Basal area increment models accounting for climate and mixture for Austrian tree species

Forest trees exhibit a large variation in basal area increment, which depends on tree specific factors, inter-tree relations and the environment. Studying the effect of the above factors on tree growth is essential for forest ecology and forest management. In this study, a basal area increment model for 22 tree species of the Austrian National Forest Inventory was parameterized. The model included effects of tree size, competition, harvesting and disturbances, climate, soil, and species mixtures. The models were fitted using generalized additive models. Basal area increment increased with tree size for all species, and competition and density reduced tree growth. Shade-tolerant tree species showed a more pronounced decrease with competition, than light-demanding tree species. Tree growth increased with disturbance intensity, but confidence intervals for high disturbance intensities became larger. Many variants of the model that included climatic effects were tested, but the best variants used long-term-mean temperature and precipitation during the growing season and the positive and negative deviations thereof, which separated climatic site effects from temperature deviations due to yearly weather conditions. Several of the tested dryness indices were not able to explain tree growth from inventory data. Soil type and soil moisture were significant for many tree species, each of them accounting for a maximum of 10% of difference in tree growth. Little growth was observed on calcareous Leptosols, whereas trees grew best on Planosols, on flysch or calcareous Cambisols. Growth was decreased on dry sites, but very moist and wet sites could result in either a decrease or an increase in growth, depending on the tree species. Mixture effects were significant for almost all mixtures with a sufficient number of observations in the data set. Notable exceptions were the mixtures of Pinus sylvestris L. with Picea abies (L.) Karst. and Abies alba Mill. and the mixtures of Quercus spp. with Pinus sylvestris L. and Fagus sylvatica L.. Mixture effects were both favorable and adverse at individual tree level. Picea abies and Fagus sylvatica were two tree species, which were mainly favored by mixture effects. Interactions of mixture with age or site could not be modelled from the data because they were confounded with other effects of the inventory data. The new basal area increment model provided a comprehensive formulation, which can be use part of individual tree growth simulators and is easily transferable to other regions and environments and can be extended in the future.