Au Sénégal, les Zules, d u n champ (13" 55 N et 15" 49 W ) ont été échantillonnés &ao& 1974 Ci ju... more Au Sénégal, les Zules, d u n champ (13" 55 N et 15" 49 W ) ont été échantillonnés &ao& 1974 Ci juillet 1975, par 22 relevés de 4 i d de surface et de 30 à 80cnz de profondeur et dans une ternzitière sur 20 c m dépaisseur en saison sèche. 6 espèces, dont une présente 2forines morphologiquement différentes, constituent 7 populations " distinctes : Graphidostreptus tumuliporus, Peridontopyge conani, P. spinosissima, Haplothysanus chapellei ( p et G) Syndosmogenus mimeuri. Leur répartition dans le champ est hétérogène. Il n'y a pas de peupleinent caractéristique autour des souches et au pied des arbres car toutes les espèces se concentrent plus ou inoins fortement dans ces biotopes. Par contre le peuplement de la couche superficielle de la termitière est constitué de quelques espèces seulement, inais très abondantes. G. tumuliporus se concentre au pied des arbres et dans les termitières mais il est rare dans le reste du champ. P. conani et les 2 formes de H. chapellei sont 2 à 3 fois plus abondants sous les arbres qu'en plein champ et les individus y sont en moyenne plus grands. Les stades âg& de ces 3 populations se concentrent en saison sèche dans les termitières oÙ leur densité est I O à 30 fois plus grande qu'en plein champ et leur bionzasse 40 6 90 fois plus grande. Par rapport aux densités en plein champ, les effectifs de P. spinosissima seraient 8 fois plus forts sous les arbres inais seulement 3 fois plus élevés autour des souches et dans les termitières. P. rubescens et S. mimeuri sont très rares dans les termitières et 3 à 4 fois plus abondants sous les arbres qu'en plein champ. En rapportant la densité des différentes populations à leur étendue dans chaque biotope on obtient une iniage globale du peuplernent du champ. P. rubescens se révélant la population dominante, compterait pour I/3 de la biomasse et de l'effectif total des Iules. S. mimeuri représeizterait le I/4 des Iules du champ inais une faible part de leur biomasse. G. tumuliporus au contraire, représenterait un effectif faible mais le 1/4 de la biomasse totale. P. conani fournirait, en poids coinine en nombre, 15 % du peuplement. Les autres populations seraient accessoires ne représentant pas I O % du peupleinent en effectif et en biomasse. Le champ nu serait le lieu où la niujarité des Iules à l'exception des G. tumuliporus, passe la saison sèche. Le pied des arbres concentrerait la plupart des individus de G. tumuliporus et une grande part de la biomasse des autres populations. Les termitières rassembleraient l'essentiel de la biomasse de G. tumuliporus et une partie de la bioinasse des 3 espèces qui colonisent aussi ce refuge. Quant aux souches avec rejets, elles ne constituent pas des biotopes particulièrement attractifs ni des refuges car si les plus jeunes stades y sont un peu plus fréquents que dans le plein champ, l'inverse est observé pour les individus de grande taille.
Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata we... more Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata were studied using near infrared reflectance spectroscopy. Compared to leaf litter, faecal pellets were significantly richer in lignin and poorer in nitrogen and readily assimilable constituents. The C:N ratio was significantly higher in the faeces. A NIRS-predicted index of litter decomposability was found to be
— The biochemical nature of leaf litter is a key factor in regulation of its decomposition.... more — The biochemical nature of leaf litter is a key factor in regulation of its decomposition. Conventional wet chemical analysis of samples is destructive, time-consuming and expensive. The objective of this study was to evaluate the potentiality of near infrared reflectance spectroscopy (NIRS) for determining litter chemistry during the decomposition process using a wide range of species and decomposition stages. The litter of 8 species of evergreen and deciduous broad-leaved trees, conifers and shrubs were used in both laboratory and field experiments. Near-infrared reflectance measurements were made with an NIRS Systems 5000 spectrophotometer over the range 1100-2500 nm. Calibration samples were analysed for ash, carbon and nitrogen. Acid-detergent fiber (ADF) and acid-detergent lignin (ADL) were determined using Van Soest procedures. Stepwise regression (SR) calibrations and partial least squares (PLSR) calibrations were developed and compared as well as the effect of scatter correction. The PLS algorithm was used to create the predictive models using all the information in the spectrum to determine the chemical concentration. Using scatter correction always gave better results. Both regression methods provided acceptable validation statistics for C, N and ash. The PLSR had better prediction accuracy for ADF and ADL. For these two constituents, the improvement of SECV was 34 and 25% respectively. Our results showed that NIRS is an effective tool to predict nitrogen, ash and proximate carbon fractions in decomposition studies and that PSLR method improves calibration compared with SR method. decomposition / leaf chemistry / litter / NIRS Résumé — Utilisation de la spectroscopie proche infrarouge dans les études de décomposition de litières. La composition biochimique des litières est un des facteurs clés de la régulation de leur décomposition. Les méthodes d'analyse