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XERIC-EDGE
Perilous life at the xeric edge: Drought-induced tree mortality in the Mediterranean
In this project, we aim to provide the necessary background knowledge to understand the causes leading to heat and drought-related large-scale tree mortality for a model tree species, Scots pine (Pinus sylvestris L.), across the Mediterranean
In the same way we aim to determine the variability in the physiological traits associated with tree mortality along gradients of water availability and tree health at three sites spanning the Mediterranean basin ; to understand the respective roles of genetic vs phenotypic variability in controlling the traits relevant for mortality in Scots pine; to assimilate the obtained information into state-of-the-art physiological models to predict the range of environmental conditions over which mortality is expected to occur.
The general questions which this proposal addresses are:
1. Which of the proximate causes of mortality (i.e., hydraulic failure, carbon starvation or phloem impairment) are the most likely to occur in the Mediterranean?
2. Are the ultimate causes of mortality caused by a lack of phenotypic plasticity or a lack of genetic adaptation? If both are involved, which one is the most important? We aim to address the following specific questions:
Q1. Do the carbon reserves and xylem and phloem transport properties a) change between periods of adequate soil water availability and periods of acute drought stress, b) differ among populations with different levels of mortality along a hydrological gradient, c) vary along a gradient of tree health (i.e., of proximity to mortality)?
Q2. Does the coupling between total non-structural carbohydrates, photosynthesis and stem/root respiration a) change between periods of adequate soil availability and periods of acute drought stress, b) differ among populations with different levels of mortality along a hydrological gradient, c) vary along a gradient of tree health (i.e., of proximity to mortality)?
Q3. Can we test the hypothesis that phloem transport limits stem/root respiration during the summer?
Q4. Does the hydraulic system of Scots pine change along regional transects because of phenotypic plasticity, because of genetic differences across populations or both?
Q5. Can we model the combination of processes leading to tree dieback and predict the range of conditions under which future large scale mortality is most likely to occur?
Conclusions
We completed the first field campaign in the Pyrenees and the data are being analysed. Initial results support both the carbon starvation and the hydraulic failure theory. A new numerical model integrating all major processes has been developed to help disentangle the interactions occurring among the major physiological processes.
