Mycorrhizas of coniferous-broadleaved forest

SUMMARY 1. Phycomycetous mycorrhizas are abundant in coniferous-broadleaved forest. Mixed infections are common and soil samples usually contain mixtures of several spore types whose distribution shows no relation to host. 2. Experimental results deny any host specificity. Size differences between plants grown with different endophytes can be ascribed to variation in speed of infection. 3. Patterns of endophyte preference vary between soils but only in one experiment was there a clear reciprocal preference for endophytes native to each soil. 4. In low-phosphate Te Anau soil Rhizophagus tenuis is an efficient symbiont, but Acaulospora laevis is effective only if infection is established in seedlings before they are transferred to this soil. 5. Compared with fine-rooted species, those with coarse roots need higher soil phosphate levels for non-mycorrhizal growth, and when mycorrhizal, remain infected at high soil phosphate levels. R. tenuis and A. laevis do not function differently when soil phosphate is not limiting. 6. Heavy shading does not affect growth of shade tolerant species but does reduce infection. Shading stunts light-demanding species but does not alter their infection levels so mycorrhizal plants accumulate phosphorus in shade. 7. Many-spored inocula result in more rapid infection than few-spored inocula, although spores often completely fail to infect. Hyphae are a better source of infection than either roots or spores and non-sporing races are common. 8. Rhizophagus tenuis appears to be a pioneer species, exhibiting a greater ability to spread than do other fungi. 9. Phycomycetous mycorrhiza-forming fungi are few in number and widely distributed yet show little ability to spread. They are the least specialised of mycorrhizas and although they have probably evolved to be highly efficient symbionts, are unlikely to show much diversity of function