NSFDEB-NERC: Addressing the plant growth C source-sink debate through observations, experiments, and modelling

Fossil fuel burning is causing atmospheric concentrations of the greenhouse gas CO2 to rise, the main driver of man-made climate change. However, the rate of CO2 rise is much slower than emissions suggest it should be. It appears that the land surface and oceans are together absorbing about 50% of annual CO2 emissions. Some field studies indicate that a large portion of the land surface uptake is due to increasing tree growth. However, the causes, locations, and future behaviour of this CO2 uptake remain highly uncertain.

A plausible hypothesis is that this land uptake is occurring because higher levels of CO2 increase plant photosynthesis, meaning more carbon in plants. However, a number of scientists believe that tree growth is not commonly limited by the rate of photosynthesis itself, but is instead controlled by other factors such as rates of cell division, nutrients, or water supply. If this is true, it implies lower future uptake of CO2 on land than is currently assumed, and so greater rates of climate change. Improving our knowledge of plant responses to CO2 is clearly essential for policy makers to be able to forecast with confidence the impacts of any controls on CO2 emissions on future climate.

When tree growth is limited by photosynthesis, we talk about 'carbon-limited' growth, whereas when it is limited by non-photosynthetic factors, it is 'sink-limited'. So, the extent to which trees are carbon-limited, and under which circumstances, is fundamental to understanding how they will respond to rising levels of CO2. Advocates of the importance of sink-limited growth point to findings of high concentrations of non-structural (storage) carbon observed in wood as evidence that carbon is abundant and not limiting, and thus growth is dominated by sinks, rather than by photosynthesis.

In this project, we propose to significantly improve our understanding of this fundamental issue using a unique combination of observations, experiments, and modelling. We will focus on mature individuals of white pine, red maple, and red oak growing in Harvard Forest, Massachusetts. This is an international collaborative project, with the modelling and detailed wood development work led by the University of Cambridge and the field and laboratory carbon measurement work led by Harvard University. The Harvard team will measure non-structural carbon concentrations and photosynthetic rates, and take microcores from tree trunks for wood development measurements at high temporal and spatial resolutions. These cores will be analysed, under the management of Cambridge, in a Swiss laboratory that is expert in studying cellular development in wood. These observations will enable us to determine the relationships between carbon sources and sinks over time.

In a highly innovative experiment, the Harvard team will also manipulate the supply of carbon to growing wood in our three experimental species at Harvard Forest by cooling the trees at particular points on their trunks. This cooling will be applied using cold collars, in which antifreeze will be circulated around the trunks of the experimental trees. Cooling will reduce the flow of sugars and we will conduct detailed measurements of the effects of changed carbon supply on wood development, and thus the extent to which growth is carbon limited.

At Cambridge, we will use these various measurements to develop a computational model of tree growth, which will be incorporated into a global model of the terrestrial carbon cycle. This model will then be used to assess the consequences of sink-limited growth for historical and future global land carbon uptake. This work has the potential to revolutionise our understanding of the role of vegetation in the global carbon cycle, the impacts of environmental change on plants, our interpretations of past climates as recorded in tree growth rings, and, because of the effect plants have on atmospheric CO2, our predictions of future climate change.

Who might benefit from this research?

Potential beneficiaries of this research include:

1) Policy-makers and politicians
2) Members of the general public
3) Members of groups traditionally under-represented in science

How might they benefit from this research?

1) Policy-makers will benefit from our project because the results will likely affect the outcome of climate change projections, providing greater scientific certainty for political negotiations and policies to mitigate and adapt to climatic changes. Our work will engage with policy makers through our links to the IPCC process via the ISI-MIP ("Inter-Sectoral Impact Model Intercomparison Project") and TRENDY ("Trends in net land atmosphere carbon exchanges") international collaborations. Our modelling within ISI-MIP has already been used in IPCC reports, and we intend for this to be reinforced in the coming years. Our HYBRID simulations have been used to examine the consequences of different levels of future climate change. Building on this work we are now, for example, using HYBRID to contribute to the COP-21 Agreement request for the IPCC to investigate the impacts of warming of 1.5 degC above pre-industrial levels through collaboration with colleagues at PIK. Our continued strong involvement with ISI-MIP and PIK will ensure that the outcomes of this project will feed through to policy makers.

2) Members of the general public will benefit from our project through our efforts to communicate our science and we will engage in a two-way dialogue with them. Through Science Pub Nights, print and video media, and a project web page, we will have the opportunity to share our stories, explain our fascination and passion for research, and transfer knowledge and understanding about fundamental topics such as "how do trees grow?" and "what does climate change mean for the forests of the future?"

3) Groups traditionally under-represented in science will benefit from our participation in the Harvard Forest Summer Research Program in Ecology, and our mentoring a total of six interns (two per year) in this program. The Summer Research Program has a phenomenal track record of attracting students from geographically, ethnically, and culturally diverse institutions, and cultivating the next generation of ecological scientists and educators. These interns will learn valuable career skills, will engage in hands-on participation in cutting edge research, and will be given responsibility for overseeing specific project tasks and objectives. The Harvard PDRA who mentors the interns will develop important skills related to supervision and personnel management.