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International audience; AbstractKey messageUsing the three characteristic points of a forest stand, dg (mean quadratic diameter), dmin(diameter of the smallest tree) anddmax (diameter of the largest tree), appears informative enough to determine the parameters of the whole diameter distribution and, hence, the standing volume, with an accuracy of 2–3%. This is related principally with a particular feature of the Weibull distribution function, and the empirical dependency of the main scale parameter α + β from the mean quadratic diameter (dg): This allows the prediction of the parameter β with an unexpectedly high likelihood. This feature could be used for growth modelling as well as inventory purposes, at least for monospecific and even-aged stands and, maybe more, because this feature is proper to the function itself.ContextOne of the most appealing applications of diameter distribution functions is to predict compliant stand diameters without needing to tally all stems, but in determining the function parameters only on the base of simple stand characteristics. This can be applied for yield model construction or inventory purposes.AimsThe aim of this paper is to compare different methods of estimating the Weibull distribution parameters, partly based on parameter recovery method (PRM). They use a remarkable, empiric property of the Weibull function. Their performances are assessed in comparison to real distributions from a wide database of permanent Swiss yield plots repeatedly measured (time series) for Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.).MethodsThe Weibull distribution offers the advantage of simple but reliable estimation procedures. One of these is the main (scale) parameter β being given at a remarkable point of the function free, i.e. independent from the shape parameter γ. Because dg lies very close to this point, it correlates empirically very tightly with this parameter and thus allows for a trustful simple estimation. We compare, with appropriate statistic tests, real distribution with such obtained with the usual maximum likelihood estimation (MLE) of the Weibull parameters and those obtained with these new procedures.ResultsThe results obtained from a set of 800 yield plots of regular spruce stands and 596 of beech in Switzerland illustrate the good performances of the two much simpler procedures. The accuracy of estimating the standing volume is about 1.4% for beech and 2.8% for spruce when the site index (SI) is known.ConclusionThe three considered characteristic stand attributes (dg, dmin and dmax) appear robust enough for determining the diameter distribution with a respectable accuracy. This is enough reason for a revival of the old, but very ingenious, method of angle count sampling of Walter Bitterlich (1947).

Aim of this study was to determine the long- term effects of thinning regime with different intensities on the stand and tree social classes development, particularly regarding the old question of compensative effect between stocking and girth improvement using the example of Douglas-fir. Data provide from a thinning experiment in one site of Swiss Central Plateau, from age 11 until 41 years containing six treatment variants (three planting densities and two level of stand density) repeatedly mea- sured six times, as well as from seven permanent yield plots times series aged between 17 and 127 years. We present as well the real observed characteristics in term of stem numbers distribution, diameter increment per d.b.h. categories, as results from simulation over the whole pro- duction time until 105 years with an appropriate growth simulator, in terms of net value increment (MAIv). Our results show that the effect of thinning works mainly on the social medium categories, in terms of number and girth improvement. Social dominant trees are more or less untouched by thinning, demonstrating that there are in every stand a quite important number of self-dominating trees which do not need particular silvicultural help to maintain their dominant status in the upper storey. The expected compensation between stem number reduction and girth improvement shows that the remaining stand density is determinant on the net value level. The so-called mass effect seems really more effective. The best way to improve a stand value appears to be artificial pruning. Because a thinning regime should not consider only the economic productivity but take in account risk, stability and resilience and particularly the way of renewal, we suggest a way for optimising the thinning regime com- bining effect of biological rationalisation and risks.

Our approach to thinning has changed over the last 50 years: why? (Essay) Thinning methods and principles have always fluctuated between an approach based on moderate interventions, aimed at maintaining a sufficient stocking level (the so-called “volume effect”) and the liberation of crowns of a limited number of trees. Nowadays Schädelin's concept of selective thinning focused on trees with high potential for the future, based on the efficient removal of those competitors which cause the most interference, is widely accepted. However, as regards the intensity and strength of thinning there are different suggestions in terms of number of trees to be removed and the extent of liberation of the crowns. Moreover, there is no consensus on the transition between the phase of compression in the young stand and the first thinnings, nor on the optimal final stand density to be aimed at. After the dramatic fall in income from forests, it was necessary to revisit the principles of tending young stands, as the costs were excessively high especially because of the high number of trees. It now appears more efficient to use the natural, and free, processes of social differentiation. Interventions founded on the so-called biological rationalisation make possible a significant reduction in costs. The results of the economic analysis of the various thinning regimes, measured by the net average increment in value, net of harvesting costs, show clearly the advantage of the volume effect and of maintaining sufficient stand density. However, it is also necessary to bear in mind stand stability, especially towards the end of the rotation period. Stand stability makes it possible to keep open, in both space and time, options for natural regeneration, which corresponds best to our goals of a multifunctional silviculture in the most efficient fashion.

