EFIMED PhD Award to the best PhD on Mediterranean forests
This was the first EFIMED Award, addressed to excel the work of young scientists which have contributed to the advance of the Mediterranean forestry sciences. Nine candidates who defended their thesis in September 2008 - August 2009 applied. The selection was based on the priorities established in the Mediterranean Forest Research Agenda, the innovative character and originality of the topic, the excellence of the related scientific publications and the significance of the contribution to the advance of sciences.
Mr. Keenan hols a Master degree in Mathematics in the Living Environment from the University of York. He has developed his doctoral work at the Center for Ecological Research and Forest Applications (CREAF) and from his research seven scientific articles have been published in renown journals. He is working nowadays at Harvard University as postdoctoral fellow.
The effect of changing climate on Mediterranean forests has already been observed and is expected to continue and perhaps accelerate. Increased temperature and changes in precipitation patterns may affect the capacity of Mediterranean forests to sequester carbon, for example through increased instances of drought, thereby providing a positive feed-back to the climate system. The processes which control the exchange of carbon and water between forests and the atmosphere during drought are, as yet, poorly understood, and terrestrial vegetation models consistently perform worse in drought stressed environments. Another important component of Mediterranean plant-atmosphere interactions comes in the form of temperature sensitive emissions of biogenic volatile organic compounds (BVOCs), which are emitted by most Mediterranean forests. Although it is not entirely clear why plants emit BVOCs, their presence has been reported to increase plant tolerance to several environmental stresses, i.e. drought stress. BVOCs play an important role in plant-insect communication and in regional air chemistry. Thus, improving our mechanistic understanding of Mediterranean forest BVOC emissions is highly desirable.
The thesis, undertaken between October 2005 and May 2009, was designed to reduce uncertainty in our understanding of and our ability to model Mediterranean forest carbon and water fluxes. Key weak points in our current knowledge of plant physiology are challenged, from knowledge of leaf scale processes to the calculation of fluxes on regional scales, both in the present and under potential future climate change. The thesis focuses on two key issues of uncertainty:
1) Understanding of Mediterranean forest canopy level responses of photosynthesis and transpiration in relation to natural drought cycles
Eddy-covariance (EC) data at six Mediterranean forest sites is used to estimate soil water content, and canopy level stomatal conductance. A new ‘scale-down embedded in scale-up’ method for estimating mesophyll conductance from EC data is proposed. Quantitative limitation analysis techniques then allow for the separation of the limitations to forest productivity imposed by changes in stomatal and mesophyll conductance during drought, shedding new light on the controlling processes behind Mediterranean forest responses to drought stress on the canopy scale. Furthermore, two vegetation models (ORCHIDEE and GOTILWA+) are parameterised and the accuracy of carbon and water fluxes from the current state of the art of terrestrial vegetation models is assessed.
2) Understanding and quantification of BVOC emissions from forest canopies.
Four different BVOC emissions models are coupled to a process-based forest model and the simulated emissions are analysed using field data, focusing on model predictions of seasonal cycles in emissions in the Mediterranean region, and model temperature and light responses. A new database of species specific emissions potentials is compiled and used to build a process based model inventory of BVOC emissions. The predictions from various modelling approaches, ranging from purely empirical to more process-based, are compared both under present conditions and under projected future climate change. The impact of seasonal changes in emissions is assessed, scaling effects from the leaf to the landscape, and a new mechanistic approach to estimating seasonal changes in emissions is proposed, based on the isoprenoid synthase enzyme.
Using a total of 21 years of continuous half-hour EC observations from six sites, results show that mesophyll conductance plays a strong role in the regulation of forest productivity during drought stress. Contrary to commonly held beliefs on modelling stomatal control of photosynthesis, artificially reducing only stomatal conductance in the Ball-Berry coupled conductance-assimilation model was shown to be insufficient to reduce forest photosynthesis. Each limitation to forest productivity was tested on a canopy scale in an eco-physiological model, showing that, with (and only with) the inclusion of mesophyll conductance, diffusive limitations can explain carbon and water flux responses to seasonal changes in soil water availability. This is the first time mesophyll conductance has been calculated on the canopy scale, and helps close the gap between studies (both model and measurement based) which report dominant roles of biological changes under water stressed conditions and those which maintain stomatal conductance limitations as the main actor in regulating forest productivity during water stress.
The BVOC emissions models agreed well on emissions from present day European forests, with average model estimates of 1.03 TgC a-1 for isoprene emission and 0.93 TgC a-1 for monoterpenes, giving a consistent emission inventory for BVOCs from European forests. Simulated estimates of future emissions of isoprenoids showed large model-dependent differences, reflecting differences in model temperature responses. This highlights the fact that we are in the early stages of the path towards a full understanding of the processes governing BVOC emissions. Many studies involving modelled future BVOC emissions may need to be revised to take into account the inherent variability introduced by the choice of the emission model used, and overestimations of emissions due to inadequately incorporated seasonality. The proposed mechanistic approach to modelling enzyme driven seasonal changes in emissions potential was shown to be applicable to any of the current models, and was responsible for a large reduction in the differences between model estimates.
This thesis contributes to improve our knowledge of Mediterranean forest carbon and water fluxes, and our ability to model them. Current uncertainty regarding the response of coupled photosynthesis-conductance models to drought conditions was shown to be due to the assumption of the sole role of stomatal conductance in the regulation of forest productivity. This has furthered our understandng of plant responses to water stress, and our ability to model them. Uncertainty in estimates of carbon fluxes through the emissions of BVOCs has been greatly reduced through the constraining of seasonal changes in emissions potentials in the Mediterranean.
Contact: tkeenan(at)oeb.harvard.edu and http://www.people.fas.harvard.edu/~tkeenan/
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