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

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Fossil fuel burning is causing atmospheric concentrations of carbon dioxide (CO2) to rise. 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. A plausible explanation is that the land uptake is occurring because higher levels of CO2 increase plant photosynthesis, meaning more plants have carbon available to support growth. However, a competing explanation proposes that tree growth is not limited by the rate of photosynthesis itself, but is instead controlled by other factors such as rates of cell division, nutrients, or water supply. The extent to which trees are carbon-limited, and under which circumstances, is fundamental to understanding how they will respond to rising levels of atmospheric CO2. This project will improve our understanding of this timely issue. Each week during the growing season, the researchers will quantify rates of wood growth and cellular development at the microscopic level. Samples will be collected from the dominant tree species at Harvard Forest, Massachusetts. This will be done for three years, to examine how year-to-year variation in weather influences the timing and amount of wood growth, and how this differs among species. The researchers will also conduct a novel experiment to manipulate the flow of carbon within the tree, and thus investigate the relationships between local carbon supply and wood growth. This project will thus directly address the question of whether or not tree growth is carbon-limited. The research has the potential to revolutionize our understanding of the role of vegetation in the global carbon cycle, the impacts of environmental change on plants, and our interpretations of past climates as recorded in tree growth rings. This project will support the advanced research training of a postdoctoral scholar, and involves a collaboration with UK scientists.Using field observations, experiments, and modeling, this project will investigate the potential for tree growth to respond to increasing atmospheric CO2. Research will focus on the contentious issue of whether tree growth is controlled by the supply of carbon (i.e. from photosynthesis and/or non-structural carbon stores in wood, hence "source-limited") or the growth processes themselves (i.e. "sink-limited"). The researchers will tease apart the controls on wood growth at the whole-tree scale for the first time through careful measurements of the internal dynamics of carbon within mature trees of three widely-distributed temperate species (i.e. white pine, red oak, and red maple) growing in Harvard Forest, Massachusetts. In an observational study, the researchers will measure non-structural carbon storage dynamics and details of wood growth at the cellular level. In an experimental study, the researchers will manipulate the transport of new photosynthetic products to growing regions using cold-block phloem chilling collars, and will thus be able to draw novel inferences concerning key relationships between wood growth and carbon supply. Subsequently, a model developed using the collected data and derived knowledge will be used to examine the implications of these results for the historical and future terrestrial carbon sink.