International audience; The specific goals of this paper are the following:1. To determine, using beech-dominated pristine forests as anexample,themaindrivingforcesofgenerationrenewal intermsoflifecycledurationandspatialrenewalpatterns; 2. Toillustratecontrastinglightrequirementsduringrenewal by comparing pristine oak and beech forests; 3. To elucidate the lower limits of stocking and canopy openness for silvicultural renewal in small clusters based on pristine forest research; 4. Topresentanapproachforimplementingthesefindingsin a liberal and versatile forest management

The basic principle of Pro Silva silviculture is to be multifunctional and to seek to combine into a harmonious whole all the benefits provided by the forest. It is thus a management system constantly adapted to follow the evolution of different needs and requirements. On the occasion of the International Year of Biodiversity, it is here explained why this form of management meets in an optimal manner the interests of biodiversity without however pushing into the background the other functions, in particular carbon management. Intrinsically it corresponds to the principles of sustainable management, maintaining the balance between economic, social and ecological interests.

The name alone of continuous cover forest demonstrates the very essence of a concept of forestry management whereby there is continuous forest cover and sudden jumps between the generations are avoided. Although this concept is highly modern, it was developed by Alfred Möller in Prussia in about 1920 in opposition to the mechanistic concept of gaining maximum profit from the soil. The ecosystem and its constituents, the trees with their individual growth characteristics, represent a production system combining ecology and economy using the natural processes of autoregulation known as bio-rationalisation. The application of the concept does however demand a high level of silvicultural competence. For finally the forester himself must determine, on the spot and with the evidence before him, which method of treatment will be best suited to the situation, taking into consideration all the prevailing conditions on the site and the structure of the stands present. This concept, which is highly suitable for multipurpose forest management, is perfectly adapted to the Swiss ecoforestry.

Since the present climate in Switzerland includes abundant rainfall, the climatic changes should not present any serious danger of the ground drying out. In fact, higher precipitation levels are predicted. So it would seem that climate is likely to become more luxuriant rather than more dry. These conditions – together with the entry of nutrients with the rainfall – favour the growth of ash and maple but not of oak. Even with an increase in stressful dry summer periods, a consideration of the broad areas of distribution of indigenous species shows they possess a sufficiently great adaptability. Research into the effects of physiological stress on fir trees from different provenances shows in particular that those with local origins are better adapted to the habitat than those from elsewhere. The danger of an increase in storms should be countered by strategies aimed at increased resistance – through mixed plantation regulation and thinning out – rather than avoidance strategies. Ecoforestry is basically well armed for this task. It should be even more orientated towards the adaptability and resilience of forests.

Canton Neuchâtel has close links with the emergence of modern plenter forest management, an integrated and a very real system in a sylvicultural sense and not to be confused with earlier plenter forestry, a disunited and uncontrollable method of management that was widespread before the introduction of modern forestry methods. We must attribute this development to the remarkable personality of Henry Biolley; it was in Gurnaud's control method that he discovered an almost experimental instrument with which to measure progress in management methods. The article traces the historic context of this emergence and the reasons why this choice is seen as a success today.

This paper examines natural mixed regeneration development in a forest gap, which was established using the irregular group regeneration technique. This allowed mixture evolution and height growth at different light gradients to be compared over the whole phase up to the thicket stage. The study was carried out on site conditions with a full base saturation and a largely sufficient water regime, i.e. in geobotanic terms, a eutrophic beech forest. Results show that, starting from a heterogeneous mixture of spontaneously regenerated seedlings, a considerable reduction in tree species in the dominant storey layer took place over 15 years. The extent of outmixing differed depending on light conditions. It was also clear that intrinsic competitive effects led more to outmixing than previously thought. Ash dominated in the bright gap centre and not only outgrew, but also suppressed other species. Beech dominated along the edge of the patch and under the old stand's shelter. The light requirements, which favour beech or ash, are determined. The reasons for this outmixing are discussed, in particular why mountain maple (Acer pseudoplatanus) was unable to compete with ash. The maintaining of uncompetitive tree species and mixtures requires very intensive precocious tending operations at thicket stage. They cost a lot more than new opportunistic tending methods.

