Multilevel logistic regression models were constructed to predict the 5-year mortality of Scots pine (Pinus sylvestris L.) and pubescent birch (Betula pubescens Ehrh.) growing in drained peatland stands in northern and central Finland. Data concerning tree mortality were obtained from two successive measurements of the National Forest Inventory-based permanent sample plot data base covering pure and mixed stands of Scots pine and pubescent birch. In the modeling data, Scots pine showed an average observed mortality of 2.73% compared to 2.98% for pubescent birch. In the model construction, stepwise logistic regression and multilevel models methods were applied, the latter making it possible to address the hierarchical data, thus obtaining unbiased estimates for model parameters. For both species, mortality was explained by tree size, competitive position, stand density, species admixture, and site quality. The expected need for ditch network maintenance or re-paludifi cation did not infl uence mortality. The multilevel models showed the lowest bias in the modeling data. The models were further validated against independent test data and by embedding them in a stand simulator. In 100-year simulations with different initial stand conditions, the models resulted in a 72% and 66% higher total mortality rate for the stem numbers of pine and birch, respectively, compared to previously used mortality models. The developed models are expected to improve the accuracy of stand forecasts in drained peatland sites.

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Models for individual-tree basal area growth were constructed for Scots pine (Pinus sylvestris L.), pubescent birch (Betula pubescens Ehrh.) and Norway spruce (Picea abies (L.) Karst.) growing in drained peatland stands. The data consisted of two separate sets of permanent sample plots forming a large sample of drained peatland stands in Finland. The dependent variable in all models was the 5-year basal area growth of a tree. The independent tree-level variables were tree dbh, tree basal area, and the sum of the basal area of trees larger than the target tree. Independent stand-level variables were stand basal area, the diameter of the tree of median basal area, and temperature sum. Categorical variables describing the site quality, as well as the condition and age of drainage, were used. Differences in tree growth were used as criteria in reclassifying the a priori site types into new yield classes by tree species. All models were constructed as mixed linear models with a random stand effect. The models were tested against the modelling data and against independent data sets.

Size structural dynamics of naturally established Norway spruce (Picea abies (L.) Karst.) stands growing on peatlands drained for forestry were investigated. The study was based on modelling of diameter at breast height (DBH) distributions of repeatedly measured stands in southern Finland. The Weibull function was used to parameterize the DBH distributions and mixed linear models were constructed to characterize the impacts of different ecological factors on stand dynamics. Initially, the positive skewness of the DBH distributions increased after drainage as a result of increases in stem numbers and a reduction in mean diameters. Simultaneously, the size inequality among trees increased. These changes were due to regeneration and (or) ingrowth and indicated only little competition from the larger trees. Subsequently, the DBH distributions changed from positively skewed to normal and finally to negatively skewed resulting from tree growth and a reduction in the number of small DBH trees. This indicated increased asymmetric intertree competition. Size inequality did not change during this later stage in stand development, suggesting a concurrent component of symmetric competition. Thinnings had little impact on DBH distribution trends. The observed stand dynamics allow the allocation of growth resources to the desired crop component by appropriate silvicultural treatments.Résumé : La dynamique de peuplements naturels d'épicéa commun (Picea abies (L.) Karst.) établis dans des tourbiè-res drainées a été analysée en termes de taille et de structure. L'étude se base sur la modélisation des distributions du diamètre à hauteur de poitrine (DHP) de peuplements mesurés à maintes reprises dans le Sud de la Finlande. La fonction de Weibull a été utilisée pour paramétrer les distributions du DHP et des modèles linéaires mixtes ont été construits de façon à caractériser les impacts de différents facteurs écologiques sur la dynamique des peuplements. À la suite du drainage, l'asymétrie gauche des distributions du DHP augmente dans un premier temps à cause de l'augmentation du nombre de tiges et de la réduction des diamètres moyens, en même temps qu'augmente l'inégalité de la taille entre les arbres. Ces changements sont dus à la régénération ou au recrutement et traduisent le peu de compétition de la part des arbres les plus grands. Les distributions du DHP passent ensuite d'une asymétrie gauche à la normalité puis à une asymétrie droite, à la suite de la croissance des arbres et de la réduction du nombre d'arbres de petit DHP. Ceci indique une compétition asymétrique plus forte entre les arbres. L'inégalité des tailles ne change plus pendant ce dernier stade de développement du peuplement, laissant entrevoir l'existence d'une composante concurrente de compétition symétrique. Les éclaircies avaient peu d'impact sur l'évolution des distributions du DHP. Les résultats de l'observation de la dynamique des peuplements permettent d'envisager une allocation des ressources de croissance vers les composantes les plus désirée...

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