Alexandre Bosc

Photosynthetic carbon isotope discrimination and its relationship to the carbon isotope signals of stem, soil and ecosystem respiration

• Photosynthetic carbon (C) isotope discrimination (Δ(Α)) labels photosynthates (δ(A) ) and atmospheric CO(2) (δ(a)) with variable C isotope compositions during fluctuating environmental conditions. In this context, the C isotope composition of respired CO(2) within ecosystems is often hypothesized to vary temporally with Δ(Α). • We investigated the relationship between Δ(Α) and the C isotope signals from stem (δ(W)), soil (δ(S)) and ecosystem (δ(E)) respired CO(2) to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. • Broad seasonal changes in Δ(Α) were reflected in δ(W,) δ(S) and δ(E). However, respired CO(2) signals had smaller short-term variations than Δ(A) and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in δ(S) did not follow Δ(A) at all times, especially during rainy periods and when there is a strong demand for C allocation above ground. • It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO(2).

A single‐substrate model to interpret intra‐annual stable isotope signals in tree‐ring cellulose

The carbon and oxygen stable isotope composition of wood cellulose (delta(13)C(cellulose) and delta(18)O(cellulose), respectively) reveal well-defined seasonal variations that contain valuable records of past climate, leaf gas exchange and carbon allocation dynamics within the trees. Here, we present a single-substrate model for wood growth to interpret seasonal isotopic signals collected in an even-aged maritime pine plantation growing in South-west France, where climate, soil and flux variables were also monitored. Observed seasonal patterns in delta(13)C(cellulose) and delta(18)O(cellulose) were different between years and individuals, and mostly captured by the model, suggesting that the single-substrate hypothesis is a good approximation for tree ring studies on Pinus pinaster, at least for the environmental conditions covered by this study. A sensitivity analysis revealed that the model was mostly affected by five isotopic discrimination factors and two leaf gas-exchange parameters. Modelled early wood signals were also very sensitive to the date when cell wall thickening begins (t(wt)). Our model could therefore be used to reconstruct t(wt) time series and improve our understanding of how climate influences this key parameter of xylogenesis.

In situ assessment of the velocity of carbon transfer by tracing 13C in trunk CO2 efflux after pulse labelling: variations among tree species and seasons

Phloem is the main pathway for transferring photosynthates belowground. In situ(13) C pulse labelling of trees 8-10 m tall was conducted in the field on 10 beech (Fagus sylvatica) trees, six sessile oak (Quercus petraea) trees and 10 maritime pine (Pinus pinaster) trees throughout the growing season. Respired (13) CO2 from trunks was tracked at different heights using tunable diode laser absorption spectrometry to determine time lags and the velocity of carbon transfer (V). The isotope composition of phloem extracts was measured on several occasions after labelling and used to estimate the rate constant of phloem sap outflux (kP ). Pulse labelling together with high-frequency measurement of the isotope composition of trunk CO2 efflux is a promising tool for studying phloem transport in the field. Seasonal variability in V was predicted in pine and oak by bivariate linear regressions with air temperature and soil water content. V differed among the three species consistently with known differences in phloem anatomy between broadleaf and coniferous trees. V increased with tree diameter in oak and beech, reflecting a nonlinear increase in volumetric flow with increasing bark cross-sectional area, which suggests changes in allocation pattern with tree diameter in broadleaf species. Discrepancies between V and kP indicate vertical changes in functional phloem properties.

Plasticity of maritime pine (Pinus pinaster) wood‐forming tissues during a growing season

The seasonal effect is the most significant external source of variation affecting vascular cambial activity and the development of newly divided cells, and hence wood properties. Here, the effect of edapho-climatic conditions on the phenotypic and molecular plasticity of differentiating secondary xylem during a growing season was investigated. Wood-forming tissues of maritime pine (Pinus pinaster) were collected from the beginning to the end of the growing season in 2003. Data from examination of fibre morphology, Fourier-transform infrared spectroscopy (FTIR), analytical pyrolysis, and gas chromatography/mass spectrometry (GC/MS) were combined to characterize the samples. Strong variation was observed in response to changes in edapho-climatic conditions. A genomic approach was used to identify genes differentially expressed during this growing season. Out of 3512 studied genes, 19% showed a significant seasonal effect. These genes were clustered into five distinct groups, the largest two representing genes over-expressed in the early- or late-wood-forming tissues, respectively. The other three clusters were characterized by responses to specific edapho-climatic conditions. This work provides new insights into the plasticity of the molecular machinery involved in wood formation, and reveals candidate genes potentially responsible for the phenotypic differences found between early- and late-wood.

