Relation between carbon isotopes of plants and soils on Marajَ Island, a large tropical island: Implications for interpretation of modern and past vegetation dynamics in the Amazon region

We assess the relation between the contrasting vegetation types of rainforest, open savanna and wooded savanna coexisting in close contact on Marajَ Island at the mouth of the Amazon River. Floristic and carbon isotopic characterizations of modern plants were combined with organic matter carbon isotope and grain size records of soil to characterize vegetation evolution at six locations on southeastern and northeastern Marajَ Island and its relations to climate changes since the late Pleistocene. C3 plants contribute 100% of the biomass in the rainforest on post-Barreiras sediments (site 1). No significant vegetation changes are evident in these places since at least ~ 7860 cal yr B.P. Rainforests on paleochannels (sites 4 and 6) are protected from flooding by slightly elevated sandy levee and have flora very similar to site 1. These forests were formed since the early-mid Holocene after channel abandonment. C3 grasses are predominant in open savanna areas (sites 4, 5 and 6), with less representation in wooded savannas (sites 2 and 3). However, C4 grasses, despite having fewer species, constitute significant biomass in the wooded (~ 60%) and open savanna vegetation areas, especially during the dry season. The reconstructions of past vegetation together with the distributions of modern vegetation allow prediction that climate changes to drier conditions can significantly influence the future Marajَ Island landscape, likely enabling expansion of C4 plants in the flooding zone and of trees in the rainforests.