dimanche 18 novembre 2012
popularité : 60%

Intensified drought is of a special concern in the Mediterranean region because scarce precipitation is already a threat to ecosystems. The response of Mediterranean vegetation to extreme water deficit conditions remains poorly understood because fully-controlled field experimentation to mimic such conditions is difficult and costly.

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A significant aspect of plant response to changing drought conditions is the alteration of the relation between their primary and secondary metabolisms. On the one hand, increased drought stress is expected to trigger loss of primary productivity and greater sensitivity to pathogens or disturbances. On the other hand, many species substantially develop their defence mechanisms such as their plant secondary metabolism (PSMs) under intense stress conditions. Both factors together determine the functioning of the ecosystem under the conditions expected later during the current century.

SEC-PRIME² aims to use a combined experimental and modelling strategy to investigate drought impacts on the functioning of a Mediterranean forest ecosystem. Its originality is to focus on plant responses to rainfall restriction through the estimation of both plant primary metabolism (PPM, e.g. plant productivity) and PSMs, as well as the trade-offs between them.PSMs may act as supplementary defences against abiotic factors (high temperature, high radiation levels, episodes of pollution and drought), which result in cell damage caused by cell oxidation, strongly scavenged (neutralized) by PSMs. In order to cope with drought episodes, plants may consequently allocate their resources to PSMs production rather than PPM and thereby increase their survival likelihood in the ecosystem. As a result, modifications of carbon allocation in response to environmental constraints could impact the carbon cycle of the entire ecosystem. Moreover, PSMs are recalcitrant for microorganisms and, consequently, an increase of litter PSMs concentration could decrease litter mineralization rates. Thus, input of carbon into the soil, immobilization of nitrogen and modified heterotrophic respiration would all impact the carbon exchange with the atmosphere and the amount of carbon stored in the ecosystem.

SEC-PRIME² is structured in three main tasks. Task 1 will study the “Benefits and drawbacks from forest investment on PSMs under intensified drought at the tree level”. Additional investment in PSMs could result in increased defences for the plant with detrimental consequences for forest productivity. At the ecosystem level, task 2 will evaluate whether such secondary metabolites, 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 nutrient availability for plants with the consequent feedback on PSMs and PPMs at the tree level. Finally, task 3 will use a generic ecosystem model (ORCHIDEE) to estimate the impact of plant investment in PSMs on the carbon budget, which will permit regional and longer-term scenario assessments of ecosystem structure and integrity.

The main hypothesis of SEC-PRIME² is that a drier climate could lead to a shift from primary to secondary metabolism in leaves. This would result in higher loads of PSMs in litter, hampering resource uptake from the soil, with subsequent feedback on the balance between primary and secondary metabolism. Moreover, higher loads of PSMs in litter could induce a decrease of the litter mineralization rate as PSMs are considered as more recalcitrant material. The altered litter composition could modify the input of carbon into the soil impacting the carbon exchanges with the atmosphere and the carbon budget.

The chosen model system is a Downy Oak (Quercus pubescens Willd.) forest in Southern France, which is assumed to be particularly responsive to climate change. Over a period of several years, we will experimentally reduce rainfall under otherwise natural conditions at the “Oak Observatory of the Observatoire de Haute Provence” (O3HP) through a dynamic rain exclusion system. This type of study is essential to understand the mechanisms behind potential shifts in species composition and survival rate, and to develop quantitative tools to characterise the vulnerability of Q. pubescens and potentially other forest ecosystems at the local and regional scale.


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