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Will nutrients limit the tropical carbon sink?

Dr. Sarah Batterman (SoG), Prof. Oliver Phillips (SoG)

Contact email: s.a.batterman@leeds.ac.uk

Tropical forests have the potential to serve as a large-scale carbon sink (Pan et al. 2011), both in enhanced carbon uptake in mature forests due to CO2 fertilization, and in the recovery of forests in deforested and degraded areas. The tropical terrestrial sink provides a critical offset to human carbon emissions, helping to slow climate change (Pacala & Socolow 2004; Le Quere et al. 2015).

Figure 1: Nutrient acquisition strategies of tropical plants: tissue stoichiometry and allocation, symbiotic nitrogen fixation and associations with fungi.

However, the recent observation of the slow-down of the sink in Amazonia (Brienen et al. 2015) raises the question of the degree to which tropical carbon uptake will persist at all (in spite of modelling assumptions that it will, e.g. Huntingford et al. 2013), and particularly if nutrient-constraints will shut it down (Hungate et al. 2003). Advances in our understanding of plant-soil nutrient feedbacks during periods of rapid carbon accumulation in secondary succession do suggest strong nutrient constraints on tropical carbon uptake (Batterman et al. 2013a). In contrast, the ability of trees to avoid nutrient limitation by evolving diverse nutrient acquisition and use strategies (Phillips et al. 2003; Fyllas et al. 2009; Batterman et al. 2013b; Sheffer et al. 2015) – including nitrogen fixation, association with mycorrhizal fungi, phosphatase enzyme release, tissue stoichiometry and allocation patterns – points to the possibility that biodiverse tropical forests are more resilient to nutrient constraints than theory and field observations suggest.

This project will examine the consequences of biodiversity and nutrient limitation for the tropical carbon sink by testing these competing hypotheses with a combination of approaches that may include large-scale field experimentation, analysis of an extensive forest dynamics dataset and theoretical modelling. 

Questions

1) How do nutrients affect the dynamics and community composition of tree species with diverse nutrient acquisition and use strategies in tropical rainforests with different net carbon uptake rates? The student will use an on-going large-scale ecosystem fertilization experiment at the Smithsonian Tropical Research Institute in Panama to examine the response of tree species with different nutrient strategies to nutrient additions. This experiment will use space for time substitution in forests at different stages of recovery from disturbance that represent different rates of carbon uptake and nutrient conditions.

2) Have species with key nutrient strategies differed in their dynamics in mature forests with distinct net carbon sink trajectories and soil nutrients?  The student will analyse long-term RAINFOR data of ~200,000 trees across 300 forest plots in South America. The student may also undergo fieldwork with targeted sampling of nutrient strategies on trees in focal forest plots.

3) What are the consequences of biodiverse nutrient strategies and potential nutrient limitation on tropical forest carbon uptake under changing climate? The student may choose to utilize their field-based findings to develop a simple ecosystem model with an evolutionary game theory approach to understand whether forests with biodiverse tree strategies are sensitive to nutrient limitation as atmospheric CO2 and climate changes.

 

Figure 2: Tropical rainforest in Panama that includes a potentially persistent carbon sink in both mature forests and forest recovering from agricultural abandonment.

Potential for high impact outcome

The project addresses one of the major challenges facing us today in understanding climate change and the tropical carbon sink. Resolving this question is critical as there is potential for a large carbon sink in the vast areas of tropical forest that are recovering from land use and in mature forests that are threatened with biodiversity loss. The research groups involved have a record of high-impact outcomes from research on tropical forest biodiversity, nutrients and the carbon sink.

Training

The student will work closely with Sarah Batterman, with additional support from Oliver Phillips at University of Leeds. The student will interact and collaborate with scientists from the Smithsonian Tropical Research Institute and the RAINFOR network. Training will include ecosystem experimentation, analysis of large datasets, field observational techniques, statistical analyses and mathematical modelling. The Ecology and Global Change group in the School of Geography at Leeds, where the student will be based, is a dynamic world-leading group on tropical ecology and biogeochemical cycling with a strong publication record.

Student profile

The student should be independent and highly motivated with a background in ecology, evolution, statistics and/or mathematical modelling, and have the ability to spend extended time in the field in tropical forest conditions. Students with previous field experience and/or a MSc or similar degree are particularly encouraged to apply.

References

Batterman, S A., L.O. Hedin, M. van Breugel, J. Ransijn, D.J. Craven, and J.S. Hall. 2013a. Key role of symbiotic dinitrogen fixation in tropical forest secondary succession. Nature 502, 224-229.

Batterman, S.A., Wurzburger, N., and Hedin, L.O. 2013b. Nitrogen and phosphorus interact to control tropical symbiotic N2 fixation: A test in Inga punctata. Journal of Ecology 101, 1400–1408. DOI: 10.1111/1365-2745.12138

Brienen RJW, Phillips OL, Feldpausch TR, et al. 2015. Long-term decline of the Amazon carbon sink. Nature 519: 344-348.

Fyllas, N.M., et al. 2009. Basin-wide variations in foliar properties of Amazonian forest: phylogeny, soils and climate. Biogeosciences 6, 2677–2708
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Hungate, BA, JS Dukes, MR Shaw, YQ Luo & CB Field. 2003. Nitrogen and climate change. Science, 302; 1512-1513.

Huntingford, C. et al. 2013. Simulated resilience of tropical rainforests to CO2-induced climate change. Nature Geoscience.

Le Quere, C. et al. 2015. Global carbon budget 2015. Earth Syst. Sci. Data 7: 349-396.

Pacala & Socolow. 2004. Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies
. Science 305.

Pan, Y. et al. 2011. A large and persistent carbon sink in the world's forests. Science 333, 988-993, doi:10.1126/science.1201609

Phillips OL, et al. 2003. Habitat association among Amazonian tree species: a landscape-scale approach. Journal of Ecology 91:757-775.

Sheffer, E., Batterman, S. A., Levin, S. A., Hedin, L. O. 2015. Biome-scale nitrogen fixation strategies selected by climatic constraints on nitrogen cycle. Nature Plants 1. doi:10.1038/nplants.2015.182