Investment in Fine Roots and Arbuscular Mycorrhizal Fungi Decrease During Succession in Three Brazilian Ecosystems

The functional groups of plants that characterize different phases of succession are expected to show differences in root distribution, fine-root traits and degrees of association with arbuscular mycorrhizal (AM) fungi. The relationship involving fine-root traits and AM fungi that regulate the nutrient acquisition potential among different plant functional groups are still not well understood. We assessed fine-root morphology, AM fungal variables and soil fertility in grassland, secondary forest and mature forest in Atlantic, Araucaria and Pantanal ecosystems in Brazil. Soil cores were collected at 010 and 1020 cm depths. Fine roots were extracted from soil by sieving and root morphological traits and AM colonization were determined. The AM spores were extracted from soil and counted. In all ecosystems, soil fertility, fine-root mass and root diameter increased with the succession, while root length, specific root length, root-hair length, root-hair incidence, AM colonization and AM spore density decreased. These results suggest that plant species from early stages of tropical succession with inherent rapid growth invest in fine roots and maintain a high degree of AM colonization in order to increase the capacity for nutrient acquisition. Conversely, fine root morphological characteristics and low degree of AM colonization exhibited by plants of the later stages of succession lead toward a low nutrient uptake capacity that combine with their typical low growth rates.