Carbon cycling in tropical ecosystems

Abstract

The diverse assemblage of ecosystems in tropical regions of the Earth holds a large fraction of the terrestrial biosphere’s carbon (C) stock (Bonan, 2008), and the annual exchange between tropical ecosystems (plants and soils) and the atmosphere is a critical controller of the CO2 concentration of the atmosphere, and hence of climate. Large-scale changes in the structure and function of tropical ecosystems, whether from the pressures of development or the impacts of drought (Meir & Woodward, 2010), can alter the balance in the annual exchange of carbon with far-reaching implications for the pace of climate change. Global models that couple the Earth’s climate system to the C cycle must, therefore, characterize well the biogeochemical and ecophysiological processes of tropical ecosystems and their sensitivity to atmospheric and climatic change. However, major uncertainties about fundamental C cycle processes in tropical ecosystems continue to hinder our progress (Dolman et al., 2010). Diverse aspects of C cycling in tropical ecosystems – from plant physiology and plant–soil interactions to human interactions and regional and global analyses – were discussed at the 23rd New Phytologist Symposium, ‘Carbon cycling in tropical ecosystems’, convened in Guangzhou, China, in November 2009, and summarized by Douglas Schaefer and Matthew Warren (http://www.newphytologist.org/carbon/23rdreport.pdf). In this issue of New Phytologist, we present four papers that emerged from that symposium. They cover the full range of topics discussed – plant physiology, soil processes, human interactions and global analysis – and, from their different perspectives, they all present analyses of C uptake, storage and release in tropical ecosystems. Hättenschwiler et al. (this issue, pp. 950–965) review the literature on leaf litter chemistry in a nutrient-poor Amazonian rainforest. They start with the recognition that, although several large-scale decomposition experiments have been invaluable for parameterizing biogeochemical and global C models, they generally do not include consideration of the influence of soil meso- and macrofauna, which may be particularly important in wet tropical ecosystems, and they often do not incorporate local variation in leaf traits, which are especially important in highly diverse tropical rainforests. Their analysis challenges the commonly held view that litter decomposition in the tropics is fast – decomposition of native litter at their site in French Guiana, and elsewhere in the tropics as shown in a literature survey, was comparatively slow. The authors speculate that natural selection favoured a leaf litter trait syndrome that leads to starvation and inhibition of decomposers, thereby altering competitive dynamics for limiting nutrients, particularly phosphorus. Salinas et al. (this issue, pp. 967–977) carried out an ambitious and comprehensive litter translocation experiment to evaluate the influence of environmental factors on decay rates of multiple species. They conducted their experiment over an elevational gradient from the tropical Andes in Peru to the adjacent Amazon lowlands in what was probably the largest-scale tropical leaf decomposition study to date. Despite the logistical challenges, this gradient had the advantages of the consistently high annual rainfall and the absence of substantial seasonality in temperature and a dormant season that would confound interpretation. They found that soil temperature explained most of the variation in decomposition of a variety of species, and concluded that recent warming in the region may have increased decomposition and nutrient mineralization by c. 10%. This increase in litter mineralization, coupled with rising atmospheric CO2, may partially explain accelerated growth and increased biomass in lowland Amazon forests. Ruiz-Jaen & Potvin (this issue, pp. 978–987) explored whether functional trait diversity or species diversity inform analyses of tree C storage in a mixed-species plantation and a natural forest in Panama. They were motivated by the potential value of using information on functional traits from different natural forests to design tree plantations for C sequestration. They concluded that the C storage capacity of natural forests could not be predicted using data from experimental plantations and, conversely, that forest managers need to be cautious when applying functional traits measured in natural populations when they design plantations for C management. Prentice et al. (this issue, pp. 988–998) approached C cycling questions from a very different temporal and spatial perspective. They assessed the realism of models of the changes in vegetation distribution and C cycling between the last glacial maximum (LGM, c. 21 000 yr ago) to the pre-industrial Holocene (PIH) using stable isotope and pollen data. They concluded that tropical forests accounted for a greater proportion of land C storage at LGM than PIH. Both model results and palaeodata supported the hypothesis that ecophysiological effects of atmospheric CO2 concentration influence tree–grass competition and plant productivity – effects that are assumed still to be in play today.