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The development trend of the diameter increment and the basal area increment of single trees and of stands is examined by means of 25 permanent trial plots of the growth and yield research. A number of these plots have been periodically measured for more than 100 years. The plots are not systematically distributed over Switzerland. The periodic resolution of the measurements lies between 4 and 10 years. In the study, the diameter increment is used for single trees and tree classes, the basal area increment for stands. These two quantities are preferred over other quantities such as height and volume because of their simple and reliable measurability. The assessment of a possible trend is done by means of a comparison of the growth with either the mean of the whole observation period, or, in those cases where it is permissible, with yield tables. Despite increasing age, a constant or, as is the case in the most recent observation period, an increase of the diameter and basal area increment can be shown for various tree species and for various forest structures in different areas of Switzerland. Relationships between the observed basal area increment and the development of the climate parameters or the silvicultural treatment can be ascertained only in some cases. Apparently the site productivity has changed or is in the process of changing. On the basis of the available data, none of the possible hypotheses - climate change, change of the CO2, content in the air, influx of substances, extension of silvicultural management etc. - can be either accepted or rejected.

Growth patterns in forests can be monitored using three approaches: assessments on permanent growth and yield plots, tree ring and stem analyses and forest inventories. Each of the three approaches has distinct advantages and disadvantages. To evaluate the effects of the three approaches on the detection and description of growth trends, they are applied to three identical areas: Switzerland, the Toppwald region in the Canton of Berne and the selection forests in the Toppwald region. The results of the case studies are discussed and the specific contribution of each approach for monitoring forest growth is highlighted. The data presented show a clear increase in growth rates in the last decades. However, these growth rates cannot be related to site productivity alone; confounding factors are very likely.

International audience; The Reineke Stand density rule relating stem numbers to the quadratic mean diameter is generally used as a reference for modelling maximal stand density.* The linearity of this relationship after double logarithmic transformation is generally assumed, but it must be questioned for untouched stands and stands with a conventional thinning regime. Curvilinearity is demonstrated for some spruce and beech stands in Switzerland and shown to be statistically representative. This relationship is independent of the site index. It can be interpreted as a change in mortality in young stages mainly due to competition and in older stages more to ageing. * A more accurate estimation of the maximal stand density needs to take into account the important variation around the mean course, known as the yield level. A simple method to assess the yield level of any stand regardless of whether it is thinned or not is presented, based on estimating the effect of a stand opening on the basal area.; La loi de Reineke reliant le nombre de tiges au diamètre quadratique moyen est généralement utilisée comme référence pour la modélisation de la densité maximale de peuplements.* On remet en question la linéarité d'une telle relation dans l'espace logarithmique, dans le cas des peuplements non traités aussi bien que pour ceux traités par des interventions conventionnelles. La curvilinéarité de cette relation est démontrée pour quelques peuplements purs d'épicéa et de hêtre en Suisse et dans sa représentativité statistique. La relation est indépendante de l'indice de fertilité et interprétée comme modification du type de mortalité au cours du développement, due pendant le jeune âge à la seule compétition, puis s'y ajoute celle due au vieillissement.* Une estimation appropriée de la densité maximale des peuplements doit tenir compte des variations importantes autour de la courbe moyenne connue comme niveau de production. Une méthode simple de détermination du niveau de production dans tous les cas de traitement antérieur est présentée, fondée sur une estimation de l'effet de l'ouverture du couvert sur la surface terrière.

International audience; Existing growth and yield plots of pure and mixed stands of Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) were aggregated in order to unify the somewhat scattered sources of information currently available, as well as to develop a sound working hypothesis about mixing effects. The database contains information from 23 long-term plots, covering an ecological gradient from nutrient poor and dry to nutrient rich and moist sites throughout Central Europe.* An empirically formed interaction model showed, that depending on the site conditions, dry mass growth in mixed stands can range from − 46% to +138 % of the growth yielded by a scaled combination of pure stands at equal mixing proportions.* Drawing from the interaction model, overyielding of the mixed stands appears to be triggered by two separate mechanisms. On poor sites, where significant overyielding is commonly found, facilitation by beech offsets nutrient-related growth limitations in spruce. In contrast, overyielding of mixed stands occurs less frequently on rich sites, and appears to be based on an admixture effect, with spruce reducing the severe intra-specific competition common in pure beech stands.* It was concluded that silviculture can accelerate growth of spruce by beech admixtures on poor sites, while growth of beech can be promoted by admixture of spruce, particularly on excellent sites.