Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites : revision of current hypotheses?

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests - one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) - and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70-80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi) arid ecosystems' carbon balances. (Résumé d'auteur

Inverse modeling of seasonal drought effects on canopy CO2/H2O exchange in three Mediterranenan ecosystems

We present a two-criteria inverse modeling approach to analyze the effects of seasonal drought on ecosystem gas exchange at three Mediterranean sites. The three sites include two nearly monospecific Quercus ilex L. forests, one on karstic limestone (Puéchabon), the other on fluvial sand with access to groundwater (Castelporziano), and a typical multispecies shrubland on limestone (Arca di Noè). A canopy gas exchange model Process Pixel Net Ecosystem Exchange (PROXELNEE), which contains the Farquhar photosynthesis model coupled to stomatal conductance via the Ball-Berry model, was inverted in order to estimate the seasonal time course of canopy parameters from hourly values of ecosystem gross carbon uptake and transpiration. It was shown that an inverse estimation of leaf-level parameters was impossible when optimizing against ecosystem H2O or CO2 fluxes alone (unidentifiable parameters). In contrast, a criterion that constrained the optimization against both H2O and CO2 fluxes yielded stable estimates of leaf-level parameters. Two separate model inversions were implemented to test two alternative hypotheses about the response to drought: a reduction in active leaf area as a result of patchy stomatal closure or a change in photosynthetic capacities. In contrast to a previously tested hypothesis of classical (uniform) stomatal control, both hypotheses were equally able to describe the seasonality of carbon uptake and transpiration on all three sites, with a decline during the drought and recovery after autumn rainfall. Large reductions of up to 80%, in either active leaf area or photosynthetic capacities, were necessary to describe the observed carbon and water fluxes at the end of the drought period. With a threshold-type relationship, soil water content was an excellent predictor of these changes. With the drought-dependent parameter changes included, the canopy model explains 80-90% of the variance of hourly gross CO2 uptake (root mean squared error (RMSE): 1.1-2.6 pmol m-2 s-1) and 70-80% of the variance of hourly transpiration (RMSE: 0.02-0.03 mm h-1) at all sites. In addition to drought effects, changes in leaf photosynthetic activity not related to water availability, i.e., high spring activity, were detected through the inverse modeling approach. Moreover, our study exemplifies a kind of multiconstraint inverse modeling that can be profitably used for calibrating ecosystem models that are meant for global applications with ecosystem flux data

Emission of Reactive Terpene Compounds from Orange Orchards and their Removal by within-Canopy Processes.

VOC emission from an orange field was measured in the frame of two field campaign carried out in Burriana (Spain) during the summer and spring season. The potential emission fluxes were scaled from enclosing the major emission sources, i.e. branches and soils of different orange varieties present in the fetch seen by micrometeorological measurements. Concurrentlyl the real fluxes of biogenicc VOC above the orange orchard were determined by relaxed eddy accumulation. A comparisonbetween emission rates and fluxes indicate that substantial removal of b-caryophyllene and linalool occurred during transport of orange emission from the canopy into the atmospheric boundary layer. While within-canopy removal of the sesquiterpene component is fully consistent with laboratory studies indicating high reactivity toward OH radicals and ozone, linalool losses are more difficult to explain. Although high fluxes of acetone suggest linalool decomposition by gas-phase reactivity, removal by heterogeneous chemistry and/or deposition cannot be ruled out.JRC.(EI)-Environment Institut

Cimicifuga racemosa (Actaea racemosa) (données phytochimiques, études pharmacologiques, activités biologiques et principales applications thérapeutiques)

LYON1-BU Santé (693882101) / SudocSudocFranceF