Will nutrients limit the tropical carbon sink?

Tropical forests have the potential to absorb large quantities of carbon (Pan et al. 2011), both by taking up extra carbon in mature forests due to CO2 fertilization, and by forest regrowth in deforested and degraded areas. Together, this “tropical terrestrial sink” currently provides a critical offset to human carbon emissions, and is helping to slow climate change (Pacala & Socolow 2004; Le Quere et al. 2015). Critically, whether or not the Earth’s major carbon sinks continue to function will determine how much of future emissions stays in the atmosphere, so any weakening of the sinks will require even stronger action on reducing fossil fuel emissions to keep global temperature rises within safe levels.

Most global models assume that tropical carbon uptake will persist (e.g. Huntingford et al. 2013). Yet, recently we have discovered a marked slow-down of the sink across Amazonia (Brienen et al. 2015), raising the question of whether the tropical carbon uptake will persist at all, and particularly if nutrient constraints will now slow growth rates (Hungate et al. 2003) given the low nutrient content of most tropical soils. Indeed, advances in our understanding of plant-soil nutrient feedbacks during secondary succession do suggest strong nutrient constraints on tropical tree growth (Batterman et al. 2013a). But whether these constraints are widespread or not remains unknown, and an alternative view suggests that among the great diversity of tropical plants, some species could overcome nutrient limitations, for example by ‘paying’ extra carbon to symbiotic bacteria and fungi in return for help acquiring nutrients.

This project will explore the consequences of nutrient limitation and biodiversity for the tropical carbon sink by testing these competing ideas. A range of different approaches may be used, including field experiments, analysis of Amazon forest dynamics, and modelling.
Potential questions to develop include:

1) How effective are plants at overcoming nutrient limitation by adjusting nutrient strategies?

2) Do nutrients explain the slowdown of the tropical carbon sink across the Amazon basin already seen?

3) Can simple mathematical models help resolve when nutrients will limit the carbon sink?

The student will have the opportunity to explore one or more of these questions, guided by the supervisors. The project will include fieldwork in Central or South America, the most biodiverse region on the planet. The supervisors already lead several projects here that can naturally support this exciting new investigation, so the student will have the opportunity to work with a variety of techniques and research colleagues.

The student may choose to take a variety of approaches, including one or more of the following:

• Work in an on-going large-scale ecosystem fertilization experiment at the Smithsonian Tropical Research Institute in Panamanian forests that span the full range of recovery from disturbance by agriculture to mature forest.

• Target data collection across the RAINFOR network of plots in the Amazon.

• Analyse the long-term RAINFOR data of ~200,000 trees across 300 forest plots in South America.

• Develop an ecosystem model with an evolutionary game theory approach, to understand whether forests with diverse tree strategies are sensitive to nutrient limitation, atmospheric CO2 and climate changes.

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/or 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 that studies tropical ecology, biogeochemical cycling and global environmental change.

http://www.nercdtp.leeds.ac.uk/projects/index.php?id=595