| Summary: | Root tip demography of a 43 year old Sitka spruce stand was determined over a two year period. A total of 35,300 root tips were examined. A total of 14 mycorrhizal and 4 non-mycorrhizal root tip types were recognised. One mycorrhizal type, formed by Tylospora fibrillosa, made up 67% of the root tips examined, the rest each made up less than 5% of the population. The level of infection was high, with c. 94% of the root tips infected with mycorrhizal fungi. There were marked short term fluctuations in the numbers and relative abundance of some of the types, as well as longer term, unidirectional increases in the dominant type and decreases in the numbers and relative abundance of non-mycorrhizal root tips. The effect of the addition of carbon and nitrogen was examined. Carbon addition had very little effect on the mycorrhizal population as a whole. Nitrogen, on the other hand, had a very marked effect on several of the root tip types. The relative abundance and the numbers of Tylospora fibrillosa mycorrhizas were both reduced by the addition of nitrogen, whereas the relative abundance and numbers of non-mycorrhizal tips increased. A large gradient of weight of organic matter and moisture existed on the site and this was shown to strongly influence the spatial distribution of the root tip types. Russula ochroleuca mycorrhizas were shown to have a positive association with organic matter and moisture. The occurrence of E-type mycorrhizas was negatively correlated with the accumulation of organic matter, which supports the idea that these mycorrhizas are primarily associated with mineral soil. Finally, the population structure of T.fibrillosa was examined using somatic incompatibility (SI) to determine the clone size of the fungus on the site. No SI was found between any of the isolates, even when collected 20m apart. This could mean that either T.fibrillosa does not exhibit SI or that the clone from which the isolates were collected was larger than 20m across.
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