Compositional trends in the primary floodplain forest of Manu National Park, Peru

Background/Question/Methods

Over the past ~20 years, stand-level assessments of undisturbed Amazonian forests have revealed an increase in stem turnover, density and basal area growth rates. However, a more detailed analysis of genus or species-level changes is required to assess the carbon dynamics of the region. One study examining undisturbed community composition at this level was undertaken by Laurance et al. (2004) near Manaus, Brazil. It revealed a directional shift towards fast growing emergents at the expense of slower-growing genera, decreasing the carbon sequestration ability of these forests. The authors cite rising atmospheric CO₂ as the only factor capable of driving such patterning. If a uniformly distributed gas as CO₂ is responsible for the observed changes, one would expect to see similar shifts across the Amazon Basin, if not pan-tropically. The primary question posited was what are the compositional changes in abundance and basal area across genera of undisturbed Amazonian forest in Manu National Park (MNP), Peru? A further question addressed whether or not any directional shift similar to that observed by Laurance et al. (2004) is occurring in MNP. The analysis examined 15 years of data across 7 undisturbed treeplots in MNP for alterations in community composition at the genus level.

Results/Conclusions

Analyses of population density and basal area across the entire lifetime of the plots (11,259 individuals over 14.025-ha analyzed) 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 by chance alone. However, an examination of corresponding wood density values revealed that these genera are not exhibiting a directional shift similar to that observed by Laurance et al. (2004). Numerous potential reasons behind the trends observed near Manaus, Brazil are posited. One such influence, local depletion of faunal seed dispersers by past hunting, is explored in further detail using a known defaunated forest near Boca Manu (BM), Peru. Data over the last 15 years in BM has shown a similar directional trend to those observed by Laurance et al. (2004), pointing to the possibility of an “empty forest” effect in their research area. Ultimately, further analyses employing life history characteristics of the compositionally altering genera within MNP are necessary to determine if other biotic or abiotic factors could be driving the changes observed.