Luxembourg Institute of Science and Technology, Luxembourg
Plants use solar energy to capture carbon dioxide (CO2) from the atmosphere and convert it into sugar, while releasing oxygen and vast amounts of water to the atmosphere. In other words, vegetation produces food to eat and oxygen to breathe for us, but it also consumes the same water we would like to drink. Therefore, a thorough understanding of how vegetation interacts with the environment and an ability to predict how it will change in response to climate or land use change is critical for our survival in the Anthropocene, where human activities have global environmental impacts. Given that climate change is exposing ecosystems to conditions never experienced before in our records, extrapolation of empirical models is not likely to lead to reliable predictions. Here I will present some predictive capabilities of the Vegetation Optimality Model (VOM), which simulates self-optimizing vegetation assuming that different plant organs adjust in a way to maximise the plants’ long-term net carbon profit (NCP), i.e. the amount of CO2 taken up by photosynthesis minus the carbon expended on the maintenance and turnover of plant organs involved in the process. The VOM simulates vegetation responses to environmental drivers at various time scales, from minutes (stomatal conductance) to seasons (foliage and root properties and distribution), decades and centuries (vegetation cover, rooting depth, water use strategies). Without any calibration, the VOM was able to reproduce temporal and spatial patterns in water and CO2 fluxes and vegetation properties along an aridity gradient in Australia. Key uncertainties are related to the trade-off between carbon investment into foliage and returns in terms of light capture for photosynthesis, and the feedback between carbon investment into root and water transport tissues, water uptake and soil moisture depletion. Key questions to address quantitatively in order to improve the VOM and likely other optimality-based models are: 1. What is the trade-off between canopy CO2 uptake and water loss under given atmospheric conditions? 2. How much carbon do the plants need to invest into their root system, as well as water transport and storage tissues in order to achieve a certain water and nutrient supply for the canopy? 3. How quickly can root systems respond to changing conditions? 4. What are the trade-offs between carbon investments into foliage, stems and roots and returns in terms of carbon uptake by photosynthesis?
Targeted experiments to address theses questions and first insights from these experiments will be discussed.