Improving models of forest nutrient export with equations that predict the nutrient concentration of tree compartments

Abstract• The objective of this study was to explore the distribution of major nutrients (N, P, K, Ca and Mg) in the aboveground compartments of an intensively managed tree species (Pinus pinaster Ait.). A total of 53 trees were cut down in even-aged stands respectively 8, 16, 26, 32 and 40 years old. The nutrient concentrations of the aboveground compartments were analysed.• Nutrient concentrations of foliage did not vary with any of the variables used, except needle age. Nutrient concentrations of living branches, stem bark, stem sapwood, stem heartwood, stemwood and stem decreased with increasing branch diameter, bark thickness, sapwood thickness and heartwood thickness, respectively. Beyond a certain value of the predictive variable (stem diameter ≈ 15 cm; branch diameter ≈ 2.5 cm), the concentration of all the nutrients stabilised.• A 50 year-old pine stand was used to obtain a validation dataset for nitrogen concentration. For this nutrient, the regression relationships gave satisfactory estimates for most compartments (mean error = 12–25%) and particularly for the stem.• A procedure is proposed to estimate the nutrient exports associated with harvests of Pinus pinaster biomass.Résumé• L’objectif de cette étude est d’explorer la distribution des éléments majeurs (N, P, K, Ca, Mg) dans les compartiments aériens d’une essence gérée de manière intensive. Au total, 53 pins maritimes (Pinus pinaster Ait.) ont été abattus parmi des peuplements équiennes de 8, 16, 26, 32 et 40 ans.• Les concentrations en nutriments du feuillage ne varient pas pour une classe d’âge d’aiguilles donnée. Les concentrations des branches vivantes, de l’écorce, de l’aubier et du duramen décroissent lorsque le diamètre ou l’épaisseur du compartiment considéré augmente. La concentration de l’ensemble des nutriments devient constante lorsque la variable prédictive (diamètre ou épaisseur) atteint une valeur plateau.• Un jeu de données de validation pour les concentrations en azote, provenant d’un peuplement équienne de pins de 50 ans, permet de confirmer les performances satisfaisantes des modèles construits (erreur moyenne = 12–25 %) et en particulier pour le tronc.• Une procédure d’estimation des exportations de nutriments associées aux récoltes de biomasse de Pinus pinaster est présentée.

Using cover measurements to estimate aboveground understorey biomass in Maritime pine stands.

Abstract• Understorey plays a major role in forest fluxes and stocks balances, however this compartment is generally poorly quantified. Our objectives were to establish models to estimate understorey biomass using vegetation cover measurements and to investigate upscaling methodologies from stand to regional level.• Understorey aboveground biomass measurements were undertaken in Maritime pine stands of mesohygric, mesic and dry moorlands in South West France.• Average biomass stock in this compartment was estimated to 3.50 t DM ha−1. The more abundant species groups varied with moorland types, with a higher relative contribution of herbaceous species (23.3%), bracken (59.2%) and mosses (31.6%) for mesohygric, mesic and dry moorlands, respectively. For each species group, we established significant relationships to estimate biomass using a volumetric index, based on cover and height measurements. No relationship between stand characteristics and understorey biomass was founded. We investigated the upscaling of these estimations to a several thousands hectare area using understorey cover measurements done along a regular spatial grid. The only significant correlation linked one satellite vegetation index to understorey biomass.• We successfully developed empirical relationships to estimate the understorey biomass at the stand level. Further investigations could focus on the analysis of understorey variability over a finer space grid and the potential use of satellite vegetation indexes.Résumé• Le sous-bois est un compartiment non négligeable dans les études de stocks et de flux des forêts; cependant il est encore mal quantifié. Les objectifs de notre étude étaient d’établir des relations permettant d’estimer la biomasse du sous-bois de peuplements et d’analyser les possibilités d’extrapolation à l’échelle du massif.• Des mesures de biomasse aérienne de sous-bois ont été réalisées sur une série de peuplements de pin maritime en Landes mésohygrophile, mésophile et sèche dans le Sud-Ouest de la France.• Nous avons estimé le stock moyen de biomasse dans ce compartiment à 3.50 t MS ha−1. Le groupe d’espèces le plus abondant diffère selon le type de landes : herbacées (23.3 %), fougères (59.2 %) et mousses (31.6 %) en landes mésohygrophile, mésophile et sèche, respectivement. Pour chaque groupe d’espèces, des relations significatives ont été mises en évidence entre un indice volumique et la biomasse de sous-bois. Aucune relation n’a été mise en évidence entre les caractéristiques du peuplement et la biomasse du sous-bois. Nous avons envisagé le calcul à l’échelle d’une zone atelier de plusieurs milliers d’hectares en utilisant une grille spatialisée de relevés de recouvrements. Seul un indice satellite de végétation a présenté une corrélation positive avec la biomasse du sous-bois.• Les relations que nous avons développées permettent d’estimer la biomasse du sous-bois à l’échelle du peuplement. L’analyse spatiale à une échelle plus fine et l’utilisation d’un indice de végétation pourraient être des pistes à explorer.

