Effects of tree species identity and mixing on below-ground resources availability and climate-growth relationships : a case study in mixtures of Scots pine and beech across Europe

On average, increasing tree species diversity in forest stands results in higher productivity and stability of productivity, as well as in a larger range of ecosystem services; however, those effects are strongly context-dependent. In my PhD, I therefore explored some mechanisms possibly involved in species-mixing effects and looked at their dependency to spatial and temporal variations of environmental conditions. More specifically, I analyzed species-mixing effects on proxies of water and nutrient use, and on climate-growth relationships for Scots pine and European beech growing in pure and mixed-species stands (triplets) using a network of sites covering a large ecological gradient across Europe. This work was conducted along three axes. The first one aimed at analyzing the effects of species interactions on water related processes using wood carbon isotopic composition as an indicator of tree water use efficiency. I found that complementarity displayed a humped-shaped curve with maximal (positive) values for dry sites and close-to-zero values for extremely dry ones. In addition, species-mixing effects on non- or moderately constrained sites were strongly dependent on water- and possibly light-related species traits. No coherent species-mixing effects on drought resistance along the gradient was found for either species. The second axis aimed at analyzing pine and beech foliar nutrient composition in pure and mixed stands along a gradient of sites nutritional richness. Significant species-mixing effects on both foliar nutrient concentrations and stoichiometry were found. Interestingly, those effects tended to be higher on richer sites. Lastly, I analyzed the large-scale variability of species-mixing effects on average climate-growth relationships and drought resistance for pine and beech across Europe. The results highlighted the strong influence of regional climate on climate-growth relationship but coherent species-mixing effects were only found in case of overwhelming climatic constraint. While no species-mixing effects on drought response was found for beech, a significant effect was highlighted for pine, yet limited to situations where local climatic conditions allowed for compensatory growth to happen during the last part of the vegetation period. Building on our results, I was able to suggest improvements to an existing framework describing variations of species-mixing effects along gradient of environmental conditions, in particular by considering non-linear relationships. The results of this thesis also highlighted the importance of considering the interactions between processes, rather than considering them independently of each other.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 202

Modélisation de la mortalité de larves d'Hylotrupes bajulus (L.) par traitement anoxique pour la conservation d'objets d'intérêt patrimonial

Au cours des dernières décénnies, les preuves scientifiques concernant les dangers des insecticides utilisés pour éliminer les insectes présents dans les œuvres d’art des points de vue de la santé du personnel et des visiteurs, des risques environnementaux et des risques pour les œuvres ont conduit le monde de la conservation patrimoniale à se tourner vers des techniques alternatives de désinsectisation. Le traitement par anoxie fait partie de ces techniques. Il s’agit de soumettre les insectes à une atmosphère à faible teneur en oxygène (généralement < 0.1%) pendant une certaine durée pour les éliminer sans causer de dégât à l’œuvre ni laisser de résidus toxiques. Cette technique souffre toutefois de deux inconvénients majeurs. Le premier concerne l’importante durée de traitement nécessaire et le second concerne la sensibilité des résultats à un grand nombre de paramètres environnementaux. Cette étude a pour but d’analyser l’impact des températures élevées comme moyen pour accélérer le traitement ainsi que d’établir des modèles de mortalité intégrant certain paramètres qui influencent l’efficacité du traitement en vue de faciliter la détermination de la durée de ce dernier. Cette étude est réalisée en collaboration avec l’Institut Royal du Patrimoine Artistique (IRPA), et a également pour but de leur fournir un protocole permettant d’assurer une efficacité de traitement proche de 100%. Il a été montré que, conformément à ce qui existe dans la littérature, la température, la durée de traitement et le poids initial des larves influencent significativement la probabilité de mortalité des larves soumises au traitement tandis que le fait que la larve se situe dans une boite de Pétri ou dans une planchette de bois ne joue pas de rôle significatif. Il est également mis en évidence que, si la déshydratation des insectes lors du traitement joue un rôle important sur la mortalité, ce n’est pas le seul mécanisme impliqué. Des recherches supplémentaires sont nécessaires à ce sujet. Des modèles ont également pu être établis et ils permettent de déterminer qu’à température ambiante et 50% d’humidité du flux de gaz, la durée de traitement nécessaire pour éliminer 99% des insectes est de 26 joursAnalyse et modélisation de la mortalité de larves d’Hylotrupes bajulus (L.) lors de traitements par anoxie en vue d’une application à la conservation d’objets d’intérêt patrimonia

Modélisation de la mortalité de larves d'Hylotrupes bajulus exposées à un traitement anoxique pour la désinfection d'oeuvres d'art

