Dependency on arbuscular mycorrhizal fungi and responsiveness of some Brazilian native woody species

Arbuscular mycorrhiza (AM) associations are of great importance in forest e cology and land rehabilitation in the tropics, but information on AM suscep tibility, host dependence, and host responsiveness to the fungi is scarce. The present study was carried out under greenhouse conditions in a low-fert ility soil with 29 woody species. There were very large differences between plant species in AM colonization, responsiveness to inoculation, mycorrhiz al dependency and efficiency of phosphorus (P) uptake. All of these paramet ers were influenced by available soil P in solution. AM colonization ranged from zero in several non-mycotrophic species to > 60% in the highly mycotr ophic ones. Ten species (34% of the total) were found to be mycorrhiza-inde pendent or non-mycotrophic, whereas the rest were highly to very highly dep endent. The level of P above which there was no AM effect, defined here as the T' value, allowed distinction between AM dependence and responsiveness of the host and was very efficient for separating species according to thes e traits. Mycorrhizal responsiveness and dependency were not related and so me species were responsive to increased P in the soil solution only when my corrhizal. Efficiency of P uptake was affected by AM and by P levels. Some species exhibited a high efficiency independent of AM, while others were ve ry inefficient even at high P. Despite differences between species, in most cases AM growth enhancement was nutritionally mediated. Differences in AM responsiveness and dependency as well as the importance of these concepts f or reforestation technology in the tropics are discussed.