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

The relationships between vegetational and arbuscular mycorrhizal (AM) dyna mics were investigated in an old-field succession in the western Italian Al ps. Vegetation and AM colonization were determined in eight sites correspon ding to different stages of successional dynamics: (a) a field under cultiv ation; (b) fields abandoned for 1, 2 and 3 years supporting ruderal vegetat ion: (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 ind ex. This index provides qualitative and quantitative information concerning the relative percentage of nonmycorrhizal, AM and ectomycorrhizal plant co ver in an entire plant community. The AM inoculum potential of each site wa s also determined using a bait approach. Farming disturbance temporarily re duced soil infectivity, Non-mycorrhizal ruderal annuals dominated after 1 y ear abandonment and covered 90-100% of the surface. After 2 or 3 years, a r apid change to AM-colonized competitive and competitive-ruderal perennials was observed. The increase in AM inoculum was associated with an increase i n floristic richness and equitability in the community. AM were also domina nt in the shrublands and early wood communities, but gave way to ectomycorr hizal species in the mature woods. The observed AM inoculum potentials are in accordance with these findings. The results of this study further emphas ize the need to take into account AM infection in plans for the renaturaliz ation of degraded areas.