Compositional Trends in the Primary Floodplain Forest of the Manu National Park, Peru

Abstract

Over the past ~20 years, various stand-level assessments of undisturbed Amazonian forests have revealed an increase in stem turnover (resulting from increases in recruitment and mortality), an increase in stem density and an increase in basal area growth rates. However, a more detailed analysis of the genus or species level changes within these forests is required to adequately assess the carbon-level dynamics of the region. The only assessment to examine undisturbed community composition at this level was undertaken in 2004 by Laurance et al. in Manaus, Brazil. This study revealed a directional shift towards fast growing, canopy emergents at the expense of slower growing genera, ultimately indicating a reduction in the carbon sequestration ability of these forests. Laurance goes on to cite rising atmospheric CO2 levels as the only capable factor of driving his observed trends. Importantly, if such a uniformly distributed gas as CO2 is responsible for the observed changes, we would expect to see similar shifts across the entire Amazon Basin, if not pan-tropically. The analysis here examines 15 years of data across 7-undisturbed treeplots in Manu National Park, Peru for alterations in community composition at the genus level. Analyses of population density and basal area across the entire lifetime of the plots have revealed that the numbers of genera found to be changing at the p <0.05 significance level are more than two times greater than would be expected from chance alone. However, an examination of corresponding wood density values reveals that these genera are not exhibiting a directional shift similar to that observed by Laurance in 2004. Numerous potential reasons behind the trends observed in Laurance’s forests, such as recent disturbance or local depletion of seed dispersers by past hunting, are explored.