We express concerns about the current trend of abandoning the policy of multi-functional forest use. The proposals of the Galileo Report for function segregation rely too much on the perceived advantages of economies of scale and mechanisation. Other methods of rationalisation are based on silvicultural skills and so-called biological rationalisation. These extensive, situative tending methods bring about a huge cost reduction comparable to those realised by mechanical rationalisation. They are also much more compatible with the delicate balancing act required by multi-functional forest use, which is demanded by society today. It is therefore important to further develop existing forest management methods, and to view them not only as a means of harmonising differing interests at a regional planning level, but also as a means of practical implementation at an operational level. The fates of forestry and the timber industry are closely linked. The main goal is no longer the mass production of timber products, but rather how to use technological innovations to develop new, high-value products.

Following measurements of dead wood (20 to 40 years) at various developmental stages of the life cycle of selected virgin forests of the 1 st to 7 th altitudinal zone the relation of its course was derived. The dynamics and course of dead wood in the life cycle of virgin forests were best represented by a polynomial of the third degree. An analysis confirmed that virgin forests consisting of stable tree species with approximately the same physical age (Boky) showed small differences between maximal increase and decrease in necromass during the whole development cycle. Virgin forests at sites rich in nutrients consisting of several tree species with various physical age had high values of necromass during their whole development cycle. Spruce stands at the upper forest boundary also had a relatively high ratio of necromass at the advanced optimum stage.

Summary In the Caspian region of northern Iran, research was carried out into the structure of pure and mixed stands of natural oriental beech (Fagus orientalis Lipsky) and the presence of regeneration in natural gaps of differing sizes. Results showed that beech stands usually occurred naturally in small groups of a similar age, whereas the age structure of larger areas (>1 ha) was more uneven. The total volume of stands investigated ranged from 480 m3 in the pure stand to 600 m3 in the mixed stand. Only one-third of total trees were classed as large timber (>55 cm diamater at breast height), which in turn accounted for two-thirds of total stand volume. Although beech regeneration within the large gaps (>0.1 ha) showed the best growth characteristics, more well-formed saplings were found within medium-sized gaps (0.02‐0.05 ha). From the results, it was concluded that the group selection system, with the removal of two to four main trees, could be a suitable method for managing oriental beech stands.

Transformation from a regular forest to a full irregular one is a long and difficult task which needs time and frequent intervention. Looking back over Swiss experience allows us to analyse the reasons for success and failure. One of the main reasons for transformation failure is that differentiation has been established too evenly over the entire surface and has been begun too late. The tendency towards forming over-regular two-storied stands increases if it is not possible to maintain adequate crown cover of the upper storey over the entire transformation period. This is linked to the ageing of the first generation main trees and to good crown formation. It is possible to establish concrete rules about transformation management and decision-making during the different transformation stages. As with plant succession, transformation follows a set sequence of stages which must occur in the correct order. The process must also start at the correct time and in the correct manner. A decision must be made whether to attempt transformation on the present generation of trees or on the subsequent one. Otherwise, one runs the risk of unwittingly opening the cover when mature trees die, before transformation has been achieved. For such purposes, we use a specific hierarchy of objectives in the decision-making process. These are principles such as stability; longevity of cover-building trees; optimising regeneration with regard to recruitment and its conforming to an irregular vertical structure. This hierarchical order must be followed. The activation of regeneration and recruitment is of utmost importance in the subsequent transformation stages.

An analysis was carried out, over a period of 40 years, of the dynamics of deadwood in two mixed virgin forests in the West Carpathians in the phytogeographic domain of pure beech and mixed fir-beech forests. Deadwood (expressed as % of dendromass, i.e. living and dead biomass) is subject to great variations at different evolutive phases. The relation between deadwood and living biomass in the aggradation phase (juvenile growing phase)is 1:2 in the Badín forest and varies between 1:2 and 1:3 in the Dobroč forest, while a maximum is reached in the optimal phase with a variation between 1:5 and 1:6.

