The distribution and mycorrhizal efficacy of fungi forming ECM associations is also influenced by climatic and edaphic factors (Bowen and Theodorou, 1973; Slankis, 1974; Harley and Smith, 1983). These fungi are generally considered to be acidophilic (preferring a low soil pH) inhabitants of litter layers near the soil surface (A horizon), but some "early stage fungi" prefer calcareous mineral soils (Section III.B.6). Isolates of ECM fungi show considerable inter- and infraspecific variations in responses to the factors listed in Table 2.