Interactions between arbuscular mycorrhizal fungi and exotic grasses differentially affect the establishment of seedlings of early- and late-successional woody species

The survival and growth of seedlings of woody species belonging to early- and late-successional stages are differentially affected by competition with the root system of Urochloa brizantha. Three trials were conducted to clarify such effect. First, seedlings of early- and late-successional woody species were grown in competition with the root system of U. brizantha. All seedlings of late-successional species survived and grew, and their ability to tolerate the competition was attributed to their long and thick roots. All seedlings of early-successional species died under competition, but 30-40 days earlier when the grass was mycorrhizal, compared with the non-mycorrhizal grass. The grass' root system seems to use the arbuscular mycorrhizal fungi (AMF) hyphae network to increase its competitiveness with the woody seedlings, reducing their lifetime. In the second trial, AMF showed to be vital for three fast-growing exotic grasses (U. brizantha, Megathyrsus maximus, and Paspalum notatum) grown in a low-fertility soil. Conversely, in a high-fertility soil, the non-mycorrhizal grasses produced three-times more shoot dry mass than mycorrhizal grasses. The growth depression of mycorrhizal grasses was attributed to the carbon drain for AMF maintenance. In the third trial, we excluded the effect of U. brizantha root competition using a steel mesh that allowed the passage of only AMF hyphae. Seedlings of early- and late-successional woody species were grown around non-mycorrhizal and mycorrhizal U. brizantha which roots were confined within the steel mesh. The AMF hyphae associated to U. brizantha passed through the net and colonized the woody seedling roots, forming a common mycorrhizal network (CMN). The seedlings of early-successional woody species grew three-times more without CMN, suggesting that the CMN associated with the root grass increased the competition and anticipated the seedlings death, as observed in the first trial. Conversely, the seedlings of late-successional woody species were not affected by the CMN. Fine root traits of the early- and late-successional woody species can explain the differences in seedlings establishment during below-ground competition with U. brizantha, whereas the AMF increase the competitiveness of the grass and make harder the survival and establishment of early-successional woody seedlings.