lundi 13 janvier 2014
popularité : 68%

“The” overall objective of SEC-PRIME² is to study the effects of intensified drought condi-tions on Mediterranean ecosystem functioning. Its originality is the focus on plant invest-ment in PSMs as defence molecules that help plants to survive in a stressful (and changing) environment. More specifically, we will examine the benefits, in terms of plant health, and drawbacks, in terms of increased plant primary metabolism costs (especially productivity losses), that result from investment in secondary chemical defences during intense drought conditions. Finally, we will estimate the impact of such changes of allocation on the carbon budget at ecosystem and regional scale.

Two approaches will be followed : (i) At the tree level, we will directly evaluate the consequences of plant investment in PSMs on plant health and productivity. We hypothesize that under high drought stress, plants allocate more energy to the synthesis of secondary metabolites at the expense of primary production. (ii) At the ecosystem level, we will evaluate whether such secondary metabolites, once in leaf litter, affect decomposer communities (i.e. soil microorganisms and fauna in terms of abundance and diversity) and then litter decomposition, resulting in an indirect effect on nutrients availability for plants with the consequent feedback on PSMs and PPMs at the tree level. Using a generic ecosystem model, we will estimate the impact of plant investment in PSM on the carbon budget at the ecosystem and at regional scales.

The main hypothesis of SEC-PRIME² is that a drier climate will result in a shift from pri-mary to secondary metabolism which results in higher loads of PSMs in leaves and litter. These PSMs will likely impact litter decomposition causing modified resource uptake from the soil, with subsequent feedbacks on the balance between primary and secondary metabolism (Fig. 3). We assume that a high load of PSMs in litter could decrease the litter mineralization rate leading to modify the input of carbon into the soil. Thus, heterotrophic respiration from litter and soil organic matter decomposition could be impacted by such changes in plant allocation leading to a modified carbon budget of the ecosystems and the exchanges with the atmosphere.

Conceptual diagram of the project

We will use a combined experimental and modelling strategy in order to isolate drought effects from other environmental conditions on vegetation functioning, by experimental modification of precipitation conditions. SEC-PRIME² assesses drought impacts on a Quer-cus pubescens ecosystem functioning under natural conditions through a dynamic rain ex-clusion system at the “Oak Observatory of the Observatoire de Haute Provence” (O3HP). Q. pubescens is a suitable species to monitor changes affecting Mediterranean forests. The study of its performance, and this, in comparison with the Q. ilex (Holm Oak, an evergreen sclerophyllous species), is particularly interesting in the context of the expected climate change.

With the data obtained during the project and those already available, in particular for P. halepensis, we will define new parameters sets to better reproduce drought effect on leaves and litter chemical composition as well as on the mineralization rate in the Mediterranean ecosystems by the ecosystem model ORCHIDEE (Krinner et al. 2005). ORCHIDEE has been developed to simulate the fluxes of water, energy and carbon at the land surface/atmosphere interface at regional and global scales. Mediterranean ecosystems are covered by three main PFTs. Using this new parameter set, we validate the output of the model and we will estimate the impact of a modification of the drought frequency on the carbon budget and on the carbon exchanges with the atmosphere at ecosystem scale. Assuming that the resulting parameters are representative for Mediterranean ecosystems, we will perform some simulations at regional scales to quantify the importance of the changes in plant allocation on the carbon budget at the regional level under different future climate conditions.


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