Introduction

The majority of natural environments which fungi inhabit are heterogeneous in both space and time with respect to many factors, and this is particularly the case in the soil habitat. Purely structural non-uniformity is exemplified by the complex spatial architecture of soil, where a myriad of connecting and blind pore networks and tortuous surfaces prevail across a wide range of scales. Temperature and moisture profiles canvary markedly both seasonally and over short timescales (Marshall &. Holmes, 1988), for example on a day with patchy cloud, temperatures near the surface can be highly dynamic. Biological heterogeneity is also the norm, with considerable spatio-temporal variation in microbial and faunal community structures being a characteristic of most soils. Nutrient resources are generally distributed patchily, and also may show seasonal variation, since they are often linked to plant growth cycles.

The degree of heterogeneity in these parameters is intimately linked to the scale under consideration. In spatial terms, all environments are heterogeneous at certain scales: at the atomic level, even a solution of salt is effectively non-uniform, and at the greatest of scales, it is apparent that the structure of the known universe is heterogeneous(Saunders et al., 1991). In mycological terms, the spatial scale of reference is not as readily definable as it might at first seem, since myceliaare composed of operational units represented by hyphae of the order of a few microns in diameter, and thalli that may extend typically several centimetres, and occasionally several kilometres (Smith, Bruhn & Anderson, 1992).