chimique par voie humide sont destructives, longues et coûteuses et ces contraintes sont rapidement limitantes dans les études en milieux hétérogènes et plurispécifiques, comme le sont les milieux forestiers spontanés méditerranéens. L'objectif de cette étude est d'évaluer les potentialités de la spectrométrie de réflexion dans le proche infrarouge (SPIR) pour l'étude et le suivi de la décomposition des litières forestières. Les échantillons utilisés proviennent d'expériences menées sur le terrain et en laboratoire sur 8 espèces méditerranéennes : feuillus caducifoliés et sempervirents, et résineux. Les spectres des litières, obtenues à différents stades de décomposition, ont été enregistrés entre 1100 et 2500 nm avec un spectrophotomètre NIRS 5000. Un tiers des échantillons a été analysé par voie humide : cendres totales, carbone, azote, ligno-cellulose et lignine (ADF et ADL méthode Van Soest). À partir de ces analyses, des modèles prédictifs de concentration de chaque composé chimique ont été établis, avec et sans correc-tion de tendance, par deux méthodes de régression : i) régression multiple pas à pas (stepwise) et ii) au moyen d'un algorithme d'ajustement par la méthode des moindres carrés PLS (partial least squares). À la différence des méthodes de régression multiples basées sur un petit nombre de longueurs d'ondes, cette méthode utilise l'ensemble de l'information spectrale. La correction de tendance améliore toujours les résultats de calibration. Les deux méthodes de régression donnent des résultats comparables pour le carbone, l'azote et les cendres. Pour la ligno-cellulose et la lignine, les erreurs standards de validation obtenues par la méthode de calibration PLS sont inférieures de 25 et 34% à celles obtenues par la méthode de régression multiple. Ces résultats montrent que la SPIR peut être utilisée dans les études de décomposition et que la méthode de calibration basée sur l'ensemble du spectre (PLS) est plus performante pour la prédiction des fractions carbonées complexes. Par sa rapidité et sa fiabilité, cette méthode réduit les contraintes analytiques et permet d'aborder les études de décomposition en milieu hétérogène. décomposition / litière / chimie du feuillage / SPIR
A laboratory experiment was carried out with the litter of 7 Mediterranean species of plants to c... more A laboratory experiment was carried out with the litter of 7 Mediterranean species of plants to compare their mass losses during the initial leaching phase of decomposition. To this end, the leaf and needle litter of deciduous, marcescent (deciduous species in which the withered leaves remain on the tree. for several months) and evergreen broad-leaved species and pine was immersed for 10 days in demineralized water, in the dark at 22°C. Samples were taken at 1,6, 24,72, 168 and 240 h. Depending on the species, the litter lost between 'T-15% of initial dry mass, and water content values were between 130-360% of dry mass. The water absorption rate was positively correlated with the initial cellulose content of the litter and negatively correlated wil:h the initial leaf density, but the water content at the end of the experiment, the leaching rate and the total mass loss were uncorrelated with any of the initial litter characteristics studied. For 100 g of litter at the start (as dry matter), the losses after 240 h of immersion in water were, when they were significant, 5-8 g of C, 70-l 30 mg of N (significant for 3 species), 4-40 mg of soluble sugars and 3 g of inorganic elements (significant for only 2 species). In contrast, the quantities of lignin and cellulose in the litter remained unchanged. Most of the leaching losses (SCrSO%) therefore consisted of C. During leaching, the litter showed a tendency to become more concentrated in cellulose and lignin and poorer in sugars. Pine needle litter was distinguished by its low water absorption and low mass loss during leaching. In general, the litters of deciduous species were more absorbent than the litters of evergreen species. In contrast, the losses of water-soluble substances were not significantly different between these 2 groups of litters. In the broad-leaved species, the losses of water-soluble substances were slower and 2-3 times lower, compared to results already published for species from mesic biomes.
Foliage moisture content (FMC in % dry-weight) was measured in seven Mediterranean evergreen spec... more Foliage moisture content (FMC in % dry-weight) was measured in seven Mediterranean evergreen species during summer 2001. All samples were dried, ground and scanned using a NIR spectrophotometer. Two methods of calibration for FMC were compared, one using stepwise regression on selected wavelengths and the other using partial least squares (PLS) regression that takes into account all the spectral information. The PLS regression method provided a better accuracy and was thus chosen. For every species except one, PLS calibrations between spectral data a nd FMC values were accurate: r2 of the linear r egression between measured values and values predicted by the model varied from 0.93 to 0.99, standard error of cross validation (SECV) values were 3 to 6 times lower than the standard deviation (SD) of the reference values. PLS Calibration of FMC in all seven species combined was also accurate: r2 = 0.94, SD/SECV > 3. It is thus possible to measure the initial moisture content of a fre...
