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Special Feature Articles: Editorial Carbon cycling in tropical ecosystems Richard J. Norby

Research reviews Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis Stephan H채ttenschwiler, Sylvain Coq, Sandra Barantal and Ira Tanya Handa

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The sensitivity of tropical leaf litter decomposition to temperature: results from a large-scale leaf translocation experiment along an elevation gradient in Peruvian forests Norma Salinas, Yadvinder Malhi, Patrick Meir, Miles Silman, Rosa Maria Roman-Cuesta, Judit Huaman, Dino Salinas, Vicky Huaman, J. Alberto Gibaja, Marlene Mamani and Filio Farfan

Can we predict carbon stocks in tropical ecosystems from tree diversity? Comparing species and functional diversity in a plantation and a natural forest. Maria C. Ruiz-Jaen and Catherine Potvin

Global vegetation and terrestrial carbon cycle changes after the last ice age Iain C. Prentice, Sandy P. Harrison and Patrick J. Bartlein

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Introduction 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. Largescale changes in the structure and function of tropical ecosystems can alter the balance of 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 climate change.

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’. 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.

Research review Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis Author for correspondence: S. H채ttenschwiler Tel: +33 467 61 22 36 Email: stephan.hattenschwiler@cefe.cnrs.fr

Stephan H채ttenschwiler, Sylvain Coq, Sandra Barantal and Ira Tanya Handa

Summary New Phytologist (2010) doi: 10.1111/j.1469-8137.2010.03483.x

Keywords: energy starvation, French Guiana, litter quality, mycorrhizas, nutrient cycling, nutrient limitation, phosphorus, soil fauna.

Proper estimates of decomposition are essential for tropical forests, given their key role in the global carbon (C) cycle. However, the current paradigm for litter decomposition is insufficient to account for recent observations and may limit model predictions for highly diverse tropical ecosystems. In light of recent findings from a nutrient-poor Amazonian rainforest, we revisit the commonly held views that: litter traits are a mere legacy of live leaf traits; nitrogen (N) and lignin are the key litter traits controlling decomposition; and favourable climatic conditions result in rapid decomposition in tropical forests. Substantial interspecific variation in litter phosphorus (P) was found to be unrelated to variation in green leaves. Litter nutrients explained no variation in decomposition, which instead was controlled primarily by nonlignin litter C compounds at low concentrations with important soil fauna effects. Despite nearoptimal climatic conditions, tropical litter decomposition proceeded more slowly than in a climatically less favourable temperate forest. We suggest that slow decomposition in the studied rainforest results from a syndrome of poor litter C quality beyond a simple lignin control, enforcing energy starvation of decomposers. We hypothesize that the litter trait syndrome in nutrient-poor tropical rainforests may have evolved to increase plant access to limiting nutrients via mycorrhizal associations.

The sensitivity of tropical leaf litter decomposition to temperature: results from a large-scale leaf translocation experiment along an elevation gradient in Peruvian forests Norma Salinas, Yadvinder Malhi, Patrick Meir, Miles Silman, Rosa Maria Roman-Cuesta, Judit Huaman, Dino Salinas, Vicky Huaman, J. Alberto Gibaja, Marlene Mamani and Filio Farfan

Summary Author for correspondence: Norma Salinas Revilla Tel: +44 7551 904389 Email: norma.salinas@ouce.ox.ac.uk

• We present the results from a litter translocation experiment along a 2800-m elevation gradient in Peruvian tropical forests. The understanding of the environmental factors controlling litter decomposition is important in the description of the carbon and nutrient cycles of tropical ecosystems, and in predicting their response to long-term increases in temperature.

New Phytologist (2010) doi: 10.1111/j.1469-8137.2010.03521.x

• Samples of litter from 15 species were transplanted across all five sites in the study, and decomposition was tracked over 448 d.

KEYWORDS: decomposition, elevational gradient, leaf litter, soil temperature, tropical forest.

