Jean-François Dhôte

Individual variability in tree allometry determines light resource allocation in forest ecosystems: a hierarchical Bayesian approach

Tree species differences in crown size and shape are often highlighted as key characteristics determining light interception strategies and successional dynamics. The phenotypic plasticity of species in response to light and space availability suggests that intraspecific variability can have potential consequences on light interception and community dynamics. Species crown size varies depending on site characteristics and other factors at the individual level which differ from competition for light and space. These factors, such as individual genetic characteristics, past disturbances or environmental micro-site effects, combine with competition-related phenotypic plasticity to determine the individual variability in crown size. Site and individual variability are typically ignored when considering crown size and light interception by trees, and residual variability is relegated to a residual error term, which is then ignored when studying ecological processes. In the present study, we structured and quantified variability at the species, site, and individual levels for three frequently used tree allometric relations using fixed and random effects in a hierarchical Bayesian framework. We focused on two species: Abies alba (silver fir) and Picea abies (Norway spruce) in nine forest stands of the western Alps. We demonstrated that species had different allometric relations from site to site and that individual variability accounted for a large part of the variation in allometric relations. Using a spatially explicit radiation transmission model on real stands, we showed that individual variability in tree allometry had a substantial impact on light resource allocation in the forest. Individual variability in tree allometry modulates species’ light-intercepting ability. It generates heterogeneous light conditions under the canopy, with high light micro-habitats that may promote the regeneration of light-demanding species and slow down successional dynamics.

Nitrogen footprint in a long-term observation of forest growth over the twentieth century

Environmental drivers of forest productivity increases have been much debated. Evidence for the suggested role of increasing nitrogen supply is lacking over long-term time scales. Tracking the footprint of environmental factors by using long-term growth records may thus prove decisive. We analysed growth chronologies of common beech in two areas of contrasting nutritional status in France. Dominant height growth was used as a proxy for productivity. Growth was compared between old and young paired stands sampled at the same sites to factor out effects of ageing and site. Growth chronologies were estimated with a statistical modelling procedure. The environmental causality of growth changes was addressed by combining (1) a comparison of growth changes between regions, (2) a regional comparison of growth chronologies with chronologies of environmental factors and (3) growth–environment relationships established from climate/soil data. Historical growth increases followed very similar courses in the two areas. Remarkably, the magnitude of change was 50% lower in the area that had reduced nutritional status and nitrogen deposition. Historical variations in environmental factors and growth were congruent with the roles of nitrogen availability and deposition, and of atmospheric CO2 increase. Low-frequency variations in climate and growth were not coincident. However, our analysis demonstrated the role of climatic anomalies in short-term growth variations. Growth–environment relationships further indicated a nitrogen constraint. These observations corroborate the enhancing role of increased nitrogen availability on forest biomass accumulation previously reported in ecosystem experiments and process-based modelling explorations.

Fagacées: a tree-centered growth and yield model for sessile oak (Quercus petraea L.) and common beech (Fagus sylvatica L.)

Abstract• IntroductionFagacées is a growth model that has been developed for pure Oak or Beech stand management in even-aged high forests and coppice with standards forests. It has been calibrated for the plain forests of northern France.• ObjectivesThis paper presents all the characteristics of this model: general structure, equations, and parameters. It can be classified as a distance-independent tree-centered model. Its main originality is that tree growth is organised according to a top–down organisation.• ResultAt first, the growth is computed at the stand level. Then it is allocated between the individual trees. The paper presents and discusses successively these two parts. In addition, the model delivers the stem profile of each tree: rings and compartments within the stem. Some other additional modelling that have been defined for batch mode purposes are also briefly presented.

Mortality of silver fir and Norway Spruce in the Western Alps - a semi-parametric approach combining size-dependent and growth-dependent mortality

