S. Lafont

Importance of vegetation feedbacks in doubled-CO2 climate experiments

The rising atmospheric concentration of carbon dioxide resulting from the burning of fossil fuels and deforestation is likely to provoke significant climate perturbations, while having far-reaching consequences for the terrestrial biosphere. Some plants could maintain the same intake of CO2 for photosynthesis by reducing their stomatal openings, thus limiting the transpiration and providing a positive feedback to the projected surface warming. Other plants could benefit from the higher CO2 level and the warmer climate to increase their productivity, which would on the contrary promote the transpiration. The relevance of these feedbacks has been investigated with the Meteo-France atmospheric general circulation model. The model has been run at the T31 spectral truncation with 19 vertical levels and is forced with sea surface temperature and sea ice anomalies provided by a transient simulation performed with the Hadley Centre coupled ocean-atmosphere model. Besides a reference doubled-CO2 experiment with no modification of the vegetation properties, two other experiments have been performed to explore the impact of changes in the physiology (stomatal resistance) and structure (leaf area index) of plants. Globally and annually averaged, the radiative impact of the CO2 doubling leads to a 2°C surface warming and a 6% precipitation increase, in keeping with previous similar experiments. The vegetation feedbacks do not greatly modify the model response on the global scale. The increase in stomatal resistance does not systematically lead to higher near-surface temperatures due to changes in the soil wetness annual cycle and the atmospheric circulation. However, both physiological and structural vegetation feedbacks are evident on the regional scale. They are liable to modify the CO2 impact on the hydrological cycle, as illustrated for the case of the European summertime climate and the Asian summer monsoon. The strong sensitivity of the climate in these areas emphasizes the large uncertainties of climate change predictions for some of the most populated regions of the world and argues for the need to include more interactive land surface processes in current generation climate models.

Impact of doubled CO2 on global‐scale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses

[1] Current increase in atmospheric CO2 is expected to modify both climate and plant function, thereby impacting plant structure and gas exchange. We investigate the effects of doubled CO2 on leaf area index (LAI) and evapotranspiration (ETR) using a global vegetation model for present-day and doubled-CO2 conditions. The model assumes that adaptation of plants to the local climate leads to an equilibrium LAI, which depends on resource availability (minimizing water stress, canopy carbon cost and self-shading). The model compares reasonably well with remote sensing estimates of LAI. Four doubled-CO2 simulations are designed to investigate the role of climate, CO2-induced stomatal closure, enhanced photosynthesis, and a combination of these effects. These simulations show that plant physiological responses to doubled CO2 are potentially more important than climate changes for LAI, and equally important for ETR. In addition, even the sign of the simulated changes in LAI and ETR varies with the assumptions on plant responsiveness to CO2. A reduction of stomatal conductance moderates or cancels the water losses caused by a warmer and drier climate, but photosynthesis stimulation counteracts this stomatal effect, especially in the mid-to-high latitudes, because of enhanced LAI. Experimental evidence of LAI and ETR response to CO2 has been reviewed and compared to the different simulations. On the basis of this confrontation we argue that plant response to CO2 doubling may have a relatively small net impact on global ETR and may cause a moderate increase of LAI. Tree stomata may be less responsive to CO2 than was previously assumed, and stimulated plant growth partly cancels the water savings caused by stomatal closure. Understanding the responses of plant canopies to CO2 is therefore critical for land surface hydrology in a CO2 rich world. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1851 Hydrology: Plant ecology; 1655 Global Change: Water cycles (1836); 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: LAI, CO2, stomatal conductance, global, evapotranspiration Citation: Kergoat, L., S. Lafont, H. Douville, B. Berthelot, G. Dedieu, S. Planton, and J.-F. Royer, Impact of doubled CO2 on globalscale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses, J. Geophys. Res., 107(D24), 4808, doi:10.1029/2001JD001245, 2002.

