VESICULAR-ARBUSCULAR MYCORRHIZAS OF EQUISETUM SPECIES IN NORWAY AND THE USA - OCCURRENCE AND MYCOTROPHY

Typical vesicular-arbuscular mycorrhizal structures occurred in the sp orophytes of Equisetum species (11 out of 12 species) collected in Nor way and U.S.A. Plants growing in wet (hydric) habitats, sampled in the U.S.A. and Norway, had lower levels of colonization by vesicular-arbu scular mycorrhizal fungi than plants growing in mesic/dry-mesic habita ts. Colonization levels ranged from 0 to 46% of the root length. Only E. variegatum found growing in a wet habitat in Illinois was consisten tly non-mycorrhizal. Equisetum species appear to be facultatively myco trophic; the degree of mycotrophy may depend on habitat conditions. Th e world-wide warming and change from hydric to mesic conditions, durin g the late Carboniferous period and thereafter, probably contributed t o the demise of the Equisetales. The demise of the Equisetales was les s likely influenced by the, mycotrophic nature of higher plants, ferns , cycads and pines.