TASK 3 : Synthesis

samedi 18 janvier 2014
popularité : 71%

Task leader : C. Fernandez - Participants : V. Baldy, W. Cramer, J. Lathière, B. Guenet, E. Ormeño
Objectives : The overall objective is to integrate the experimental findings on PPMs/PSMs dynamics under water stress in a regional sensitivity study of oak forest functioning under conditions of climate change. For this purpose, we first to define a set of suitable parameters for the ORCHIDEE model corresponding to the effect of a modification on drought frequency on plant and ecosystem functioning. These parameters will then be used to perform different simulations at ecosystem and regional scales to estimate the impact of changes in plant allocation due to a modification of drought frequency on the carbon budget.
Hypothesis : We assume that an increase of the PSMs load in leaves could induce an increase in the PSMs load of litter. PSMs are recalcitrant and consequently litter mineralization rate would decrease decreasing the heterotrophic respiration and the incorporation of carbon into the soil. As a consequence, more carbon would be stored in the litter. Furthermore, the input of carbon into the soil would also be more recalcitrant leading to decrease the mineralization rate of the soil organic matter. Consequently, more carbon would also be stored in the soil. Finally, soil moisture is studied as the major driver of mineralization. The increase of drought frequency would lead to decrease the soil moisture and therefore decrease the mineralization.

Subtask 3-1 : Define a parameter set based on the data obtained in the task 1 and 2 suitable for the ORCHIDEE model
To improve the representation of the PSMs and their impact on the carbon budget in the model several parameters must be defined and incorporated into the model. To allow a validation only half of the plot will be used to define parameters, the other will be used for validation.
Using the data obtained during the subtask 1-1, we will modify the proportion of dead leaves distributed in the structural litter pool depending on the leaf age and on the droughts frequency.
Using the data obtained during the subtask 2-1 we could derive the mineralization rate of the aboveground litter modulated by temperature and moisture. Then, thanks the PSMs load measured during the sub task 1-1 we could estimate the fraction of metabolic and structural litter and thus the mineralization rate of each pools.

Subtask 3-2 : Use the new parameters to estimate the impact of the changes in plant allocation due to drought frequency increase on the carbon budget at ecosystem and regional scales
The ORCHIDEE model defines the plant taxa through 12 plant functional types according to systematic, physiological, phenological, climate conditions. In the Mediterranean region mainly three of them are present : the C3 herbaceous (crop or grass), the temperate needle-leaf evergreen and temperate broad-leaved summergreen. Q. pubescens corresponds to the last one. Using the parameters defined during the subtask 3-1, we propose to perform different simulations.
Once the model validated, we could perform sensitivity analysis of the model to the presence of the new parameters over the O3HP site and estimate the role and the influence of the PSMs in the carbon budget at the ecosystem scale.
We also propose to perform some predictive simulations under different scenarios de-fined by the IPCC. In particular, we could estimate the impact of different drought frequencies on the Mediterranean Oak forest with a particular focus on the carbon budget and the exchanges between land surface and the atmosphere.
Finally, using other parameters obtained in the O3HP and in particular for P. halepensis we could perform the same analysis with other ecosystems but also over the entire Mediterranean region.


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