Not all forests are expanding over central Brazilian savannas

Abstract

Recently we reported on the expansion of riparian forests into savannas in central Brazil. To enlarge the scope of the earlier study we investigated whether upland deciduous and xeromorphic forests behaved similarly. We investigated past vegetation changes that occurred in forest/savanna transitions using carbon isotope ratios (δ13C) measured in the soil organic matter as a tracer. We analyzed the 14C activity where δ13C showed major shifts in vegetation. The role of soil chemical and physical attributes in defining vegetation distribution is discussed. Structural changes in vegetation were found to be associated with shifts in the isotope composition (δ13C) of soil organic matter. This was attributed to intrinsic differences in the biomass of trees and grasses and allowed for the determination of past shifts in vegetation by evaluating δ13C at different depths. The deciduous forest decreased in area approximately 980 years ago. Tree cover increased in the xeromorphic forest, but the border stayed stable through time. The deciduous forest and adjacent savanna have eutrophic soils while the xeromorphic forest and adjacent savanna have dystrophic soils. However, greater organic carbon, nitrogen and phosphorus concentrations are observed in the forests. We provide concrete evidence of deciduous forest retreat unlike the stability observed in the xeromorphic forest/savanna boundary. These results contrast with the expansion of riparian forests recently reported in the same region.