Christophe François

Climate change impacts on tree ranges: model intercomparison facilitates understanding and quantification of uncertainty

Model-based projections of shifts in tree species range due to climate change are becoming an important decision support tool for forest management. However, poorly evaluated sources of uncertainty require more scrutiny before relying heavily on models for decision-making. We evaluated uncertainty arising from differences in model formulations of tree response to climate change based on a rigorous intercomparison of projections of tree distributions in France. We compared eight models ranging from niche-based to process-based models. On average, models project large range contractions of temperate tree species in lowlands due to climate change. There was substantial disagreement between models for temperate broadleaf deciduous tree species, but differences in the capacity of models to account for rising CO(2) impacts explained much of the disagreement. There was good quantitative agreement among models concerning the range contractions for Scots pine. For the dominant Mediterranean tree species, Holm oak, all models foresee substantial range expansion.

Analytical parameterization of canopy directional emissivity and directional radiance in the thermal infrared. Application on the retrieval of soil and foliage temperatures using two directional measurements

Abstract This work is aimed at deriving canopy component (soil and foliage) temperatures from remote sensing measurements. A simulation study above sparse, partial and dense vegetation canopies has been performed to improve the knowledge of the behaviour of the composite radiative temperature and emissivity. Canopy structural parameters have been introduced in the analytical parameterization of the directional canopy emissivity and directional canopy radiance:namely, the leaf area index (LAI), directional gap fraction and angular cavity effect coefficient. The parameterization has been physically defined allowing its extension to a wide range of Leaf Inclination Distribution Functions (LIDF). When single values are used as leaves and soil temperatures, they prove to be retrieved with insignificant errors from two directional measurements of the canopy radiance (namely at 0 and 55 from nadir), provided that the canopy structure parameters are known. A sensitivity study to the different parameters shows the...

Sequential Assimilation of ERS-1 SAR Data into a Coupled Land Surface-Hydrological Model Using an Extended Kalman Filter

Abstract A first attempt to sequentially assimilate European Space Agency (ESA) Remote Sensing Satellite (ERS) synthetic aperture radar (SAR) estimations of surface soil moisture in the production scheme of a lumped rainfall–runoff model has been conducted. The methodology developed is based on the use of an extended Kalman filter to assimilate the SAR retrievals in a land surface scheme (a two-layer hydrological model). This study was performed in the Orgeval agricultural river basin (104 km2), a subcatchment of the Marne River, 70 km east of Paris, France. Assimilation was tested over a 2-yr period (1996 and 1997), corresponding to 25 SAR measurements. The improvements observed in simulating flood events demonstrate the potential of sequential assimilation techniques for monitoring surface functioning models with remote sensing data. It was demonstrated that the method could correct for some errors or uncertainties in the input data (precipitation and evapotranspiration), provided that these errors are ...

Atmospheric corrections in the thermal infrared: global and water vapor dependent split-window algorithms-applications to ATSR and AVHRR data

The split-window method is an appropriate way to perform atmospheric corrections of satellite brightness temperatures in order to retrieve the surface temperature. A climatological data set of 1761 different radio soundings, the TIGR database, has been used to develop two different split-window methods. A global quadratic (QUAD) method, with global coefficients to be applied on a worldwide scale, and a water vapor dependent (WVD) algorithm. The first method includes a quadratic term in the split-window equation that roughly accounts for the water vapor amount. The other method explicitly includes the water vapor amount in each split-window coefficient. When applied to the 1761 radio soundings, the latter method gives better results than the global one, especially when the surface emissivity is far from unity (0.95 or less) and when the water vapor reaches great values. Both algorithms have been tested on ATSR/ERSI and AVHRR/NOAA data over sea pixels. The QUAD algorithm gives correct results for simulations (the standard error is 0.2 K) and experimental data (the bias ranges from -0.1 to 0.4 K). The WVD algorithm appears to be more accurate for both simulations (the standard error is less than 0.1 K) and AVHRR experimental data when climatological water vapor contents are used (the bias ranges from -0.2 to 0.1 K).