Combined effects of defoliation and water stress on pine growth and non-structural carbohydrates.

Climate change is expected to increase both pest insect damage and the occurrence of severe drought. There is therefore a need to better understand the combined effects of biotic and abiotic damage on tree growth in order to predict the multi-factorial effect of climate change on forest ecosystem productivity. Indeed, the effect of stress interactions on tree growth is an increasingly important topic that greatly lacks experiments and data, and it is unlikely that the impact of combined stresses can be extrapolated from the outcomes of studies that focused on a single stress. We developed an original manipulative study under real field conditions where we applied artificial defoliation and induced water stress on 10-year-old (∼10 m high) maritime pine trees (Pinus pinaster Ait.). Tree response to combined stresses was quantitatively assessed following tree secondary growth and carbohydrate pools. Such a design allowed us to address the crucial question of combined stresses on trees under stand conditions, sharing soil supplies with neighboring trees. Our initial hypotheses were that (i) moderate defoliation can limit the impact of water stress on tree growth through reduced transpiration demand by a tree canopy partly defoliated and that (ii) defoliation results in reduced non-structural carbohydrate (NSC) pools, affecting tree tolerance to drought. Our results showed additive effects of defoliation and water stress on tree growth and contradict our initial hypothesis. Indeed, under stand conditions, we found that partial defoliation does not limit the impact of water stress through reduced transpiration. Our study also highlighted that, even if NSC in all organs were affected by defoliation, tree response to water stress was not triggered. We found that stem NSC were maintained or increased during the entire growing season, supporting literature-based hypotheses such as an active maintenance of the hydraulic system or another limiting resource for tree growth under defoliation. We also observed a significant decrease in root carbohydrates, which suggests a shift in the root carbon balance under defoliation. The decrease in carbohydrate supply under defoliation may not counterbalance the carbon use for mineral and water uptakes or a translocation to other tissues.

Paired comparison of water, energy and carbon exchanges over two young maritime pine stands (Pinus pinaster Ait.): effects of thinning and weeding in the early stage of tree growth.

The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO(2)), H(2)O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m(-2) on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R(E)). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m(-2), respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.

Two-year dynamics of foliage labelling in 8-year-old Pinus pinaster trees with 15N, 26Mg and 42Ca—simulation of Ca transport in xylem using an upscaling approach

Abstract• IntroductionAtmospheric deposition is an important input of major nutrients into forest ecosystems. The long-term goal of this work was to apply stable isotope methodology to assess atmospheric nutrient deposition in forest systems.• Materials and methodsA labelling experiment of foliage with stable isotopes of primary and secondary macro nutrients (15N, 26Mg and 42Ca injected into the stem sapwood) was carried on standing trees to monitor interactions between canopy and precipitations. 15N rapidly reached the foliage; however, Mg and Ca were not detected in foliage until more than a year after injection.• Results and discussionThe delay in mobilization of Mg and Ca prevented us from accurately modelling deposition contributions of these two elements. Nonetheless, an upscaling approach based on published results on Ca transport in shoots xylem was used to simulate our results. These simulations of Ca transport at the tree scale were consistent with our experimental data.• ConclusionThis consistency suggested that mechanisms of nutrient transport are the same at the different scales. Nitrogen was rapidly transported in the xylem to foliage, probably mainly by mass flow. Conversely, transport of Mg and particularly Ca was considerably delayed, probably due to successive cation exchanges along the xylem vessels.

Insect – Tree Interactions in Thaumetopoea pityocampa

The pine processionary moth is, by far, the most important insect defoliator of pine forests in Southern Europe and North Africa, both in terms of its temporal occurrence, geographic range and socioeconomic impact. Monitoring and pest management actions are therefore required on a regular basis, to ensure the detection, evaluation and mitigation of potential risks to forest and public health. However, we still lack some of the basic knowledge required for relevant analyses of the risk posed by the pine processionary moth. Pest risk is defined as a combination of three components: (1) pest occurrence, which depends on the spatiotemporal dynamics of populations; (2) plant vulnerability to the pest, resulting in a certain amount of damage; and (3) the socioeconomic impact of damage, depending on the potential value of the plants damaged (Jactel et al. 2012). The population dynamics of the processionary moth has been extensively studied, in particular within the context of climate change (see Battisti et al. 2014, Chap. 2, this volume). Several studies have recently addressed the question of tree and forest vulnerability to pine processionary attacks but a comprehensive review of evidence was missing. This is the first objective of this chapter. In particular we were interested in a better understanding of the ecological mechanisms responsible for the host tree selection, at both the species and individual tree levels. In a second part we show that pine susceptibility to the pine processionary moth could be reduced by improving forest diversity at different spatial scales. In the last part of this chapter we provide quantitative estimate of the growth losses caused by defoliations of the pine processionary moth. Altogether this information paves the way for quantitative risk analyses on pine processionary moth infestations based on forest growth models.