• Species’ type had a large influence on the decomposition rate (k), most probably through its influence on leaf quality and morphology. When samples were pooled across species and elevations, soil temperature explained 95% of the variation in the decomposition rate, but no direct relationship was observed with either soil moisture or rainfall. The sensitivity of the decay rate to temperature (κT) varied seven-fold across species, between 0.024 and 0.169°C−1, with a mean value of 0.118 ± 0.009°C−1 (SE). This is equivalent to a temperature sensitivity parameter (Q10) for litter decay of 3.06 ± 0.28, higher than that frequently assumed for heterotrophic processes. • Our results suggest that the warming of approx. 0.9°C experienced in the region in recent decades may have increased decomposition and nutrient mineralization rates by c. 10%.

Can we predict carbon stocks in tropical ecosystems from tree diversity? Comparing species and functional diversity in a plantation and a natural forest Maria C. Ruiz-Jaen and Catherine Potvin

Summary Author for correspondence: Maria C. Ruiz-Jaen Tel: +1 514 3986726 Email: maria.ruizjaen@mail.mcgill.ca

New Phytologist (2010) doi: 10.1111/j.1469-8137.2010.03501.x

KEYWORDS: dominance, functional trait diversity, functional traits, mixed-species plantations, Panama, species diversity, tree carbon storage, tropical forests.

• Linking tree diversity to carbon storage can provide further motivation to conserve tropical forests and to design carbon-enriched plantations. Here, we examine the role of tree diversity and functional traits in determining carbon storage in a mixed-species plantation and in a natural tropical forest in Panama. • We used species richness, functional trait diversity, species dominance and functional trait dominance to predict tree carbon storage across these two forests. Then we compared the species ranking based on wood density, maximum diameter, maximum height, and leaf mass per area (LMA) between sites to reveal how these values changed between different forests. • Increased species richness, a higher proportion of nitrogen fixers and species with low LMA increased carbon storage in the mixed-species plantation, while a higher proportion of large trees and species with high LMA increased tree carbon storage in the natural forest. Furthermore, we found that tree species varied greatly in their absolute and relative values between study sites. • Different results in different forests mean that we cannot easily predict carbon storage capacity in natural forests using data from experimental plantations. Managers should be cautious when applying functional traits measured in natural populations in the design of carbon-enriched plantations.

Global vegetation and terrestrial carbon cycle changes after the last ice age Iain C. Prentice, Sandy P. Harrison and Patrick J. Bartlein

Summary Author for correspondence:I. C. Prentice Tel: +61 2 9850 4227 Email: colin.prentice@mq.edu.au

• In current models, the ecophysiological effects of CO2 create both woody thickening and terrestrial carbon uptake, as observed now, and forest cover and terrestrial carbon storage increases that took place after the last glacial maximum (LGM). Here, we aimed to assess the realism of modelled vegetation and carbon storage changes between LGM and the pre-industrial Holocene (PIH).

New Phytologist (2010) doi: 10.1111/j.1469-8137.2010.03620.x

• We applied Land Processes and eXchanges (LPX), a dynamic global vegetation model (DGVM), with lowered CO2 and LGM climate anomalies from the Palaeoclimate Modelling Intercomparison Project (PMIP II), and compared the model results with palaeodata.

KEYWORDS: CO2, dynamic global vegetation model (DGVM), forest, last glacial maximum (LGM), productivity, sink, stable isotopes, woody thickening

• Modelled global gross primary production was reduced by 27–36% and carbon storage by 550–694 Pg C compared with PIH. Comparable reductions have been estimated from stable isotopes. The modelled areal reduction of forests is broadly consistent with pollen records. Despite reduced productivity and biomass, tropical forests accounted for a greater proportion of modelled land carbon storage at LGM (28–32%) than at PIH (25%). • The agreement between palaeodata and model results for LGM is consistent with the hypothesis that the ecophysiological effects of CO2 influence tree–grass competition and vegetation productivity, and suggests that these effects are also at work today.

27th New Phytologist Symposium

Stoichiometric flexibility in terrestrial ecosystems under global change Biosphere 2, Oracle, AZ, USA 25–28 September 2011

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Carbon cycling in tropical ecosystems