Semantically speaking, the term «near-natural» has found its way into the language because of a humanist approach to nature, namely, how nature relates to culture and mankind; this is in contrast to more fundamental ideologies where nature alone is the central focus. The Swiss approach to this kind of silviculture, expounded by Schädelin and Leibundgut, has been a recognised school of thought in Switzerland for over 70 years. It advocates an all-encompassing outlook on forest management, and is a more comprehensive method than being restricted by one particular silvicultural technique. It is a liberal, pragmatic form of forest utilisation, which promotes not only renewal on a tree-by-tree basis (plentering), but also the renewal and growth of trees in discrete generations using progressive group felling; all this with the same goal in mind, that is, the creation of structured mixed forest. The following six principles characterise this approach: use of local yield potential; silvicultural intervention combining timber use and tending goals; continual renewal; maximising the use of mixed tree populations; use of natural regeneration; and a liberal felling policy. The procedure is characterised by the liberal felling policy, which can be used on a per-situation basis to decide which felling technique should be used. Near-natural silviculture is a direct response to new demands which consider the forest as having its own living space. However,the main challenge for Swiss forestry today lies more in resolving its economic problems rather than improving its proximity to nature. Liberalisation of trade has placed the viability of wood production in a precarious situation. Future silviculture must be more cost-efficient, and must be based on biological rationalisation. This requires a fundamental shift in silviculture towards a less intrusive policy which relies more on natural automation than in the past. It must also focus on individual trees and situative interventions, rather than focusing on a stand scale; this allows varying development potential of individual trees to be taken into consideration. In summary, real multipurpose use calls for flexible solutions, which can integrate silvicultural interventions on a much more varied scale than in the past.

Current economic trends call for a review of the costefficiency of silvicultural systems. This article considers how ecological rationalisation can work alongside organisational and technical measures to minimize the cost of tending work.

Although the majority of the world’s forest ecosystems are dominated by uneven-sized mixed species stands, forest management practice and theory have focused on the development of plantation monocultures to maximize the supply of timber at low cost. Societal expectations are changing, however, and uneven-aged multi-species ecosystems are often believed to be superior to monocultures in addressing a wide range of expectations. This chapter attempts to clarify terminology and define, albeit somewhat simplistically, continuous cover forestry (CCF) as opposed to rotation management (RFM). RFM is characterized by three distinct development phases: planting–thinning–clearfelling. At least two of these phases, and sometimes all three, occur simultaneously in CCF. The second section of this chapter describes the development of the European Plenter Forest system, which, being based on a sound theory of sustainable use and an extended period of practical applications, is often considered to be the “Archetype” of CCF. Prompted by a rising interest in continuous cover forestry, specific methods have been developed in many other regions. Accordingly, the third section briefly covers some approaches in China, Chile, Mexico, the United States and South Africa. The forth section introduces adaptive approaches to CCF. The ideal J-shaped target structure is an artificial construct which is easy to implement and thus useful for management. The problem is to define it. This problem is aggravated when foresters wish to (a) mimic natural structures which do not resemble inverse J-shaped diameter distributions and (b) when there is a need to respond to changing economic and environmental conditions.

In the development of pure beech and pure spruce pristine forests, we assume there should be a continuous transition from full stocking to successive disintegration and, finally, total replacement of the first generational wave at the end of its lifetime, dependent on mortality due to overcrowding as well as ageing, leading newly recruited cohorts to grow up from below. A survey of stem number distribution from permanent plots measured repeatedly in six pristine beech reserves and four spruce reserves, mainly in Slovakia, was considered as well as surveys from yield plots in untouched stands in Switzerland on comparable site conditions. From 64 (54, respectively) surveys in virgin forests, we separated (if necessary) the original regeneration waves from successive waves for calculation of stand density development. Comparison with a previous model for full stand closure according to Reineke’s density rule allowed the risk of stand release to be evaluated and determined. The risk of stand closure release is substantially higher for spruce than for beech because of differences in longevity and in vulnerability to storms. Decay does not seem to be the decisive factor, until advanced ageing is reached. Consequences for the understanding of natural forest dynamics, especially reinitiation through successive waves of regeneration are drawn on the basis of silvicultural expertise from continuous cover systems. The density threshold limit for the accession of a second growth in the upper storey is attained at an age of about 180 years for beech and 140 years for spruce. This corresponds to about 70% or 40% of the life cycle, respectively. Vulnerability to stand release should be considered as a gradual (discrete) rather than a continuous phenomenon that depends on the complex relationship between the collective cohesion within a stand and a propensity for weakening factors. This suggests that the establishment of large disclosure patches could be the driving force in the renewal of these virgin forests.

