Intense mycorrhizal root colonization in a human-modified landscape of the Caatinga dry forest

Arbuscular mycorrhizal fungi (AMF) symbioses are thought to help plants to cope with harsh environments and to affect not only plant fitness, but also ecological organization from population to ecosystem level. Here, we investigated to what extent this association was present, and what the major environmental drivers were in a human-modified landscape of the Caatinga dry forest in north-eastern Brazil. AMF spore density in the soil and the frequency of AMF root colonization were examined at the species and forest-stand level for the nine most abundant and widespread woody plant species in 18 forest plots across gradients of chronic anthropogenic disturbance, rainfall and soil attributes. Soil spore density was low (1.0 +/- 0.6 spore/g soil) across all conditions. However, AMF were present across all species, and colonization frequencies were high (50%, on average) throughout the entire environmental gradients. At species level, AMF colonization frequency only responded to environmental variables (disturbance, rainfall, soil available phosphorus, soil organic matter and soil pH) in one species (Cnidoscolus pubescens). M forest-stand level, colonization frequency responded positively to both soil pH and rainfall, but not to chronic anthropogenic disturbance. Our results suggest AMF symbiosis is widespread among the plant species and ecological conditions in the Caatinga. Moreover, this symbiosis, which in this case was mediated by rainfall, is likely affecting the resilience of the Caatinga dry forest as it is associated with the most abundant and biomass-relevant forest species and in the harshest conditions for plant survival and reproduction (i.e. nutrient-poor soils and dry habitats).