Past vegetation changes in the Brazilian Pantanal arboreal–grassy savanna ecotone by using carbon isotopes in the soil organic matter

Abstract

Measurements of the organic carbon inventory, its stable isotopic composition and radiocarbon content were used to deduce vegetation history from two soil profiles in arboreal and grassy savanna ecotones in the Brazilian Pantanal. The Pantanal is a large floodplain area with grass-dominated lowlands subject to seasonal flooding, and arboreal savanna uplands which are only rarely flooded. Organic carbon inventories were lower in the grassy savanna site than in the upland arboreal savanna site, with carbon decreasing exponentially with depth from the surface in both profiles. Changes in ^^C of soil organic matter (SOM) with depth differed markedly between the two sites. Differences in surface SOM "C values reflect the change from C3 to C4 plants between the sites, as confirmed by measurements of '^C of vegetation and the soil surface along a transect between the upland closed-canopy forest and lowland grassy savanna. Changes of ^^C in SOM with depth at both sites are larger than the 3-4 per mil increases expected from fractionation associated with organic matter decomposition. We interpret these as recording past changes in the relative abundance of C3 and C4 plants at these sites. Mass balances with ^'*C and "C suggest that past vegetational changes from C3 to C4 plants in the grassy savanna, and in the deeper part of the arboreal savanna, occurred between 4600 and 11 400 BP, when major climatic changes were also observed in several places of the South American Continent. The change from C4 to C3, observed only in the upper part of the arboreal savanna, was much more recent (1400 BP), and was probably caused by a local change in the flooding regime.

Carbon-13 variation with depth in soils of Brazil and climate change during the Quaternary

“…The NhecoJandia profile came from the Brazilian Pantanal, a large floodplain, highly dynamic in terms of geomorphology (Adamoli 1982), with a landscape that alternates high land forested areas with low land areas of savannah. This dynamism is reflected in the dt 3C values, with reach a maximum of almost 6%0 at 100 cm depth, s uggesting the replacement of the forest by a C4 savannah, and a decrease to almost 0.5%o at 170 cm depth, indicating the dominance of a C3 vegetation type again (Victoria et al 1995). The profiles from Tunas and Salitre (Sal-I and Sal-2) show ~13C increasing from 4.0 to 6.0%0, within the upper 30 cm.…”

“…In such cases the most likely cause for this large J3C enrichment with depth is the existence of prior C4 vegetation (Dzurec et al 1985;Mondenesi et al 1986;Schwartz et al 1987;Volkoff and Cerri 1987;Martin et al 1990;Desjardins et al 1991;McPherson et al 1993;Wang et al 1993;Mariotti and Peterschmitt 1994;Victoria et al 1995). Both profiles of Salitre, NhecoJandia, and Tunas had intermediate 613C values varying approximately from 4 .0 to 6.0%0 (Fig.…”

“…Radiocarbon ages. expressed as years before present (years BP), were available for profiles located at Piracicaba, Londrina, Altamira, Salitre, Nhecolandia and Para- • Average rainfall (mm/yr) O ECOLOGIA 106 ( 1996) Valencia ( 1993) for the first three profiles, and by Pessenda et al (1996), Victoria et al (1995), a nd Trumbore et al ( 1995) for the three last profiles, respectively.…”

Dinâmica do ecótono floresta-campo no sul do estado do Amazonas no Holoceno, através de estudos isotópicos e fitossociológicos

“…Estudos palinológicos (Absy & Van der Hammen, 1976;Absy, 1985;Sanford et al, 1985;Ledru, 1992Ledru, , 1993Behling & Costa, 2000, geomorfológicos (Servant et al, 1981), sedimentológicos Turcq et al, 1993;Sifeddine et al, 1994Sifeddine et al, , 2001Sifeddine et al, , 2003 e isotópicos (Victoria et al, 1995;Desjardins et al, 1996;Pessenda et al, 1996aPessenda et al, ,b,c e 1998aPessenda et al, , 2001Pessenda et al, , 2004Gouveia et al, 1997Gouveia et al, , 1999Gouveia et al, , 2002Freitas et al, 2001) indicam a ocorrência de variações paleoclimáticas na Bacia Amazônica e em outras áreas do Brasil e da América do Sul, durante o Quaternário tardio. Paleoclimas mais secos que o atual, provavelmente, dominou algumas áreas da Bacia Amazônica, favorecendo a substituição da floresta tropical por vegetação do tipo campo.…”

Century-scale environmental reconstruction by using stable carbon isotopes: just one method from the big bag of tricks

“…Because of the power to detect C 3 / C 4 change it has been applied widely in tropical and semi-arid areas (Schwartz et al 1986;Tieszen and Archer 1991;McPherson et al 1993;Bond et al 1994;Nordt et al 1994;Archer 1995;Victoria et al 1995;Desjardins et al 1996;Boutton et al 1999;Bowman and Cook 2001).…”