral nutrients when new resources are encountered during mycelial foraging or when new resources are added to mature systems. Mycelial-cord systems encountering heterogeneously distributed organic substrata coordinate use of available resources so that demands are met by 'spreading the load' through the system. The persistent networks, ability to forage for new resources, and ability to reallocate mycelial biomass and nutrients exhibited by saprotrophic cord-forming fungi provide a solution to the problems of life in a spatially and temporally heterogenous environment. Response to the heterogenous environ ment in which these fungi exist generates heterogeneity in themselves, and they in turn generate heterogeneity in their environment, both in space and time.