L'arrivee des profileurs a scie circulaire a permis d'augmenter considerablement les cadences de sciage et induit, parce que le rendement de cette forme de premiere transformation se fonde sur les economies d'echelles, une forte demande de petits bois. De la a faire croire qu'il ne faut plus viser la production traditionnelle de gros bois, il y a un pas que l'on demontre ne pas devoir faire. Dans une perspective de multi-fonctionnalite et de retention du gaz carbonique, principal gaz a effet de serre, il devient imperatif de favoriser l'utilisation du bois dans l'habitation, si possible sous une forme facilement recyclable. L'avenir du bois depend plus de la promotion de la meilleure valeur ajoutee et de l'innovation technique fondee sur le savoir-faire que les economies d'echelles. Cela est possible avec des produits modernes tels que lamelle-colle, lamelle-feuillete, aussi bien pour les resineux que les feuillus. La gageure est de produire des elements de haute utilite avec du bois de moyenne voire mediocre qualite. Il n'y a aucune ineluctabilite technique a la transformation de gros bois et encore moins a leur production dans les concepts sylvicoles. Il faut donc proposer pour nos conditions de production des modeles de transformation se basant sur l'optimisation des debouches par le tri de la qualite, a differents niveaux, et l'acheminement vers des produits finaux de haute valeur ajoutee, et adapter la technique de transformation a ces ressources.

European Journal of Forest Research, 125 (3), ISSN:1612-4677, ISSN:1612-4669

The aim of this study is to scrutinise whether a dynamic equilibrium model based on sustainability of the demography is valid for pure beech plenter forests. Broad-leaved trees in general, and beech in particular react differently to individualisation and to shade than conifers, because of differences in space occupancy and their reaction to shade. Therefore application of the plenter system (or selection system) presents other constraints than for conifers forests. Sustainability must be assessed at stand level, because the plenter system functions without cover interruption and so needs continuous recruitment growing from the stand bottom up. The algorithm used for determination of demographic equilibrium depends on knowledge of the recruitment from below, stem migration over time (dependent on diameter increment), and removal for cultural and harvesting purposes and their dependency from stand density. Data used in this study comes from three permanent research plots in the pure beech plenter forests of Langula (Thuringia basin, Eastern Germany). The determination of the optimal stand density ensuring equilibrium uses an incremental growth model based on a basal area oriented density index (GCUM). It emphasises the phenomenon of growth extinction with increasing stand closure. Equilibrium is gained for a standing volume about 250 m3/ha and Basal area 22 m2/ha. This occurs at a much lower stand density than for classical coniferous plenter forests. The reasons for these differences and the silvicultural consequences are discussed., Modélisation de l'équilibre démographique en futaie jardinée de hêtre en Allemagne de l'Est. L'objectif de cette étude est de vérifier l'applicabilité d'un modèle d'équilibre démographique aux forêts jardinées de feuillus. Les feuillus réagissent différemment des conifères à l'individualisation due à l'irrégularité car ils utilisent différemment l'espace vital et l'ombrage. L'application des principes du jardinage est donc liée à des contraintes limitantes différentes que pour les forêts jardinées classiques de conifères. L'équilibre se détermine au niveau du peuplement, car en futaie jardinée il n'y a jamais interruption de couvert. Il faut donc une continuité du recrutement sous couvert. L'algorithme utilisé pour déterminer l'équilibre démographique se fonde sur la mesure du recrutement, de la migration des tiges dépendant de la croissance radiale et des éliminations à des fins culturales et de récolte, le tout dépendant de la densité des peuplements. Les données proviennent de trois parcelles d'essai de futaies jardinées de hêtre de Langula en Thuringe (Allemagne de l'Est). Le modèle incrémentiel de détermination de la densité optimale de peuplement qui assure l'équilibre démographique utilise un indice de densité variable GCUM fondé sur la surface terrière des plus gros. Il met en évidence un phénomène d'extinction du recrutement avec l'augmentation de la fermeture des peuplements. L'équilibre jardiné s'obtient pour des volumes sur pied de l'ordre de 250 m3/ha et une surface terrière de 22 m2, ce qui est nettement plus bas que dans le cas des forêts jardinées de conifères. On discute les raisons de cette différence et les conséquences sylvicoles.

