Vegetation Disturbance and Human Population in Colombia-A Regional Reconstruction

Article excerpt

Introduction

Students of environmental change world wide are increasingly concerned with the effects of human intervention on vegetation, especially since 8000 BC (the Holocene). Numerous palaeoecological investigations reconstructing vegetation dynamics from pollen invoke the forcing factor of human activity to explain vegetation change. The nature of the human impact ranges from complete regional deforestation (Marchant & Taylor 1998) to promoting the abundances of certain components of the naturally occurring vegetation, such as oil-bearing palms (Vincens et al. 1999). Problems of interpretation are common-place when a combination of anthropogenic and natural factors operate in parallel. However, unravelling the complexities of human-environmental interactions, and how the signals are commuted to the sedimentary record, are central to understanding past changes in our environment, the mechanisms behind these changes and what bearing these may have on current and future responses. For example, explaining Amazonian biodiversity patterns and its present ecosystem requires a knowledge of human activity, both past and present (Roosevelt et al. 1996).

Paleoecological study in Colombia

Late Quaternary vegetation dynamics in Colombia have been investigated at a number of spatial and temporal scales, ranging from some of the first site-specific studies in the tropics (Van der Hammen & Gonzalez 1965) to high-resolution studies combining latest techniques of dating control (Wille et al. 2000). Colombia is eminent in Latin America, and indeed the wider tropics, for the excellent spatial distribution of high quality pollen-based records with a wide application of radiocarbon data. Initial research focussed on the abundant sedimentary basins located at high altitudes where the sites are relatively accessible and the climate temperate, but this has been complemented recently with a range of site-specific investigations from the lowlands (Berrio et al. 2002; Behling & Hooghiemstra 1998, 1999; Wille et al. 2000). The locations of 43 sites with pollen data are mapped on Figure 1. The sites are listed with their altitude, latitude and longitude in Table 1, which will be found online at http://antiquity.ac.uk/ProjGall/marchant.

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The climatic background

The climate of north-western South America is influenced by the annual migration of the Inter-Tropical Convergence Zone (ITCZ) that produces an annual migration of rainfall (Hastenrath 1991). This has a sinusoidal profile over northern South America, due to the influence of the Pacific westerlies, and the sharply rising topography of the Andes. Precipitation is highest (> 10,000 mm/year) in the Choco Pacific region due to the close proximity of Pacific-based moisture source and the steeply rising ground of the Western Cordillera. Another important climate system brings moisture from the Atlantic Ocean; precipitation being highest on the eastern slopes of the Andes as rising air condenses, the concave nature of the topography acting as a natural receptacle for moisture. Low rainfall is recorded within rain shadow areas, resulting in dry inter-Andean plains (Berrio etal. 2002a). Andean climate can be classified as tropical diurnal (Kuhry 1988). At a given location differences in monthly temperature are small (< 3[degrees]C) although daily fluctuations may be large (20[degrees]C), especially in dry seasons. The three Cordilleras of the northern Andes rise to approximately 5000 m, applying a lapse rate of 6.6[degrees]C per1000m (Van der Hammen & Gonzalez 1965). This altitudinal rise equates to a temperature change of nearly 30[degrees]C resulting in a change in vegetation from diverse lowland tropical rainforest to high altitude grasslands. Additional complexity to the South American climate system is concerned with the E1 Nino Southern Oscillation (ENSO). The tropical Pacific oscillates somewhat irregularly (three to seven years) between an El Nino phase (warm tropical waters upwelling off the South American Pacific coast) and a La Nina phase (cold tropical waters dominating). …