RESUMEN
Vegetation cover changes in African drylands are often thought to result from population growth, social factors and aridification. Here we show that long-term vegetation proxy records can help disentangling these main driving factors. Taking the case of North Ethiopia, we performed an integrated investigation of land cover changes over the last four centuries around the endorheic Lake Ashenge, as derived from pollen analysis and repeat photography complemented with information from historical sources. Pollen and sediment analysis of radiocarbon-dated lake deposits shows a phase of environmental destabilization during the 18th century, after a more stable previous period. This is evidenced by decreases of tree pollen (Juniperus, Olea, Celtis, Podocarpus<5%), increases in Poaceae (>40%) and deposition of coarser silt lake sediments (>70%). Quantitative analysis of 30 repeated landscape photographs around the lake indicates a gradual decline of the vegetation cover since a relative maximum during the mid-19th Century. Vegetation cover declined sharply between the 1950s and the 1980s, but has since begun to recover. Overall, the data from around Lake Ashenge reveal a nonlinear pattern of deforestation and forest regrowth with several periods of vegetation cover change over the past four centuries. While there is forcing of regional drought and the regional land tenure system, the cyclic changes do not support a simplified focus on aridification or population growth.
Asunto(s)
Cambio Climático , Monitoreo del Ambiente , Pradera , Conservación de los Recursos Naturales , Etiopía , Lagos/químicaRESUMEN
As quantitative or spatially distributed studies of environmental change over truly long-term periods of more than 100 years are extremely rare, we re-photographed 361 landscapes that appear on historical photographs (1868-1994) within a 40,000 km(2) study area in northern Ethiopia. Visible evidence of environmental changes apparent from the paired photographs was analyzed using an expert rating system. The conditions of the woody vegetation, soil and water conservation structures and land management were worse in the earlier periods compared to their present conditions. The cover by indigenous trees is a notable exception: it peaked in the 1930s, declined afterwards and then achieved a second peak in the early 21st century. Particularly in areas with greater population densities, there has been a significant increase in woody vegetation and soil and water conservation structures over the course of the study period. We conclude that except for an apparent upward movement of the upper tree limit, the direct human impacts on the environment are overriding the effects of climate change in the north Ethiopian highlands and that the northern Ethiopian highlands are currently greener than at any other time in the last 145 years.