We combined chemical and dendroecological analyses to understand the mechanisms that are involved... more We combined chemical and dendroecological analyses to understand the mechanisms that are involved in escaping deer browse by young Sitka spruce (Picea sitchensis) exposed to browsing by Sitka black-tailed deer (Odocoileus hemionus sitchensis) on Haida Gwaii (British Columbia, Canada). We compared chemical defences (terpenes), nutritive compounds (nitrogen, non-structural constituents, cellulose, and lignin), as well as age and radial growth of two young spruce categories growing side by side: (1) stunted spruces that were heavily browsed, shorter than the browse line, and (2) escaped spruces that were taller than the browse line but still browsed below the browse line. Escaped and stunted spruces did not differ in terpene concentrations, or in nutritive compound contents, suggesting that they had similar palatability. Escaped spruces were older that stunted spruces. Stunted and escaped trees had similar slow growth when young, suggesting no difference in initial browsing between the two spruce categories. For escaped spruce, there was a dramatic increase in radial growth at about 12-13 years old, suggesting that the apex of the trees had escaped deer browse. Because the two categories of spruces were equally accessible and did not differ in chemical defences or in nutritive compounds, and because escaped spruces were older than stunted trees and had a similar slow radial growth in their first 12-13 years, we conclude that morphological differences between stunted and escaped browsed trees are due to age and that it is only a matter of time before spruce escape deer on Haida Gwaii.
Remobilization of internal resources is an important mechanism enabling plants to be partly indep... more Remobilization of internal resources is an important mechanism enabling plants to be partly independent of external nutrient availability. We assessed resource remobilization during the growing period in woody and foliar tissues of leafy branches of mature evergreen Mediterranean oak (Quercus ilex L.) at three field sites. We compared nonstructural carbohydrates, lipids, nitrogen and phosphorus pools in leaves and stems before bud burst (March) and at the end of the growing period (July). We also experimentally defoliated leafy branches to determine the storage function of old leaves. Changes in pools of carbon compounds in leaves and stems during spring and in response to defoliation indicated that foliar and woody tissues could provide carbon to support shoot growth. Independently of stem age, soluble sugar and lipid pools decreased significantly during spring. Changes in leaf pools between March and July involved all compounds measured except starch and were accompanied by a 5% decrease in mean leaf biomass. During the same period, 15% of the nitrogen and 25% of the phosphorus were removed from leaves. In contrast, woody tissues did not remobilize nitrogen or phosphorus. Our results support earlier hypotheses that leaves of evergreen species have a primary role in resource remobilization.
Page 1. REGULAR ARTICLE Plant traits, litter quality and decomposition in a Mediterranean old-fie... more Page 1. REGULAR ARTICLE Plant traits, litter quality and decomposition in a Mediterranean old-field succession Jacques Cortez Æ Eric Garnier Æ Natalia Pérez-Harguindeguy Æ Max Debussche Æ Dominique Gillon Received ...
L. 2002. Disentangling the causes of damage variation by deer browsing on young Thuja plicata. -O... more L. 2002. Disentangling the causes of damage variation by deer browsing on young Thuja plicata. -Oikos 98: 271-283.
The aim of this study was to produce calibration equations between near-infrared re¯ectance (NIR)... more The aim of this study was to produce calibration equations between near-infrared re¯ectance (NIR) spectra and the concentrations of carbon, nitrogen, and phosphorus in heterogeneous material: from living needles to litter in Pinus halepensis stands subjected to prescribed burnings. The aim was to determine whether calibrations should be conducted within each stage in the transformation of needles (local calibrations), giving relationships that were accurate but valid only for each particular stage, or whether it was possible to integrate the various forms of variation in needles (global calibrations) while retaining an acceptable accuracy. A principal component analysis calculated from the sample spectral data was used to distinguish three dierent sets, each sharing spectral characteristics and corresponding to three categories of needle: needles collected on the pines (N), falling needles (F), and litter (L), and each containing samples collected from the burnt sites and a control site. Samples representative of all the forms of variation in spectral properties were selected from within each category and their carbon, nitrogen, and phosphorus concentrations were measured using standard wet chemistry methods; these constituted the calibration sets n, f, and l. Calibrations were produced between the nutrient concentrations and the NIR spectra of the calibration sets n, f, and l and the grouped sets (n+f, f+l, n+f+l). The results of local calibrations made from each individual category showed that the carbon, nitrogen, and phosphorus concentrations were accurately predictable by NIR spectra. The global calibrations made by lumping together several categories were valid for a wider range of concentrations and for spectrally heterogeneous materials and in most cases were just as accurate as the local calibrations produced from each individual category.
