From branch to forest to globe: How do trees choices regarding growth affect forest responses to increased carbon dioxide levels?

Forests of the future will be subjected to levels of carbon dioxide for which there is no analogue in the current world. There is compelling evidence extra carbon dioxide (CO2) will cause trees to photosynthesise more and grow larger, storing more carbon, and helping to reduce the rate of carbon accumulation in the atmosphere. However, recent experiments have indicated that whilst trees may take up more carbon through photosynthesis under high CO2 conditions, the strategies by which they invest this additional carbon in enhanced growth are complex and variable. An important knowledge gap is how wood growth is affected by elevated CO2. For increased photosynthesis to translate into larger carbon sinks, sequestration in long-lived woody carbon pools such as the stems of growing trees is key. However, woody carbon is much more than just stems, also including branches, twigs and bark, both living and dead. The rate at which carbon is channelled into each of these woody pools, and lost from them through processes such as mortality, herbivory and decomposition, is poorly understood. The governing processes may be affected directly or indirectly by rising CO2, and by environmental extremes such as droughts. For instance, plant material produced under elevated CO2 levels often differ in the chemical composition, reflecting physiological responses of the plants, with cascading effect on wood function when alive and decomposition rates after death.
This project will open the ‘black box’ of the woody carbon pool of mature forest ecosystems to disentangle the different woody compartments, key biological processes governing their formation and loss, and the responses of those processes to environmental drivers, particularly atmospheric CO2, water availability and soil nutrient status. Two ground-breaking experimental forests in contrasting environments of Australia and the United Kingdom will be compared. These experiments subject mature and intact forest stands to the levels of carbon dioxide expected in the atmosphere in the year 2050. The student will carry out measurements and analysis at both locations with a view to developing new understanding that will inform parameterisations of forest function in computer models used to project future forest carbon uptake.