The potential of directional radiometric temperatures for monitoring soil and leaf temperature and soil moisture status

Issues related to thermal infrared radiative transfer modelisation and retrieval of component temperatures from directional remote sensing measurements are addressed in this paper. We compare five models and assess their accuracy in direct mode (directional radiative temperature simulations) and inverse mode (soil and leaf temperature retrievals), for a large set of simulations. We write all models using a single pattern, and we propose a unique inversion scheme based on this pattern. In this context, some precisions are given regarding previous parameterizations and results. Two new efficient and simple models appear to successfully compare to the reference model. The relation between radiative temperature and soil moisture is also examined using a coupled process-radiative transfer model and it is shown that directional measurements dramatically improve the soil moisture detection capability in the thermal infrared.

Definition of a radiosounding database for sea surface brightness temperature simulations: Application to sea surface temperature retrieval algorithm determination

Abstract The Ocean and Sea Ice Satellite Application Facility (O&SI SAF) project includes the development of Sea Surface Temperature (SST) retrieval algorithms for infrared radiometers. The algorithm definition stage presented in this paper includes three steps: building of representative simulated brightness temperature databases, definition and analysis of formalisms on simulated brightness temperatures, and tests of these formalisms on a Matchup Database (MDB). A cloud-free, latitude equally distributed simulation database has been built to determine SST algorithms, which were further checked on a NOAA-14 latitude equally distributed validation database build from the Pathfinder MDBs. The tests showed that the presence of clouds in the training database increases the bias, while an underrepresentation of low-latitude profiles in the training database increases the standard deviation. The use of a cloud-free, latitude equally distributed training database reduced both the standard deviation in the algorithms and the bias consistently over the whole range of latitudes. The effect of the instrumental noise has been assessed and algorithms using noisy simulated data have been tested: they appear much more useful at daytime than at nighttime. Indications suggest that such noise algorithms should be more adaptive in atypical situations and that they should perform better than noise-free algorithms in the presence of cloud contamination or aerosols. The systematic procedure allowed us to test a large number of different algorithms. We determined that the best possible theoretical accuracy that may be obtained with NOAA-14 derived SSTs against buoy measurements on a global scale is 0.4 K. The successive steps that were performed in this study (cloud-free training database, algorithm determination, and noise scheme) allowed us to obtain retrievals with a 0.57 K standard deviation at both daytime and nighttime. These performances appears to be comparable or better than those obtained with algorithms directly derived by regression against in situ measurements. This proves that simulation-based SST retrievals are feasible and accurate when sufficient care is taken in the constitution of the training database. The effect of maritime aerosols has also been assessed and was found to be generally of little consequences for SST retrievals: a positive bias found to be lower than 0.08 K and no effect on the standard deviation. This bias is overestimated when maritime aerosols are used in the training of the algorithms using radiative transfer models. Current SST products (North Atlantic Regional and high latitudes) based on NOAA-14 and NOAA-16 data and routine validations against drifting buoys are available at 〈 http://www.meteorologie.eu.org/safo 〉.

Impact of carbohydrate supply on stem growth, wood and respired CO2 δ13C: assessment by experimental girdling

The present study examines the impact of the C source (reserves vs current assimilates) on tree C isotope signals and stem growth, using experimental girdling to stop the supply of C from leaves to stem. Two-year-old sessile oaks (Quercus petraea) were girdled at three different phenological periods during the leafy period: during early wood growth (Girdling Period 1), during late wood growth (Girdling Period 2) and just after growth cessation (Girdling Period 3). The measured variables included stem respiration rates, stem radial increment, delta(13)C of respired CO(2) and contents of starch and water-soluble fraction in stems (below the girdle) and leaves. Girdling stopped growth, even early in the growing season, leading to a decrease in stem CO(2) efflux (CO(2R)). Shift in substrate use from recently fixed carbohydrate to reserves (i.e., starch) induced (13)C enrichment of CO(2) respired by stem. However, change in substrate type was insufficient to explain alone all the observed CO(2R) delta(13)C variations, especially at the period corresponding to large growth rate of control trees. The below-girdle mass balance suggested that, during girdling periods, stem C was invested in metabolic pathways other than respiration and stem growth. After Girdling Period 1, the girdle healed and the effects of girdling on stem respiration were reversed. Stem growth restarted and total radial increment was similar to the control one, indicating that growth can be delayed when a stress event occurs early in the growth period. Concerning tree ring, seasonal shift in substrate use from reserves (i.e., starch) to recently fixed carbohydrate is sufficient to explain the observed (13)C depletion of tree ring during the early wood growth. However, the inter-tree intra-ring delta(13)C variability needs to be resolved in order to improve the interpretation of intra-seasonal ring signals in terms of climatic or ecophysiological information. This study highlighted, via carbohydrate availability effects, the importance of the characterization of stem metabolic pathways for a complete understanding of the delta(13)C signals.