Journal of Experimental Marine Biology and Ecology, 2003
Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to inves... more Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to investigate decomposition processes in terrestrial ecosystems. The objectives of this research were to determine the potential of NIRS to predict (1) the halophytic litter chemistry (i.e., carbon and nitrogen content) during decomposition, and (2) the stage of decomposition of halophytic litter. Decomposition experiments were conducted in the laboratory with microcosms placed under a wide range of physical characteristics and in the field with litterbags located along the elevation gradient (i.e., low to upper marsh). Microcosm experiments were used to calibrate the predictive equations. These calibration equations were then applied to the field data to test their capacity to predict %C, %N, and litter mass loss (LML). NIRS can be successfully applied to predict chemical composition of halophyte litter during decomposition processes. We hypothesized that the use of litterbags in the field might lead to a 20 -40% overestimation of the decay rate as fine organic debris are lost through the meshes of the litterbags. NIRS can be used as a fast and nondestructive method to more accurately predict decay rates, and thus microbial consumption in aquatic environments. D
Because soil macroinvertebrates strongly modify decomposition processes, it is important to know ... more Because soil macroinvertebrates strongly modify decomposition processes, it is important to know how their abundance will respond to global change. We investigated in laboratory microcosms, the effects of elevated temperatures and reduced leaf litter quality on the life-history traits of a saprophagous macroarthropod (development time, growth, survival and reproduction). Millipedes (Polydesmus angustus) from an Atlantic temperate forest were reared throughout their life cycle ( ! 16 months) under two temperature regimes differing on average by 3.3 1C; in a factorial design, they were fed either on Atlantic leaf litter or on Mediterranean leaf litter with a higher C : N ratio; humidity was consistently high. The components of the population growth rate (r) were affected positively by the temperature rise and negatively by the switch from Atlantic to Mediterranean leaf litter. When both treatments were combined, litter effects offset temperature effects. These results show that the short-term response of saprophagous macroarthropods to warming is positive but depends on the availability of high-quality litter, which is difficult to predict in the global change context. In a parallel experiment, conspecific millipedes from a Mediterranean population, which have evolved for a long time in a warmer climate and on poor-quality litter, were reared at elevated temperatures on Mediterranean leaf litter. All components of r were higher than in the Atlantic population under the same conditions. This suggests that in the longer term, macroarthropods can overcome detrimental trophic interactions. Based on our study and the literature, we conclude that for decades the positive effects of warming on saprophagous macrofauna should exceed the negative effects of changes in litter quality. The abundance of those organisms in temperate forests could increase, which is confirmed by latitudinal patterns in Europe. Studies aimed at predicting the impacts of global change on decomposition will need to consider interactions with soil macroinvertebrates.
The effects of two herbicides (atrazine and alachlore) and two insecticides (fipronil and carbofu... more The effects of two herbicides (atrazine and alachlore) and two insecticides (fipronil and carbofuran) on organic matter decomposition and soil mesofauna were evaluated in a maize field under normal agriculture conditions. Soil mesofauna were studied using the litterbag method. Near infrared reflectance spectrophotometry was used to study the effects on decomposition parameters (litter mass and nitrogen dynamic). The effects on soil microarthropods varied depending on both taxon and pesticide, with fipronil having a particularly strong effect. Differences were also observed between treatments for organic matter decomposition parameters, especially with the use of alachlore. Therefore, the analysis of microarthropods and the decomposition parameters provides complementary information on the effects of these chemical treatments.
Several decomposition experiments were used to explore the potential of near infrared reflectance... more Several decomposition experiments were used to explore the potential of near infrared reflectance spectroscopy (NIRS) for predicting litter decomposability. A first experiment was conducted on a calibration set, and predictive equations were established between the NIR spectral data of the initial litters and the decay descriptors. These equations were then applied to two validation sets. The 34 litters of the calibration set were incubated for eight weeks in microcosms in the laboratory. The values of litter mass remaining (LMR) in relation to incubation time were fitted to the single-exponential decay model (LMR ϭ e Ϫkt ). The litter mass loss (LML) values at different sampling dates and the rate constant k were strongly correlated with the initial litter characteristics. The calibrations carried out between measurements of the NIR spectra on the initial litters and the different descriptors of the decomposition patterns of the 34 litters showed that the LML values at the different sampling dates and the rate constant k were correlated with the spectra of the initial litters more closely than with their initial characteristics. The descriptors of litter decomposition patterns could therefore be predicted by NIRS. Among these descriptors, which were all correlated with one another, the easiest to measure (LML after one week incubation in the microcosm) was tested as a litter decomposability index (LDI). LDI was predicted by NIRS on the initial litters in two validation sets, the first consisting of 12 litters incubated under the same conditions in microcosms over a longer period (14 mo), the second consisting of four litters incubated in the field for one year at three stations situated along a climatic gradient. The results showed that the values of LDI predicted by NIRS were significantly correlated with the decay rate constant k in the two validation sets. The values of LDI predicted by NIRS therefore expressed a decomposability scale that was valid for litters decomposing over a longer period under the same incubation conditions and for litters subjected to different incubation conditions. The possibility of predicting litter decomposability by NIRS provides many opportunities, firstly for studying spatial and temporal variability in the rate of recycling of organic matter, and secondly for characterizing the gradual changes in litter quality during decomposition.