Nitrogen controls plant canopy light‐use efficiency in temperate and boreal ecosystems

Optimum daily light-use efficiency (LUE) and normalized canopy photosynthesis (GEE*) rate, a proxy for LUE, have been derived from eddy covariance CO2 flux measurements obtained at a range of sites located in the mid to high latitudes. These two variables were analyzed with respect to environmental conditions, plant functional types (PFT) and leaf nitrogen concentration, in an attempt to characterize their variability and their potential drivers. LUE averaged 0.0182 mol/mol with a coefficient of variation of 37% (42% for GEE*). Foliar nitrogen N of the dominant plant species was found to explain 71% of LUE (n = 26) and 62% of GEE* (n = 44) variance, across all PFTs and sites. Mean Annual Temperature, MAT, explained 27% of LUE variance, and the two factors (MAT and N) combined in a simple linear model explain 80% of LUE and 76% GEE* variance. These results showed that plant canopies in the temperate, boreal and arctic zones fit into a general scheme closely related to the one, which had been established for plant leaves worldwide. The N-MAT- LUE relationships offer perspectives for LUE-based models of terrestrial photosynthesis based on remote sensing. On a continental scale, the decrease of LUE from the temperate to the arctic zone found in the data derived from flux measurements is not in line with LUE resulting from inversion of atmospheric CO2. (Less)

Incoming Solar and Infrared Radiation Derived from METEOSAT: Impact on the Modeled Land Water and Energy Budget over France

AbstractThe Land Surface Analysis Satellite Applications Facility (LSA SAF) project radiation fluxes, derived from the Meteosat Second Generation (MSG) geostationary satellite, were used in the Interactions between Soil, Biosphere, and Atmosphere (ISBA) land surface model (LSM), which is a component of the Surface Externalisee (SURFEX) modeling platform. The Systeme d’Analyze Fournissant des Renseignements Atmospheriques a la Neige (SAFRAN) atmospheric analysis provides high-resolution atmospheric variables used to drive LSMs over France. The impact of using the incoming solar and infrared radiation fluxes [downwelling surface shortwave (DSSF) and longwave (DSLF), respectively] from either SAFRAN or LSA SAF, in ISBA, was investigated over France for 2006. In situ observations from the Flux Network (FLUXNET) were used for the verification. Daily differences between SAFRAN and LSA SAF radiation fluxes averaged over the whole year 2006 were 3.75 and 2.61 W m−2 for DSSF and DSLF, respectively, representing 2....

A vegetation radiative transfer scheme in the ISBA-A-GS interactive vegetation model

Vegetation surfaces play an important role in the Earths energy balance and have a significant impact on the global carbon cycle. The fraction of solar radiation that is absorbed by a vegetation canopy will assess the rate of photosynthesis and besides the amount of carbon flux further fixed or released by this same canopy layer. The radiative transfer scheme within the canopy of ISBA-Ags interactive vegetation model was defined in 1998 by Calvet et al [2] according to a self-shading approach. The incident fluxes at the top of the canopy go through a multilayer vegetation cover. Then, the attenuated flux in the PAR wavelength domain of each layer is used by the Jacobs Model [3] to evaluate the net assimilation (An) of the leaf. Net assimilation of the leaf estimated for each layer is mixed together to derive the average An quantity of the total vegetation cover. A detailed description of the vegetation radiative transfer scheme within the canopy is given in Appendix B of Calvet et al. [2]. The objective of the present study is the improvement of the current scheme and its evaluation. The impact of the new absorbed radiation on the photosynthesis module is considered, in order to obtain better simulations of the Leaf Area Index (LAI) and Gross Primary Production (GPP). These evaluations sustain the value added of an advanced representation of the radiative transfer within the canopy in order to estimate the photosynthesis.

Land surface albedo and downwelling shortwave radiation from MSG geostationary satellite: Method for retrieval, validation, and application

The European Meteorological Satellite Organization (EUMETSAT) maintains a number of decentralized processing centers dedicated to different scientific themes. The Portuguese Meteorological Institute hosts the Satellite Application Facility on Land Surface Analysis (LSA-SAF). The primary objective of the LSA-SAF is to provide added-value products for the meteorological and environmental science communities with main applications in the fields of climate modeling, environmental management, natural hazards management, and climate change detection. Since 2005 data from Meteosat Second Generation satellite are routinely processed in near real time by the LSA-SAF operational system in Lisbon. Presently, the delivered operational products comprise land surface albedo and temperature, shortwave and long-wave downwelling radiation fluxes, vegetation parameters and snow cover. After more than ten years (1999–2011) of research, development, and progressive operational activities, a summary of characteristics and performances of albedo and downwelling shortwave (DSSF) products is presented (Sections 2 and 3). The relevance of LSA-SAF albedo product is analyzed through a weather forecast model (ALADIN) in order to account for the inter-annual spatial and temporal variability (Section 4). The added value brought by the use of LSA-SAF shortwave and long-wave products is also diagnosed through SURFEX Land Surface Models (LSM) simulations with the surface temperature, the water content and the energy fluxes (Section 4). Perspectives of the LSA-SAF project are finally stressed (Section 5–6).