Estimation of total atmospheric water vapor content from split-window radiance measurements

Abstract Different studies have shown that the estimation of the surface temperature from split-window algorithms can be greatly improved by a rough knowledge of the total atnwspheric water vapor. In this article, we have tested the split-window covariance variance ratio technique to estimate the total columnar water vapor content. This method has been applied over sea surfaces and over land on two databases. The first dataset contains AVHRR/NOAA9 and ATSR-IR/ERSI data coincident with radio soundings over Europe (land areas). The second one includes ATSR-IR images and water vapor content estimations derived from the microwave radiometer ATSR-MW on board ERSI over the Atlantic Ocean and the Mediterranean Sea. The results show the necessity of using two different algorithms: one for land and another for sea pixels because of the effects c f surface cini.ssivity. Then, two algorithms have been calibrated and validated. The conclusion is that the columnar water vapor content may be estinuzted from thermal infrared .split-window channels with an accuracy better than 0.5 g/cm 2 , which is, in most cases, enough to improve surface temperature retrievals with infrared radiometers.

Leaf and twig δ13C during growth in relation to biochemical composition and respired CO2

In deciduous trees, the delta(13)C values of leaves are known to diverge during growth from those of woody organs. The main purpose of this study is to determine whether the divergence in delta(13)C between leaves and current-year twigs of Fagus sylvatica (L.) is influenced by changes (i) in the relative contents of organic matter fractions and (ii) in the delta(13)C of respired CO(2). The delta(13)C values of bulk matter, extractive-free matter, lignin, holocellulose, starch, soluble sugars, water-soluble fraction and respired CO(2), as well as their relative contents in bulk matter were determined. The delta(13)C values of biochemical fractions and respired CO(2) showed very similar temporal variations for both leaves and twigs. Variations in bulk matter delta(13)C during growth were, therefore, poorly explained by changes in biochemical composition or in respiratory fractionation and were attributed to the transition from (13)C-enriched reserves (mainly starch) to (13)C-depleted new photoassimilates. The divergence between leaves and twigs was related to higher values of soluble sugar delta(13)C in twigs. However, the difference between lignin and holocellulose delta(13)C varied during growth. This phenomenon was attributed to the delay between holocellulose and lignin deposition. These results may have implications for analysis of organic matter delta(13)C in trees and forest ecosystems.

PROSPECT+SAIL: 15 Years of Use for Land Surface Characterization

The combined PROSPECT leaf optical properties model and SAIL canopy bidirectional reflectance model, i.e. PROSAIL, has been used for about fifteen years to increase our understanding of plant canopy spectral and bidirectional reflectance in the solar domain and to develop new methods of vegetation biophysical properties retrieval. It links the spectral variation of canopy reflectance with its directional variation. This link is the key to simultaneously estimate biophysical/structural canopy variables for applications in agriculture, plant physiology, and forestry at different scales. PROSPECT and SAIL are still evolving: they have undergone recent improvements both at the leaf and the plant levels and became one of the most popular radiative transfer tools in these domains due to their ease of use, their robustness, and because they have been validated by many lab/field/space experiments over the years. This paper is intended to review this subject, which has been extensively researched in optical remote sensing.