Au Sénégal, les Zules, d u n champ (13" 55 N et 15" 49 W ) ont été échantillonnés &ao& 1974 Ci ju... more Au Sénégal, les Zules, d u n champ (13" 55 N et 15" 49 W ) ont été échantillonnés &ao& 1974 Ci juillet 1975, par 22 relevés de 4 i d de surface et de 30 à 80cnz de profondeur et dans une ternzitière sur 20 c m dépaisseur en saison sèche. 6 espèces, dont une présente 2forines morphologiquement différentes, constituent 7 populations " distinctes : Graphidostreptus tumuliporus, Peridontopyge conani, P. spinosissima, Haplothysanus chapellei ( p et G) Syndosmogenus mimeuri. Leur répartition dans le champ est hétérogène. Il n'y a pas de peupleinent caractéristique autour des souches et au pied des arbres car toutes les espèces se concentrent plus ou inoins fortement dans ces biotopes. Par contre le peuplement de la couche superficielle de la termitière est constitué de quelques espèces seulement, inais très abondantes. G. tumuliporus se concentre au pied des arbres et dans les termitières mais il est rare dans le reste du champ. P. conani et les 2 formes de H. chapellei sont 2 à 3 fois plus abondants sous les arbres qu'en plein champ et les individus y sont en moyenne plus grands. Les stades âg& de ces 3 populations se concentrent en saison sèche dans les termitières oÙ leur densité est I O à 30 fois plus grande qu'en plein champ et leur bionzasse 40 6 90 fois plus grande. Par rapport aux densités en plein champ, les effectifs de P. spinosissima seraient 8 fois plus forts sous les arbres inais seulement 3 fois plus élevés autour des souches et dans les termitières. P. rubescens et S. mimeuri sont très rares dans les termitières et 3 à 4 fois plus abondants sous les arbres qu'en plein champ. En rapportant la densité des différentes populations à leur étendue dans chaque biotope on obtient une iniage globale du peuplernent du champ. P. rubescens se révélant la population dominante, compterait pour I/3 de la biomasse et de l'effectif total des Iules. S. mimeuri représeizterait le I/4 des Iules du champ inais une faible part de leur biomasse. G. tumuliporus au contraire, représenterait un effectif faible mais le 1/4 de la biomasse totale. P. conani fournirait, en poids coinine en nombre, 15 % du peuplement. Les autres populations seraient accessoires ne représentant pas I O % du peupleinent en effectif et en biomasse. Le champ nu serait le lieu où la niujarité des Iules à l'exception des G. tumuliporus, passe la saison sèche. Le pied des arbres concentrerait la plupart des individus de G. tumuliporus et une grande part de la biomasse des autres populations. Les termitières rassembleraient l'essentiel de la biomasse de G. tumuliporus et une partie de la bioinasse des 3 espèces qui colonisent aussi ce refuge. Quant aux souches avec rejets, elles ne constituent pas des biotopes particulièrement attractifs ni des refuges car si les plus jeunes stades y sont un peu plus fréquents que dans le plein champ, l'inverse est observé pour les individus de grande taille.
Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata we... more Chemical changes in holm oak leaf litter that was consumed by the millipede Glomeris marginata were studied using near infrared reflectance spectroscopy. Compared to leaf litter, faecal pellets were significantly richer in lignin and poorer in nitrogen and readily assimilable constituents. The C:N ratio was significantly higher in the faeces. A NIRS-predicted index of litter decomposability was found to be
— The biochemical nature of leaf litter is a key factor in regulation of its decomposition.... more — The biochemical nature of leaf litter is a key factor in regulation of its decomposition. Conventional wet chemical analysis of samples is destructive, time-consuming and expensive. The objective of this study was to evaluate the potentiality of near infrared reflectance spectroscopy (NIRS) for determining litter chemistry during the decomposition process using a wide range of species and decomposition stages. The litter of 8 species of evergreen and deciduous broad-leaved trees, conifers and shrubs were used in both laboratory and field experiments. Near-infrared reflectance measurements were made with an NIRS Systems 5000 spectrophotometer over the range 1100-2500 nm. Calibration samples were analysed for ash, carbon and nitrogen. Acid-detergent fiber (ADF) and acid-detergent lignin (ADL) were determined using Van Soest procedures. Stepwise regression (SR) calibrations and partial least squares (PLSR) calibrations were developed and compared as well as the effect of scatter correction. The PLS algorithm was used to create the predictive models using all the information in the spectrum to determine the chemical concentration. Using scatter correction always gave better results. Both regression methods provided acceptable validation statistics for C, N and ash. The PLSR had better prediction accuracy for ADF and ADL. For these two constituents, the improvement of SECV was 34 and 25% respectively. Our results showed that NIRS is an effective tool to predict nitrogen, ash and proximate carbon fractions in decomposition studies and that PSLR method improves calibration compared with SR method. decomposition / leaf chemistry / litter / NIRS Résumé — Utilisation de la spectroscopie proche infrarouge dans les études de décomposition de litières. La composition biochimique des litières est un des facteurs clés de la régulation de leur décomposition. Les