International audience; This paper outlines the prerequisites for research into natural and opportunistic methods of vegetation control. These methods must be chemical-free and respect naturalness, but must still be acceptable in terms of cost. These alternative regeneration control methods are discussed and illustrated. Firstly, weed control is influenced by the degree of shading during the regeneration process. The principle of antibiosis can also be used by sowing an admixture of herbs: these are invasive enough to flourish but favour the soil's biological activity (or other factors) enough to reduce weed development and so assist the regeneration process. In the long term, natural plant dynamics in temperate Central Europe lead to fairly regular stands, which are composed of relatively poor species, due to natural outmixing processes. This paper examines natural regeneration development in a 0.25 ha gap. Results show that starting from a very heterogeneous mixture of spontaneously regenerated seedlings (20 species), a considerable reduction in tree species in the dominant stand layer has taken place after 15 years. Outmixing differs depending on light conditions. Ash, which is an intolerant species, dominates in the gap centre, and beech, a tolerant species, dominates along the edge of the patch and under the old stand's shelter.; Méthodes opportunistes de contrôle du développement de la végétation de parterre de coupe, inspirées de la dynamique naturelle de la succession végétale avec mention particulière des effets de démélange dans une régénération en trouée. Le présent texte souligne les conditions-cadre de la recherche de méthodes opportunistes de contrôle de la végétation du parterre de coupe. Elles se veulent libres d'intrants chimiques, respectueuses de la naturalité et acceptables en termes de coûts. On présente et illustre de telles méthodes alternatives de contrôle de la végétation. C'est tout d'abord l'utilisation de l'ombrage pendant le processus de régénération qui permet de bien gérer la végétation adventive. D'autres possibilités sont l'utilisation des principes d'antibiose et notamment l'installation par semis dirigé d'un mélange de plantes herbacées suffisamment prospère pour empêcher la prolifération d'autres plantes plus gênantes et pas trop incompatible avec un bon développement des ligneux désirés et ceci tout en agissant de façon favorable sur l'activité biologique du sol. À long terme le développement de la végétation ligneuse aboutit pour les conditions d'Europe centrale tempérée à la constitution de peuplements relativement pauvres en essences à cause de phénomènes de démélange. On présente les résultats du développement d'un rajeunissement naturel dans une trouée moyenne (d'environ 25 ares), constitué au départ de 20 espèces qui après 15 années démontre un effet très significatif de réduction du mélange d'essences dans l'étage dominant. Ce processus de démélange diffère en fonction de l'ombrage. Le frêne espèce héliophile domine les autres essences au centre de la trouée et le hêtre, sciaphile, dans les bords et en lisière.

Forestry, 75 (4), ISSN:0015-752X, ISSN:1464-3626

"Forêts et tempête" Expertise collective sur les tempêtes, la sensibilité des forêts et sur leur reconstitution; National audience

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

Small populations are prone to genetic drift as a consequence of random sampling effects. We investigated whether we could detect such random sampling effects in the English yew (Taxus baccata), a dioecious conifer species occurring in scattered populations in Switzerland. Seven pairs of small and large populations were analyzed using random amplified polymorphic DNA (RAPD) marker bands from 20 individuals per population. Several genetic parameters (mean marker band frequency deviation, molecular variance, population differentiation) indicated that small populations experienced genetic drift. These genetic differences between small and large populations of yew were paralleled by an increased sex ratio bias towards a higher number of females in the small populations. Our findings support earlier assumptions that the Swiss occurrences of yew may be described as metapopulation dynamics, characterized by local colonization and extinction events leading to the observed genetic drift.