Origin and dynamics of the northern South American coastal savanna belt during the Holocene – the role of climate, sea-level, fire and humans
Introduction
Present-day savannas occupy approximately 16.1 million km2, or 11.5% of the global landmass. The proportion of trees and grasses in savannas is related to climate and land use practice. In addition, extensive biomass burning during the dry season plays an important role to inhibit tree growth (Scholes and Hall, 1996, Murphy and Bowman, 2012). Considering the large difference between above ground carbon storage capacity of treeless grasslands (2 tons C/ha) and woodland savannas (30 tons C/ha), besides the huge amount of CO2 that may be emitted to atmosphere by biomass burning of savannas (estimated at 0.5–4.2 Gt C per year globally) (Grace et al., 2006), plans for CO2 management need to consider climatic/anthropogenic influences which trigger change from savanna with arboreal taxa (cerradão) to treeless grasslands (campo limpo).
A narrow strip of savanna known as “coastal savanna belt” in northern South America is found along the coast of British Guiana, Surinam, French Guiana, and in State of Amapá, Marajo Island and in part in the state of Pará. This discontinuous belt about 2000 km long is disrupted locally by other types of coastal vegetation (Fig. 1a). Based on the meteorological data (NOAA), annual precipitation in the coastal savanna belt is in a range similar to most of Amazon regions (between 1750 and 3500 mm) (Snow, 1976, Nimer, 1989, Weischet, 1996). Therefore instead of savanna presence of Amazon rainforest would be expected. Because of its considerable area the savanna's existence and its dynamics should have a substantial effect on the regional carbon budget.
Several palaeo-environmental studies using pollen and charcoal analyses have previously been carried out to investigate the history of the coastal area in northern South America. Although these studies are mainly focused on mangrove development, useful clues can be obtained regarding savanna/forest/mangrove interaction. Records from Guyana (Van der Hammen, 1963), Suriname (Wijmstra, 1971) and French Guiana (Tissot and Marius, 1992) show expansion of savanna during the full glacial period (and shoreline regression) and mangrove development during the interglacial period (and sea level transgression). Swamp savanna with dominance of Poaceae and Cyperaceae was present during the last c 5700 cal yr BP in the coastal regions of Guyana, Suriname, and French Guiana (Behling and Hooghiemstra, 2001). In Amapá State lacustrine littoral records from lakes Tapera and Marcio (Toledo and Bush, 2007) also demonstrate changes in vegetation from closed forests with swamp taxa to open flooded savanna at c 4750 cal yr BP. In the pollen record of Lago Arari, on Marajó Island in the mouth of the Amazon River in northeastern Pará there is a marked change from the more or less closed to open swamp savanna and forest at ca 7400 cal yr BP (Absy, 1985). Another study from Lake Arari investigated four sediment cores, which reveal replacement of mangrove by herbaceous vegetation at 2300 cal yr BP and an expansion of herbs during the last 1000 years (Smith et al., 2012). In the Southern Hemispheric part of the coastal area, in Lago Crispim (Behling and Costa, 2001), Lagoa da Curuca (Behling, 2001) and Lagoa do Caco (Ledru et al., 2001, Pessenda et al., 2005) Holocene started with arboreal taxa dominating in the vegetation, which with different timing (due to different latitudinal position) gradually were accompanied by swamp trees and finally replaced by open vegetation.
In addition to being interesting for palaeoenvironmental research, lands on the eastern Amazonia on the Amazon River bank or near the river mouth host many archeological surveys. Late Pleistocene Paleo-Indian camp side at Monte Alegre in the eastern Brazilian Amazon documents presence of ecologically adapted foragers with presumably limited big-game hunting habit (Roosevelt et al., 1996). In another study on Marajo Island, Roosevelt and her colleagues (1991) found that mound builders have occupied the alluvial floodplains of the Lower Amazon from A.D. 400–1300.
This work presents the analysis of a 750 cm-long sediment core taken from a key area of the coastal savanna belt near the mouth of the Amazon River, which forms an ecotone between Amazon rainforest and coastal vegetation. In order to test different hypotheses concerning probable driving forces for the development of a savanna belt, such as sea level change, climate, fire, human and edaphic factors, this continuous pollen and charcoal record was analyzed and compared with other results from northern South America.
Section snippets
Study area
The core named Curiau (CUR), was collected from a small Mauritia swamp 15 m in diameter (00°12′30.3″ N, 51°01′12.1″ W, 5 m a.s.l) located 16 km north of Macapá City in the south of Amapá State near the mouth of Amazon river (Fig. 1b). The studied swamp is surrounded by small hills which have an elevation up to 22 m a.s.l.
Material and methods
The core CUR was taken from a Mauritia swamp in November 2004 using a Russian Corer and was transported to the Federal University of Pará and stored in a refrigerator after subsampling.
Stratigraphy and chronology
In the sediment core CUR two major types of deposits are recognizable: The bottom of the core (750–700 cm) contains gray sandy sediments with small amounts of organic material. Between 700 and 0 cm the sediment is composed of dark gray material with a high organic content. In the uppermost 30 cm some bioturbation may occur. In general, from the base to the top of the core accumulation rate decreased. However, between 700 and 550 cm accumulation rate is three-fold and shows a strong change in
Beginning of the Holocene (Zone CUR-I, 11,500–11,200 cal yr BP)
The pollen assemblages of this period indicate a landscape occupied by a mono-dominant arboreal community composed of Micropholis and mixed with sparse trees of Protium and Myrtaceae family, which probably grew in the depressions. Small patches of savanna probably covered the top of the hills along the Pleistocene plains. Today Micropholis is found mainly both in the lowland and upland rainforest of South America (Lorenzi, 1992, Lorenzi, 1998). Therefore it can be concluded that the formation
Discussion
The data reveal that during the last 11,500 years, savanna was an integral part of the landscape in the Amapá coastal region. However, it expanded after the extinction of the specific arboreal vegetation, which covered the area at the beginning of the Holocene (>11,500–11,200 cal yr BP). During the early Holocene (11,200–8500 cal yr BP) savanna was dominant and occurred on the elevated areas of the hills along Pleistocene plain and forest swamp established along palaeo-channels. At
Conclusion
The following main conclusions can be made from this study:
- 1
Similar to many regions in the lowland Amazonia, a unique forest type without modern analog grew near the mouth of Amazon River at the beginning of the Holocene.
- 2
Savanna had been continuously present in the coastal area of Amapá State at least since the beginning of the Holocene (11,500 cal yr BP).
- 3
In this region savanna has an intermediate position between interhemispheric shifts of atmospheric convection systems, because the early
Acknowledgment
We would like to thank Dr Thomas Giesecke (University of Göttingen), who kindly supervised statistical analysis of this investigation. We sincerely appreciate Professor Vera Markgarf (INSTAAR, University of Colorado Boulder) for her sage advice on both language and scientific aspect of the paper. We thank also Suzette G.A. Flantua manager of Latin American Pollen Database from the Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam, for the useful advice on the
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