méthodes d'analyse chimique par voie humide sont destructives, longues et coûteuses et ces contraintes sont rapidement limitantes dans les études en milieux hétérogènes et plurispécifiques, comme le sont les milieux forestiers spontanés méditerranéens. L'objectif de cette étude est d'évaluer les potentialités de la spectrométrie de réflexion dans le proche infrarouge (SPIR) pour l'étude et le suivi de la décomposition des litières forestières. Les échantillons utilisés proviennent d'expériences menées sur le terrain et en laboratoire sur 8 espèces méditerranéennes : feuillus caducifoliés et sempervirents, et résineux. Les spectres des litières, obtenues à différents stades de décomposition, ont été enregistrés entre 1100 et 2500 nm avec un spectrophotomètre NIRS 5000. Un tiers des échantillons a été analysé par voie humide : cendres totales, carbone, azote, ligno-cellulose et lignine (ADF et ADL méthode Van Soest). À partir de ces analyses, des modèles prédictifs de concentration de chaque composé chimique ont été établis, avec et sans correc-tion de tendance, par deux méthodes de régression : i) régression multiple pas à pas (stepwise) et ii) au moyen d'un algorithme d'ajustement par la méthode des moindres carrés PLS (partial least squares). À la différence des méthodes de régression multiples basées sur un petit nombre de longueurs d'ondes, cette méthode utilise l'ensemble de l'information spectrale. La correction de tendance améliore toujours les résultats de calibration. Les deux méthodes de régression donnent des résultats comparables pour le carbone, l'azote et les cendres. Pour la ligno-cellulose et la lignine, les erreurs standards de validation obtenues par la méthode de calibration PLS sont inférieures de 25 et 34% à celles obtenues par la méthode de régression multiple. Ces résultats montrent que la SPIR peut être utilisée dans les études de décomposition et que la méthode de calibration basée sur l'ensemble du spectre (PLS) est plus performante pour la prédiction des fractions carbonées complexes. Par sa rapidité et sa fiabilité, cette méthode réduit les contraintes analytiques et permet d'aborder les études de décomposition en milieu hétérogène. décomposition / litière / chimie du feuillage / SPIR
A laboratory experiment was carried out with the litter of 7 Mediterranean species of plants to c... more A laboratory experiment was carried out with the litter of 7 Mediterranean species of plants to compare their mass losses during the initial leaching phase of decomposition. To this end, the leaf and needle litter of deciduous, marcescent (deciduous species in which the withered leaves remain on the tree. for several months) and evergreen broad-leaved species and pine was immersed for 10 days in demineralized water, in the dark at 22°C. Samples were taken at 1,6, 24,72, 168 and 240 h. Depending on the species, the litter lost between 'T-15% of initial dry mass, and water content values were between 130-360% of dry mass. The water absorption rate was positively correlated with the initial cellulose content of the litter and negatively correlated wil:h the initial leaf density, but the water content at the end of the experiment, the leaching rate and the total mass loss were uncorrelated with any of the initial litter characteristics studied. For 100 g of litter at the start (as dry matter), the losses after 240 h of immersion in water were, when they were significant, 5-8 g of C, 70-l 30 mg of N (significant for 3 species), 4-40 mg of soluble sugars and 3 g of inorganic elements (significant for only 2 species). In contrast, the quantities of lignin and cellulose in the litter remained unchanged. Most of the leaching losses (SCrSO%) therefore consisted of C. During leaching, the litter showed a tendency to become more concentrated in cellulose and lignin and poorer in sugars. Pine needle litter was distinguished by its low water absorption and low mass loss during leaching. In general, the litters of deciduous species were more absorbent than the litters of evergreen species. In contrast, the losses of water-soluble substances were not significantly different between these 2 groups of litters. In the broad-leaved species, the losses of water-soluble substances were slower and 2-3 times lower, compared to results already published for species from mesic biomes.
Foliage moisture content (FMC in % dry-weight) was measured in seven Mediterranean evergreen spec... more Foliage moisture content (FMC in % dry-weight) was measured in seven Mediterranean evergreen species during summer 2001. All samples were dried, ground and scanned using a NIR spectrophotometer. Two methods of calibration for FMC were compared, one using stepwise regression on selected wavelengths and the other using partial least squares (PLS) regression that takes into account all the spectral information. The PLS regression method provided a better accuracy and was thus chosen. For every species except one, PLS calibrations between spectral data a nd FMC values were accurate: r2 of the linear r egression between measured values and values predicted by the model varied from 0.93 to 0.99, standard error of cross validation (SECV) values were 3 to 6 times lower than the standard deviation (SD) of the reference values. PLS Calibration of FMC in all seven species combined was also accurate: r2 = 0.94, SD/SECV > 3. It is thus possible to measure the initial moisture content of a fre...
