Patrick Vallet

Overyielding in mixed forests decreases with site productivity

Summary There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two-species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak (Quercus petraea Liebl.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst). Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is European beech–Norway spruce, European beech–silver fir and to a lesser extent, silver fir–Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak–Scots pine and sessile oak–European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress-gradient hypothesis.

Characterization of the structure, dynamics, and productivity of mixed-species stands: review and perspectives

The growth and yield of mixed-species stands has become an important topic of research since there are certain advantages of this type of forest as regards functions and services. However, the concepts and methods used to characterize mixed stands need to be understood, as well as harmonized and standardized. In this review we have compiled a set of measures, indices, and methods at stand level to characterize the structure, dynamics, and productivity of mixed stands, and we discuss the pros and cons of their application in growth and yield studies. Parameters for the characterization of mixed stand structure such as stand density, species composition, horizontal (intermingling) and vertical tree distribution pattern, tree size distribution, and age composition are described, detailing the potential as well as the constraints of these parameters for understanding resource capture, use, and efficiency in mixed stands. Furthermore, a set of stand-level parameters was evaluated to characterize the dynamics of mixed stands, e.g. height growth and space partitioning, self- and alien-thinning, and growth partitioning among trees. The deviations and changes in the behaviour of the analysed parameters in comparison with pure stand conditions due to inter-specific interactions are of particular interest. As regards stand productivity, we reviewed site productivity indices, the growth–density relationship in mixed stands as well as methods to compare productivity in mixed versus monospecific stands. Finally, we discuss the main problems associated with the methodology such as up-scaling from tree to stand level as well as the relevance of standardized measures and methods for improving forest growth and yield research in mixed stands. The main challenges are also outlined, especially the need for qualitatively sound data.

Difference in shade tolerance drives the mixture effect on oak productivity

Assessing how species productivity in mixtures is influenced by species shade tolerance and phylogeny would be helpful to improve our general understanding of the relationship between tree species diversity and productivity in forests. We investigated the effects of differences in shade tolerance and phylogenetic distances between pairs of species on the productivity of Quercus petraea growing in 18 mixtures in lowland temperate forests. We calculated the mixture effect as the difference in productivity of Q. petraea in mixed vs. pure stands. Our analyses were based on data from seven annual campaigns of the French National Forest Inventory covering 1,573 plots. The mixture effect on Q. petraea productivity increased when the shade tolerance of the companion species decreased. Compared to its productivity in pure stands, Q. petraea productivity in mixed coniferous stands varied from -14.6% up to +39.6% as the shade tolerance of the companion species inversely varied from highest to lowest. With broadleaved companion species, the mixture effect varied from -10% up to +13.9% with decreasing shade tolerance. We found no effect of phylogenetic distance between Q. petraea and the companion species on the mixture effect. Synthesis: Our results confirm that shade tolerance is an important driver of the diversity effect on productivity at species level in temperate forests and that phylogenetic distance is not a relevant proxy for species functional dissimilarity. This article is protected by copyright. All rights reserved.

Des modèles robustes et génériques de Biomasse. Exemple du hêtre.

Cette etude presente un exemple de modeles de biomasse empiriques etablis pour le Hetre et repondant a des caracteristiques d’exhaustivite (prise en compte des branches et des fines decoupes), de performance (evaluation la plus precise possible) et de robustesse (face aux changements de productivite et aux interventions sylvicoles). Ce niveau de precision a pu etre atteint en ajoutant l’âge des peuplements aux variables explicatives traditionnelles des modeles de biomasse (diametre a 1,30 m et hauteur totale de l’arbre), et en adoptant une forme d’equation allometrique permettant de relier les parametres du modele aux phenomenes biologiques regissant la croissance des arbres. Les modeles ont ete appliques a une chronosequence situee en Bretagne. Les resultats obtenus ont ete compares avec ceux des tarifs de Bouchon (1982) actuellement utilises par les gestionnaires forestiers pour les estimations des stocks de biomasse. Il apparait que les outils utilises actuellement dans la pratique sous-estiment fortement les stocks de houppier qui representent pourtant l’essentiel de la biomasse potentiellement utilisable pour satisfaire les besoins en bois energie. Pour l’estimation des stocks de carbone a l’echelle nationale, l’emploi d’equations de biomasse permet d’augmenter la precision des estimations de 10 % par rapport aux calculs successifs d’un volume puis d’une masse.

Disponibilités en biomasse ligneuse en forêt, dans les peupleraies et dans les haies pour les différents usages du bois

This article provides an assessment of the quantities of potentially exploitable timber in French forests, poplar plantations and hedges for the period 2007 to 2020. The first step consisted in computing the gross available quantities of timber, prior to deduction of the various current consumptions. This was done applying the reference silvicultural scenarios to all the plots in the French national forest inventory, on the basis of their features (species, structure, fertility, age, observed per hectare volume). Current consumption was then subtracted from these quantities. It was estimated using the annual sectoral “forest exploitation” survey in industry and an estimation of fuelwood consumption by households. The outcome is an excess availability of more than 28 million cubic metres of timber per year for bioenergy or pulp uses, and nearly 15 million cubic metres of workable timber, essentially hardwoods.

Une base de données unique en France de cubages d’arbres individuels (volumes, biomasses) au service d’une modélisation générique de la ressource en bois énergie

A base for volume and biomass data of the aerial woody compartments of individual trees was constituted under the ANR EMERGE (French acronym for ‘designing models for a robust, all-purpose estimation of wood energy’) project by seven bodies involved in forest research and management in France (ONF, INRA, Cemagref, IFN, FCBA, CNPF and CIRAD) so as to come up with a robust model of woody resources in forests. These measurements, taken for various logging diameters, sometimes down to the slenderest branches, have been performed since the beginning of the 20th century forming a unique data base in France. The data cover a broad range of age groups, sizes, species, stand structures and environmental factors (soil, climate). Of particular interest is the distribution of the available data in relation to French resources as estimated in the National Forest Inventory so as to use sampling campaigns to collect missing information under the EMERGE project. Another significant feature is the potential for designing robust, all-purpose models of volumes and biomass for various logging diameters depending on species, stage of development, geographic gradients, variations in silvicultural practises and possible effect of date of measurement.