QuestionTree mortality can be modeled using two complementary covariates, tree size and tree growth. Tree growth is an integrative measure of tree vitality while tree diameter is a good index of sensitivity to disturbances and can be considered as a proxy for tree age which may indicate senescence. Few mortality models integrate both covariates because classical model calibration requires large permanent plot data-sets which are rare. How then can we calibrate a multivariate mortality model including size and growth when permanent plots data are not available?LocationTo answer this question, we studied Abies alba and Picea abies mortality in the French Swiss and Italian Alps.MethodOur study proposes an alternative semi-parametric method which includes a random sample of living and dead trees with diameter and growth measurements.ResultsWe were able to calibrate a mortality model combining both size-dependent and growth-dependent mortality. We demonstrated that A. alba had a lower annual mortality rate (10%) than P. abies (18%) for low growth (< 0.2 mmyear−1). We also demonstrated that for higher diameters (DBH ≥ 70 cm), P. abies had a higher mortality rate (0.45%) than A. alba (0.32%).ConclusionOur results are consistent with the mechanisms of colonization-competition trade-off and of successional niche theory which may explain the coexistence of these two species in the Alps. The method we developed should be useful for forecasting tree mortality and can improve the efficiency of forest dynamics models.RésuméQuestionIl est possible de modéliser la mortalité des arbres en utilisant deux covariables complémentaires : la taille et la croissance de l’arbre. La croissance est une mesure synthétique de la vitalité alors que le diamètre est un bon indicateur de la sensibilité aux perturbations et est très fortement corrélé à l’âge de l’arbre, qui détermine la sénescence. Peu de modèles de mortalité intègrent les deux covariables, car cela nécessite, pour les approches classiques, une calibration à partir de données de placettes permanentes qui sont rares. Comment obtenir un modèle de mortalité multivarié, incluant la taille et la croissance, lorsque des données de placettes permanentes ne sont pas disponibles?Localisation géographiquePour répondre à cette question, nous avons étudié la mortalité du sapin pectiné (Abies alba) et de l’epicéa commmun (Picea abies) dans les Alpes suisses françaises et italiennes.MéthodeNotre étude propose une méthode semi-parametrique alternative s’appuyant sur un échantillon d’arbres morts et vivants avec des mesures de diamètre et de croissance.RésultatsNous avons obtenu un modèle combinant la mortalité dépendant à la fois de la taille et de la croissance. Nous avons démontré qu’A. alba avait un taux de mortalité inférieur (10 %) à celui de P. abies (18 %) pour une faible croissance (< 0.2 mman−1). De plus, pour de larges diamètres (DBH >- 70 cm), P. abies a un taux de mortalité supérieur (0.45 %) à A. alba (0.32 %).ConclusionNos résultats sont en accord avec les mécanismes de niche de succession et de compromis entre colonisation et compétition qui sont invoqués pour expliquer la coexistence des deux espéces dans les Alpes. Notre méthode devrait contribuer à améliorer la prédiction du taux de mortalité et la précision des modèles de dynamique forestière.

A flexible radial increment model for individual trees in pure even-aged stands

Un modele generique de croissance radiale darbre en peuplements purs et reguliers. Un modele generique de croissance de type arbre, pour des peuplements purs et reguliers, est propose en utilisant les points communs de differents modeles de la litterature et en tenant compte a travers un parametre des differentes formes observees selon les essences. Lutilisation de ce modele etant une implementation informatique dans un simulateur de sylviculture, sa forme a ete contrainte pour un comportement robuste en dehors de la zone de validite, tandis que les variables directrices ont ete choisies pour faciliter lutilisation par des gestionnaires forestiers. La premiere etape basee sur un jeune peuplement clinal depicea a permis dintroduire simplement leffet de la competition a laide de deux variables explicatives (surface terriere et hauteur dominante). Lutilisation dun large reseau dessais a ensuite permis de prendre en compte leffet simultane de la sylviculture et de la fertilite. Enfin ce modele a directement ete ajuste sur une base de donnees de croissance de Douglas et sest parfaitement adapte a une forme de croissance plus progressive.

Hypotheses about competition for light and water in even-aged common beech (Fagus silvatica L.)

Abstract A distance-independent tree model was built, in order to simulate the dynamics of pure even-aged stands of beech, under various silvicultural scenarios. The formulation and calibration of this model were based on data from permanent plots, installed around the beginning of the century. The modeling method was largely empirical. However, to overcome some limitations of the database, the main features of the model were justified by means of simple biological assumptions. The influence of tree social status on growth is featured with a segmented linear differential equation, organized around a threshold size. The properties of this threshold size (increasing with age, independent of stand density and structure) are interpreted as the consequence of a predominant aerial competition for light, between trees vertically structured in two storys (upper and lower storys). Additional trials performed to test the validity of the model reveal that the threshold may have much more complex temporal patterns; in the case of remarkable climatic sequences the possibility of an underground competition for water uptake is discussed. Finally some reflections about the attitude of growth and yield models with regard to the physiological and ecological knowledge are proposed. Because all the complexity of basic mechanisms is not fully described, or described at time and spatial scales which are not appropriate for growth and yield problems, the empirical phase can probably not be avoided in the building of growth models. At least, an effort for simplifying their mathematical machinery will allow us to be more lucid with regard to their biological content and more confident in their qualitative behavior.

A model bridging distance-dependent and distance-independent tree models to simulate the growth of mixed forests