We combined chemical and dendroecological analyses to understand the mechanisms that are involved... more We combined chemical and dendroecological analyses to understand the mechanisms that are involved in escaping deer browse by young Sitka spruce (Picea sitchensis) exposed to browsing by Sitka black-tailed deer (Odocoileus hemionus sitchensis) on Haida Gwaii (British Columbia, Canada). We compared chemical defences (terpenes), nutritive compounds (nitrogen, non-structural constituents, cellulose, and lignin), as well as age and radial growth of two young spruce categories growing side by side: (1) stunted spruces that were heavily browsed, shorter than the browse line, and (2) escaped spruces that were taller than the browse line but still browsed below the browse line. Escaped and stunted spruces did not differ in terpene concentrations, or in nutritive compound contents, suggesting that they had similar palatability. Escaped spruces were older that stunted spruces. Stunted and escaped trees had similar slow growth when young, suggesting no difference in initial browsing between the two spruce categories. For escaped spruce, there was a dramatic increase in radial growth at about 12-13 years old, suggesting that the apex of the trees had escaped deer browse. Because the two categories of spruces were equally accessible and did not differ in chemical defences or in nutritive compounds, and because escaped spruces were older than stunted trees and had a similar slow radial growth in their first 12-13 years, we conclude that morphological differences between stunted and escaped browsed trees are due to age and that it is only a matter of time before spruce escape deer on Haida Gwaii.
Remobilization of internal resources is an important mechanism enabling plants to be partly indep... more Remobilization of internal resources is an important mechanism enabling plants to be partly independent of external nutrient availability. We assessed resource remobilization during the growing period in woody and foliar tissues of leafy branches of mature evergreen Mediterranean oak (Quercus ilex L.) at three field sites. We compared nonstructural carbohydrates, lipids, nitrogen and phosphorus pools in leaves and stems before bud burst (March) and at the end of the growing period (July). We also experimentally defoliated leafy branches to determine the storage function of old leaves. Changes in pools of carbon compounds in leaves and stems during spring and in response to defoliation indicated that foliar and woody tissues could provide carbon to support shoot growth. Independently of stem age, soluble sugar and lipid pools decreased significantly during spring. Changes in leaf pools between March and July involved all compounds measured except starch and were accompanied by a 5% decrease in mean leaf biomass. During the same period, 15% of the nitrogen and 25% of the phosphorus were removed from leaves. In contrast, woody tissues did not remobilize nitrogen or phosphorus. Our results support earlier hypotheses that leaves of evergreen species have a primary role in resource remobilization.
Page 1. REGULAR ARTICLE Plant traits, litter quality and decomposition in a Mediterranean old-fie... more Page 1. REGULAR ARTICLE Plant traits, litter quality and decomposition in a Mediterranean old-field succession Jacques Cortez Æ Eric Garnier Æ Natalia Pérez-Harguindeguy Æ Max Debussche Æ Dominique Gillon Received ...
L. 2002. Disentangling the causes of damage variation by deer browsing on young Thuja plicata. -O... more L. 2002. Disentangling the causes of damage variation by deer browsing on young Thuja plicata. -Oikos 98: 271-283.
The aim of this study was to produce calibration equations between near-infrared re¯ectance (NIR)... more The aim of this study was to produce calibration equations between near-infrared re¯ectance (NIR) spectra and the concentrations of carbon, nitrogen, and phosphorus in heterogeneous material: from living needles to litter in Pinus halepensis stands subjected to prescribed burnings. The aim was to determine whether calibrations should be conducted within each stage in the transformation of needles (local calibrations), giving relationships that were accurate but valid only for each particular stage, or whether it was possible to integrate the various forms of variation in needles (global calibrations) while retaining an acceptable accuracy. A principal component analysis calculated from the sample spectral data was used to distinguish three dierent sets, each sharing spectral characteristics and corresponding to three categories of needle: needles collected on the pines (N), falling needles (F), and litter (L), and each containing samples collected from the burnt sites and a control site. Samples representative of all the forms of variation in spectral properties were selected from within each category and their carbon, nitrogen, and phosphorus concentrations were measured using standard wet chemistry methods; these constituted the calibration sets n, f, and l. Calibrations were produced between the nutrient concentrations and the NIR spectra of the calibration sets n, f, and l and the grouped sets (n+f, f+l, n+f+l). The results of local calibrations made from each individual category showed that the carbon, nitrogen, and phosphorus concentrations were accurately predictable by NIR spectra. The global calibrations made by lumping together several categories were valid for a wider range of concentrations and for spectrally heterogeneous materials and in most cases were just as accurate as the local calibrations produced from each individual category.
