*information taken from paper printed in Oecologia (1991)
The species composition of vesicular-arbuscular mycorrhizal (VAM) fungal communities
changed during secondary succession of abandoned fields based on a field to forest chronosequence. Twenty-five VAM fungal species were identified. Seven species were clearly early successional and five species were clearly late successional. The total number of VAM fungal species did not increase with successional time, but diversity as measured by the Shannon-Wiener index tended to increase, primarily because the community became more even as a single species, Glomus aggregatum, became less dominant in the older sites. Diversity of the VAM fungal community was positively correlated with soil C and N. The density of VAM fungi, as measured by infectivity and total spore count, first increased with time since abandonment and then decreased in the late successional forest sites. Within 12 abandoned fields, VAM fungal density increased with increasing soil pH, H2O soluble soil C, and root biomass, but was inversely related to extractable soil P and percent cover of non-host plant species. The lower abundance of VAM fungi in the forest sites compared with the field sites agrees with the findings of other workers and corresponds with a shift in the dominant vegetation from herbaceous VAM hosts to woody ectomycorrhizal hosts.
Although there have been many studies reporting the dynamics of plant species and soil properties during old field succession, the dynamics of an important plant-soil intermediary, vesicular-arbuscular mycorrhizae, have rarely been studied. Vesicular-arbuscular mycorrhizae are symbiotic associations between plant roots and a group of ubiquitous soil zygomycetes. Most vascular plants form vesicular-arbuscular mycorrhizae, and the zygomycetes involved in these associations are frequently the most abundant fungi in the soil (Gerdemann and Nicolson 1963). Mycorrhizae often improve plant growth and survival by facilitating uptake of nutrients and increasing drought tolerance (e.g., Mosse 1973; Nelsen 1987). It has been suggested that recovery of disturbed ecosystems may depend upon the reestablishment of mycorrhizal fungi (Reeves et al. 1979; Janos 1980; Allen and Allen 1980; Perry et al. 1989). Therefore, a study of mycorrhizal dynamics during succession may provide insights into the factors and processes regulating ecosystem development.
Nearly 150 species of vesicular-arbuscular mycorrhizal (VAM) fungi have been described worldwide (Schenck and Perez 1990). Natural distributions of these species appear to be influenced by edaphic factors (Hayman 1982; Anderson et al. 1984; Porter et al. 1987 Gibson and Hetrick 1988; Henkel et al. 1989), and plant community composition (Schenck and Kinlock 1980;
McGraw and Hendrix 1984; Johnson et al. I 991). Consequently, the species composition of VAM fungal communities may change in response to the changes in soil properties and plant community composition occurring during succession. Successional patterns in species of ectomycorrhizal fungi have been described (Mason et al. 1983; Gardner and Malajczuk 1985), but such patterns were not found in the VAM fungal communities studied by Benjamin et al. (1989).
We hypothesized that VAM fungal communities change during old field succession, and that these changes are related to plant species replacement and nutrient accrual. To test this hypothesis, we studied mycorrhizae in a chronosequence of old fields and forest sites in order to (1) compare the composition of VAM fungal communities of early and late successional sites
and (2) assess the relationships between the density and species of VAM fungi and various soil and plant community properties.