ARBUSCULAR MYCORRHIZAE IN A TROPICAL SAND DUNE ECOSYSTEM ON THE GULF-OF-MEXICO .1. MYCORRHIZAL STATUS AND INOCULUM POTENTIAL ALONG A SUCCESSIONAL GRADIENT

Root samples of 37 species distributed on the beach and along a succes sional gradient (from mobile to stabilized areas) in a tropical sand d une system on the Gulf of Mexico showed that 97% of the species were m ycorrhizal. The mycorrhizal inoculum potential of the sand from severa l dune areas was compared using two different bioassays. Firstly, the field rate of colonization by arbuscular mycorrhizal fungi of Chamaecr ista chamaecristoides seedlings transplanted to random plots in the fo redunes and in the mobile area was measured. The seedlings were harves ted at intervals during 3 weeks to record mycorrhizal structures. In t he mobile area, no mycorrhizal colonization was observed during the ex periment. In the foredunes, hyphae and external mycelium were present in 40% of the seedlings as early as 8 days after transplanting. After 15 days, arbuscules and vesicles were observed in 60 and 20% of the se edlings, respectively, and after 21 days, 100, 46 and 20% of the seedl ings showed hyphae, arbuscules and vesicles, respectively. Secondly, m aize seedlings were transplanted to pots previously filled with sand f rom the foredunes, mobile dunes, grassland and a Dyphisa robinoides sh rub area. After 1 month, the lowest mycorrhizal inoculum potential was recorded for the mobile dunes and the highest for the shrub area. As expected, mycorrhizal inoculum potential increased with dune stabiliza tion.