Abstract
In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16–52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha−1 y−1, 95% confidence intervals [0.07, 0.39] MgC ha−1 y−1), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y−1) compared with the tree community as a whole (+0.15 % y−1); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y−1), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.
Highlights
Tropical rain forests play a major role in the global carbon cycle: they encompass over a third of terrestrial carbon stocks [1], and they contribute approximately 30% of terrestrial net primary productivity [2]
Recent studies have reported major changes in mature tropical forests, with increases in both forest biomass and net primary productivity, as well as shifts in plant species composition that favour fast-growing species over slow-growing ones. These pervasive alterations were attributed to global environmental change, and may result in dramatic shifts in the functioning of tropical forest ecosystems
We found that tree biomass increased at seven of our ten plots, and showed a large decrease at a single plot
Summary
Tropical rain forests play a major role in the global carbon cycle: they encompass over a third of terrestrial carbon stocks [1], and they contribute approximately 30% of terrestrial net primary productivity [2]. Are many tropical forests under direct threat from land-use changes and logging [3,4,5], but it has been suggested that pristine, apparently undisturbed rainforests may be undergoing widespread shifts in carbon stocks and floristic composition as a result of large-scale anthropogenic environmental changes. Models suggest that plants in general, and tropical forest plants in particular, are sensitive to environmental changes such as increased atmospheric CO2 concentration, nitrogen deposi-. ¤b Current address: Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, California, United States of America,. ¤c Current address: Wake Forest University, Department of Biology, Winston-Salem, North Carolina, United States of America. ¤d Current address: Organization for Tropical Studies, Durham, North Carolina, United States of America
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