Abstract• It is widely believed that distance-independent tree models fail to take into account the complexity of mixed stands due to the fact that spatial structure often has a greater impact on growth and dynamics in mixed stands than in pure stands. On the other hand, distance-dependent tree models are difficult to use because they require a map of the stand, which is not only very costly but also impracticable in a routine management context.• This paper reports the development of a model bridging distance-dependent and distanceindependent tree models, and that is designed to simulate the growth of a mixed forest. The model used distributions of the number of neighbours to reconstruct tree neighbourhoods and compute the competition indices needed as inputs to the growth model.• Data were collected from a mixed forest of sessile oak and Scots pine in central France. The study showed that local competition indices explained a significant proportion of growth variability and that intraspecific competition was greater than interspecific competition. The model based on neighbourhood distributions gave consistent predictions compared to a distance-dependent model.• This type of model could be used instead of distance-dependent models in management contexts.Résumé• On considère généralement que les modèles arbre indépendant des distances ne permettent pas de rendre compte de la complexité des peuplements mélangés. En effet, la structure spatiale a souvent un rôle plus important sur la croissance et la dynamique dans ces peuplements que dans les peuplements purs. Les modèles arbre dépendant des distances sont quant à eux difficile à utiliser, car ils nécessitent une cartographie du peuplement qui est une information très coûteuse à obtenir et qui n’est pas disponible dans un cadre de gestion courante.• Cet article présente un modèle intermédiaire entre un modèle arbre indépendant des distances et un modèle arbre dépendant des distances. Ce modèle a été développé pour simuler la croissance de peuplements mélangés. Il utilise des distributions de nombre de voisins pour reconstruire le voisinage des arbres. Ces voisinages reconstruits permettent ensuite de calculer les indices de compétition nécessaires dans l’équation de croissance.• Les données ont été récoltées dans des peuplements mélangés de chêne sessile et de pin sylvestre dans le centre de la France. Ce travail montre que des indices de compétition locaux expliquent une part significative de la croissance individuelle et que la compétition intraspécifique est supérieure à la compétition interspécifique. Le modèle basé sur les distributions de voisinage donne des prédictions cohérentes par rapport au modèle arbre dépendant des distances.• Ce type de modèle pourrait être utilisé à la place des modèles arbre dépendant des distances dans des contextes de gestion.

Biases in the estimation of size-dependent mortality models: advantages of a semiparametric approach

Mortality rate is thought to show a U-shape relationship to tree size. This shape could result from a decrease of competition-related mortality as diameter increases, followed by an increase of senescence and disturbance-related mortality for large trees. Modeling mortality rate as a function of diameter is nevertheless difficult, first because this relationship is strongly nonlinear, and second because data can be unbalanced, with few observations for large trees. Parametric functions, which are inflexible and sensitive to the distribution of observations, tend to introduce biases in mortality rate estimates. In this study we use mortality data for Abies alba Mill. and Picea abies (L.) Karst. to demonstrate that mortality rate estimates for extreme diameters were biased when using classical parametric functions. We then propose a semiparametric approach allowing a more flexible relationship between mortality and diameter. We show that the relatively shade tolerant A. alba has a lower annual mortality rate (2.75%) than P. abies (3.78%) for small trees (DBH <15 cm). Picea abies , supposedly more sensitive to bark beetle attacks and windthrows, had a higher mortality rate (up to 0.46%) than A. alba (up to 0.30%) for large trees (DBH ‡ 50 cm).

Epicormic ontogeny on Quercus petraea trunks and thinning effects quantified with the epicormic composition

Abstract• Effects of thinning on epicormics have rarely been demonstrated especially due to inaccurate surveying methods.• Our objective was to assess the effect of contrasted thinnings on the ontogeny of epicormics on sessile oak. We used the epicormic composition defined as the arrangement of epicormics in different classes (isolated bud, clustered buds, short and long shoots and “picots”) and quantified by the total frequency of epicormics and the proportion of each class.• Epicormic composition was recorded in a silvicultural experiment testing highly contrasted thinnings, at 3 different stand stages and on lower (0.5–3 m) and upper (3–6 m) boles. Ageing provoked an accumulation of bud clusters and of picots. After thinning, epicormic shoots emerged mainly from (1) still present short epicormic shoots, (2) buds either isolated or in clusters depending on the stand stage. Upper boles bore epicormic compositions close to those observed on lower boles in the few preceding years. Upper logs were more reactive than lower boles.• To conclude, the epicormic composition was a relevant tool to follow the dynamics of the epicormic ontogeny and to demonstrate the effects of thinning on it.

Résistance aux vents forts des peuplements forestiers, et notamment des mélanges d’espèces

Cet article propose un bilan des etudes empiriques, phenomenologiques et mecanistes sur la resistance aux vents forts des peuplements forestiers, notamment des melanges d’especes. La resistance des peuplements purs depend en premier lieu des caracteristiques de l’evenement climatique, de l’essence principale, de la hauteur des peuplements et de la profondeur de sol prospectee par les racines. Des eclaircies fortes et recentes aggravent le risque. La structure interne des peuplements (densite, structure verticale...), qui modifie relativement peu les champs de vitesse de vent et l’intensite des turbulences, intervient dans une moindre mesure dans l’explication des degâts. Un classement de resistance des essences en peuplement pur est propose. Bien que, en melange, les processus d’imbrication des houppiers et des racines puissent moduler cette resistance, les etudes phenomenologiques realisees jusqu’ici montrent que le taux de degâts en peuplements melanges s’explique avant tout par la stabilite intrinseque (c’est-a-dire observee en peuplement pur) des essences en presence et par leur proportion relative. Le melange peut donc avoir des effets positifs ou negatifs, selon les cas. En revanche, il permet de limiter la frequence des grandes arasees. Quelques recommandations sylvicoles sont proposees, a integrer dans une analyse globale economique et ecologique. L’introduction d’essences resistantes en melange est particulierement recommandee dans les situations de forte vulnerabilite ou a forts enjeux. Un diagnostic de la resistance des essences devra etre realise sur la base du classement propose et des caracteristiques stationnelles. Enfin, tout comme pour les peuplements purs, une intensification de la sylviculture permettra de limiter les risques.