Journal of Experimental Marine Biology and Ecology, 2003
Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to inves... more Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to investigate decomposition processes in terrestrial ecosystems. The objectives of this research were to determine the potential of NIRS to predict (1) the halophytic litter chemistry (i.e., carbon and nitrogen content) during decomposition, and (2) the stage of decomposition of halophytic litter. Decomposition experiments were conducted in the laboratory with microcosms placed under a wide range of physical characteristics and in the field with litterbags located along the elevation gradient (i.e., low to upper marsh). Microcosm experiments were used to calibrate the predictive equations. These calibration equations were then applied to the field data to test their capacity to predict %C, %N, and litter mass loss (LML). NIRS can be successfully applied to predict chemical composition of halophyte litter during decomposition processes. We hypothesized that the use of litterbags in the field might lead to a 20 -40% overestimation of the decay rate as fine organic debris are lost through the meshes of the litterbags. NIRS can be used as a fast and nondestructive method to more accurately predict decay rates, and thus microbial consumption in aquatic environments. D
Because soil macroinvertebrates strongly modify decomposition processes, it is important to know ... more Because soil macroinvertebrates strongly modify decomposition processes, it is important to know how their abundance will respond to global change. We investigated in laboratory microcosms, the effects of elevated temperatures and reduced leaf litter quality on the life-history traits of a saprophagous macroarthropod (development time, growth, survival and reproduction). Millipedes (Polydesmus angustus) from an Atlantic temperate forest were reared throughout their life cycle ( ! 16 months) under two temperature regimes differing on average by 3.3 1C; in a factorial design, they were fed either on Atlantic leaf litter or on Mediterranean leaf litter with a higher C : N ratio; humidity was consistently high. The components of the population growth rate (r) were affected positively by the temperature rise and negatively by the switch from Atlantic to Mediterranean leaf litter. When both treatments were combined, litter effects offset temperature effects. These results show that the short-term response of saprophagous macroarthropods to warming is positive but depends on the availability of high-quality litter, which is difficult to predict in the global change context. In a parallel experiment, conspecific millipedes from a Mediterranean population, which have evolved for a long time in a warmer climate and on poor-quality litter, were reared at elevated temperatures on Mediterranean leaf litter. All components of r were higher than in the Atlantic population under the same conditions. This suggests that in the longer term, macroarthropods can overcome detrimental trophic interactions. Based on our study and the literature, we conclude that for decades the positive effects of warming on saprophagous macrofauna should exceed the negative effects of changes in litter quality. The abundance of those organisms in temperate forests could increase, which is confirmed by latitudinal patterns in Europe. Studies aimed at predicting the impacts of global change on decomposition will need to consider interactions with soil macroinvertebrates.
The effects of two herbicides (atrazine and alachlore) and two insecticides (fipronil and carbofu... more The effects of two herbicides (atrazine and alachlore) and two insecticides (fipronil and carbofuran) on organic matter decomposition and soil mesofauna were evaluated in a maize field under normal agriculture conditions. Soil mesofauna were studied using the litterbag method. Near infrared reflectance spectrophotometry was used to study the effects on decomposition parameters (litter mass and nitrogen dynamic). The effects on soil microarthropods varied depending on both taxon and pesticide, with fipronil having a particularly strong effect. Differences were also observed between treatments for organic matter decomposition parameters, especially with the use of alachlore. Therefore, the analysis of microarthropods and the decomposition parameters provides complementary information on the effects of these chemical treatments.
Several decomposition experiments were used to explore the potential of near infrared reflectance... more Several decomposition experiments were used to explore the potential of near infrared reflectance spectroscopy (NIRS) for predicting litter decomposability. A first experiment was conducted on a calibration set, and predictive equations were established between the NIR spectral data of the initial litters and the decay descriptors. These equations were then applied to two validation sets. The 34 litters of the calibration set were incubated for eight weeks in microcosms in the laboratory. The values of litter mass remaining (LMR) in relation to incubation time were fitted to the single-exponential decay model (LMR ϭ e Ϫkt ). The litter mass loss (LML) values at different sampling dates and the rate constant k were strongly correlated with the initial litter characteristics. The calibrations carried out between measurements of the NIR spectra on the initial litters and the different descriptors of the decomposition patterns of the 34 litters showed that the LML values at the different sampling dates and the rate constant k were correlated with the spectra of the initial litters more closely than with their initial characteristics. The descriptors of litter decomposition patterns could therefore be predicted by NIRS. Among these descriptors, which were all correlated with one another, the easiest to measure (LML after one week incubation in the microcosm) was tested as a litter decomposability index (LDI). LDI was predicted by NIRS on the initial litters in two validation sets, the first consisting of 12 litters incubated under the same conditions in microcosms over a longer period (14 mo), the second consisting of four litters incubated in the field for one year at three stations situated along a climatic gradient. The results showed that the values of LDI predicted by NIRS were significantly correlated with the decay rate constant k in the two validation sets. The values of LDI predicted by NIRS therefore expressed a decomposability scale that was valid for litters decomposing over a longer period under the same incubation conditions and for litters subjected to different incubation conditions. The possibility of predicting litter decomposability by NIRS provides many opportunities, firstly for studying spatial and temporal variability in the rate of recycling of organic matter, and secondly for characterizing the gradual changes in litter quality during decomposition.
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Papers by Dominique Gillon