Abstract
Abstract. Pollen data collected in Africa at high (Kuruyange, valley swamp, Burundi) and low altitude (Victoria, lake, Uganda; Ngamakala, pond, Congo) showed that after 6 ky before present (BP), pollen of deciduous trees increase their relative percentage, suggesting thus the reduction of the annual amount of precipitation and/or an increase of in the length of the dry season. Until now, pollen-climate transfer functions only investigated mean annual precipitation, due to the absence of modern pollen-assemblage analogs under diversified precipitation regimes. Hence these functions omit the potential effect of a change in precipitation seasonality modifying thus the length of the dry season. In the present study, we use an equilibrium biosphere model (i.e. BIOME3.5) to estimate the sensitivity of equatorial African vegetation, at specific sites, to such changes. Climatic scenarios, differing only in the monthly distribution of the current annual amount of precipitation, are examined at the above three locations in equatorial Africa. Soil characteristics, monthly temperatures and cloudiness are kept constant at their present-day values. Good agreement is shown between model simulations and current biomes assemblages, as inferred from pollen data. To date, the increase of the deciduous forest component in the palaeodata around 6 ky BP has been interpreted as the beginning of a drier climate period. However, our results demonstrate that a change in the seasonal distribution of precipitation could also induce the observed changes in vegetation types. This study confirms the importance of taking into account seasonal changes in the hydrological balance. Palaeoecologists can greatly benefit from the use of dynamic process based vegetation models to acccount for modification of the length of the dry season when they wish to reconstruct vegetation composition or to infer quantitative climate parameters, such as temperature and precipitation, from pollen or vegetation proxy.
Highlights
One of the fundamental assumptions in plant ecology is that, on continental or global scales, the distribution and composition of vegetation is strongly controlled by climatic conditions (Woodward, 1987; Stephenson, 1990) through the key processes of photosynthesis, respiration and transpiration
Pollen data collected in Africa at high (Kuruyange, valley swamp, Burundi) and low altitude (Victoria, lake, Uganda; Ngamakala, pond, Congo) showed that after 6 ky before present (BP), pollen of deciduous trees increase their relative percentage, suggesting the reduction of the annual amount of precipitation and/or an increase of in the length of the dry season
Precipitation regimes are characterised by two main aspects modifying the duration and the intensity of the dry season(s) which have to be considered in defining the suitable bioclimatic limits of a vegetation type (Street-Perrott and Perrott, 1993)
Summary
One of the fundamental assumptions in plant ecology is that, on continental or global scales, the distribution and composition of vegetation is strongly controlled by climatic conditions (Woodward, 1987; Stephenson, 1990) through the key processes of photosynthesis, respiration and transpiration. The relation between vegetation composition and climate is often described in a simple way using annual descriptors such as temperature and precipitation (Bonnefille et al, 1990). Precipitation regimes are characterised by two main aspects (i.e. annual mean and month to month variability) modifying the duration and the intensity of the dry season(s) which have to be considered in defining the suitable bioclimatic limits of a vegetation type (Street-Perrott and Perrott, 1993). Climatic zones related to the upper-level circulation tend to occur as symmetrical belts on either side of the equator (Thompson, 1965). The tropical climate is governed by the seasonal movement of the meteorological equator (i.e. intertropical convergence zone, ITCZ) in response to changes in Published by Copernicus Publications on behalf of the European Geosciences Union
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