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Catastrophic natural origin of a species-poor tree community in the world's richest forest

Abstract:

Upper Amazonian tree communities are famous for their very high alpha-diversity. This paper describes an anomalous forest just 6 km south of the equator in lowland Ecuador that is structurally mature, surrounded by hyperdiverse forest, but strikingly poor in species. To investigate the anomaly, a 1-ha tree inventory and soil analysis were carried out and compared with 15 similar surveys of upland forest in the same region. The anomalous forest contains only 102 tree species ha-1, compared with a regional mean of 239 ± 28 species ha-1. It is structurally indistinguishable from richer forests, and closest in composition to upland forest, but lacks the uplands' typically rich understorey tree community. Three hypotheses for its origin are examined: recovery from anthropogenic disturbance, unique soil conditions and recovery from a large-scale natural catastrophe. The third hypothesis is the best supported. Mineralogical, geological and remote-sensing evidence, and 14C-dating suggest that the forest grows on a vast debris plain left by a catastrophic flooding event roughly 500 y ago. The forest's low diversity today is most likely due to the failure of a full complement of the region's tree species - especially understorey taxa - to recolonize the outwash plain in the time since the disaster. Copyright © 2005 Cambridge University Press.