Experiments were conducted to quantify the effect of several variables on the mortality of insects exposed to an anoxic treatment in order to generate a model linking mortality to these variables. This study aims to explore the possible interest of using such a model to determine the characteristics of treatment (especially duration) needed to guarantee insect mortality with a given level of probability. Trials were performed on Hylotrupes bajulus larvae, which is a widespread species known for its high tolerance to anoxic conditions. The studied variables are the initial mass of the larvae, the treatment temperature (21, 30 and 40 C), the treatment duration (four durations for each temperature tested) and whether the larva is held in wood or in a petri dish (directly exposed to anoxic atmosphere) during the experiment. It was found that, while the last variable is not correlated with mortality, treatment duration and temperature are significantly and positively correlated with it. Larvae with higher body mass were also shown to have a better resistance to the treatment. Based on these results, a model including insect initial mass, treatment temperature and duration, together with the interaction between these two variables, was determined. This relatively simple model appeared to be a useful tool in overcoming the difficulty in defining the modalities for anoxic treatment in order to reach a given level of mortality.Traitement par anoxie des ooeuvres d'ar

Analyse et modélisation de la mortalité de larves d'Hylotrupes bajulus (L.) lors de traitements par anoxie en vue d'une application à la conservation d'objets d'intérêt patrimonial

Nous avons conçu des expérimentations visant à établir un modèle quantifiant les effets de certaines variables clef sur la mortalité. Cette étude a pour but d’explorer les potentialités d’avoir recours à une approche de modalisation pour faciliter la détermination des paramètres de traitement à appliquer lors d’un traitement par anoxie pour atteindre un objectif donné de mortalité des larves. Les expériences ont été réalisées sur Hylotrupes bajulus (L.) (Capricorne des maisons), une espèce largement répandue dans les collections et connue pour sa grande résistance au traitement par anoxie. Les variables considérées comprennent la masse initiale des larves, la température de traitement (21, 30 ou 40°C), la durée de traitement et l’environnement dans lequel la larve est maintenue lors du traitement (en boite de Pétri ou implantée en planchette de bois). Nous n’avons pas mis en évidence de lien entre l’environnement et la mortalité. En revanche, la durée de traitement ainsi que la température sont toutes deux positivement corrélées à la mortalité. La masse initiale des larves est également un facteur important puisque les larves dont la masse initiale était plus importante ont montré une résistance accrue au traitement. Sur base de ces résultats, un modèle intégrant la masse initiale, la température de traitement, la durée du traitement ainsi que l’interaction entre ces deux dernières variables a été établi. Ce modèle relativement simple semble être un outil intéressant pour résoudre la difficulté que consiste le fait de déterminer les paramètres de traitement nécessaires pour atteindre un certain niveau de mortalité.Désinsectisation des œuvres d'arts par traitement par anoxi

Modelling the mortality of Hylotrupes bajulus (L.) larvae exposed to anoxic treatment for disinfestation of wooden art objects (conference IPM)

Experiments were conducted to quantify the effect of several variables on the mortality of insects exposed to an anoxic treatment in order to generate a model linking mortality to these variables. This study aims to explore the possible interest of using such a model to determine the characteristics of treatment (especially duration) needed to guarantee insect mortality with a given level of probability. Trials were performed on Hylotrupes bajulus larvae, which is a widespread species known for its high tolerance to anoxic conditions. The studied variables are the initial mass of the larvae, the treatment temperature (21, 30 and 40 _C), the treatment duration (four durations for each temperature tested) and whether the larva is held in wood or in a petri dish (directly exposed to anoxic atmosphere) during the experiment. It was found that, while the last variable is not correlated with mortality, treatment duration and temperature are significantly and positively correlated with it. Larvae with higher body mass were also shown to have a better resistance to the treatment. Based on these results, a model including insect initial mass, treatment temperature and duration, together with the interaction between these two variables, was determined. This relatively simple model appeared to be a useful tool in overcoming the difficulty in defining the modalities for anoxic treatment in order to reach a given level of mortality

Species proportions by area in mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.)

Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) dominate many of the European forest stands. Also, mixtures of European beech and Scots pine more or less occur over all European countries, but have been scarcely investigated. The area occupied by each species is of high relevance, especially for growth evaluation and comparison of different species in mixed and monospecific stands. Thus, we studied different methods to describe species proportions and their definition as proportion by area. 25 triplets consisting of mixed and monospecific stands were established across Europe ranging 0from Lithuania to Spain in northern to southern direction and from Bulgaria to Belgium in eastern to western direction. On stand level, the conclusive method for estimating the species proportion as a fraction of the stand area relates the observed density (tree number or basal area) to its potential. This stand-level estimation makes use of the potential from comparable neighboring 0monospecific stands or from maximum density lines derived from other data, e.g. forest inventories or permanent observations plots. At tree level, the fraction of the stand area occupied by a species can be derived from the proportions of their crown projection area or of their leaf area. The estimates of the potentials obtained from neighboring monospecific stands, especially in older stands, were poorer than those from the maximum density line depending on the Martonne aridity index. Therefore, the stand-level method in combination with the Martonne ridity index for potential densities can be highly recommended. The species’ proportions estimated with this method are best approximated by the proportions of the species’ leaf areas. In forest practice, the most commonly applied method is an ocular estimation of the proportions by crown projection area. Even though the proportions of pine were calculated here by measuring crown projection areas in the field, we found this method to underestimate the proportion by 25% compared to the stand-level approach

Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe

Tree species-mixing has been suggested as one option to counteract the adverse effects of global change on tree mineral nutrition, yet the effect of mixing on nutrient availability remains poorly documented. We therefore analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and ilr balances (isometric log transformed ratios between elements or groups of elements) for 261 European beech and 248 Scots pine trees from 15 sites, each consisting of one beech-pine mixed stand and the respective monocultures, across a gradient of environmental conditions in Europe. We hypothesized an overall positive effect of mixing on tree foliar nutrient content, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models and multivariate linear regression models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether the species-mixing effect was related to site fertility. The nutrient composition of beech leaves and pine needles differed significantly for all ilr balances. For both species, significant mixing effects were detected for some nutrients and ilr balances; those effects, however, could not be consistently related to contrasted nutrient composition between species. For most nutrients and ilr balances, the mixing effect was influenced by the site nutritional status, but the pattern differed from expectation: absence or minor differences between monocultures and mixtures at the lower end of the chemical fertility gradient, and maximum differences in rich soils. The contrasting foliar nutrient composition of pine and beech trees and the site nutrient status only partly explained the mixing effects on tree mineral nutrition. Our results claim for a better understanding of nutrientrelated mechanisms associated with complementarity and points towards the need to further expand the existing frameworks to account for the multivariate nature of tree nutrition

Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe

The mineral nutrition of major broadleaved and coniferous tree species is deteriorating in Europe. In that context, species-mixing could be an efficient management tool to improve stand nutrition through effects of species interactions on the availability, uptake or use efficiency of nutrients. We analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and balances of 260 beech and 248 pine trees from 15 pure and two-species stands across a gradient of environmental conditions throughout Europe. We hypothesized an overall positive effect of mixing on tree nutrition, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether mixing was related to site fertility. The composition of beech leaves and pine needles differed significantly for all balances. For both species, significant mixing effects were detected for some nutrients and balances; those effects could however not be related to interspecific differences in foliar nutrient composition. For most nutrients and balances, the mixing effect was affected by the site nutritional status; however, the magnitude of this effect was low, and no consistent pattern could be detected. The contrasting foliar elemental composition of pine and beech trees, and the difference in nutrient status between sites proved insufficient to explain the mixing effects on tree nutrition. While our results suggest limited impact of mixing for those two species, they claim for a better understanding of nutrient-related mechanisms associated with complementarityMiškų ir ekologijos fakultetasVytauto Didžiojo universiteta

Species proportions by area in mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.)

Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) dominate many of the European forest stands. Also, mixtures of European beech and Scots pine more or less occur over all European countries, but have been scarcely investigated. The area occupied by each species is of high relevance, especially for growth evaluation and comparison of different species in mixed and monospecific stands. Thus, we studied different methods to describe species proportions and their definition as proportion by area. 25 triplets consisting of mixed and monospecific stands were established across Europe ranging from Lithuania to Spain in northern to southern direction and from Bulgaria to Belgium in eastern to western direction. On stand level, the conclusive method for estimating the species proportion as a fraction of the stand area relates the observed density (tree number or basal area) to its potential. This stand-level estimation makes use of the potential from comparable neighboring monospecific stands or from maximum density lines derived from other data, e.g. forest inventories or permanent observations plots. At tree level, the fraction of the stand area occupied by a species can be derived from the proportions of their crown projection area or of their leaf area. The estimates of the potentials obtained from neighboring monospecific stands, especially in older stands, were poorer than those from the maximum density line depending on the Martonne aridity index. Therefore, the stand-level method in combination with the Martonne aridity index for potential densities can be highly recommended. The species’ proportions estimated with this method are best approximated by the proportions of the species’ leaf areas. In forest practice, the most commonly applied method is an ocular estimation of the proportions by crown projection areaVytauto Didžiojo universitetasŽemės ūkio akademij