Vegetation dynamics and arbuscular mycorrhiza in old-field successions of the western Italian Alps

The relationships between vegetational and arbuscular mycorrhizal (AM) dynamics were investigated in an old-field succession in the western Italian Alps. Vegetation and AM colonization were determined in eight sites corresponding to different stages of successional dynamics: (a) a field under cultivation; (b) fields abandoned for 1, 2 and 3 years supporting ruderal vegetation; (c) grasslands; (d) shrublands; (e) early wood communities; (f) mature woods. AM colonization was evaluated on the roots of representative plants from each community. The data thus obtained, together with those from the literature, were then used to calculate the plant community mycorrhizal index. This index provides qualitative and quantitative information concerning the relative percentage of non-mycorrhizal, AM and ectomycorrhizal plant cover in an entire plant community. The AM inoculum potential of each site was also determined using a bait approach. Farming disturbance temporarily reduced soil infectivity. Non-mycorrhizal ruderal annuals dominated after 1 year abandonment and covered 90–100 % of the surface. After 2 or 3 years, a rapid change to AM-colonized competitive and competitive-ruderal perennials was observed. The increase in AM inoculum was associated with an increase in floristic richness and equitability in the community. AM were also dominant in the shrublands and early wood communities, but gave way to ectomycorrhizal species in the mature woods. The observed AM inoculum potentials are in accordance with these findings. The results of this study further emphasize the need to take into account AM infection in plans for the renaturalization of degraded areas.