A basal area increment model for individual trees growing in even- and uneven-aged forest stands in Austria

Abstract

Because of the gradual shift from pure even-aged forest management in central Europe, existing yield tables are becoming increasingly unreliable for forest management decisions. Individual tree-based stand growth modeling can make accurate stand growth predictions for the full range of conditions between pure even-aged and mixed-species uneven-aged stands. The central model in such a simulator is basal area increment for individual trees. Spatial information is not needed, and age and site index are intentionally not used to gain generality for all possible stand conditions. A basal area increment model is developed for all the main forest species in Austria: spruce (Picea abies), fir (Abies alba), larch (Larix decidua), Scots pine (Pinus sylvestris), black pine (Pinus nigra), stone pine (Pinus cembra), beech (Fagus silvatica), oak (Quercus robur, Quercus petraea and Quercus cerris), and for all other broadleaf species combined. The Austrian National Forest Inventory provided 5-year basal area increment from 44 761 remeasured trees growing on 5416 forested plots in the 1980s. This large sample is representative of forest conditions and forest management practices throughout Austria and therefore provides an excellent data base for the development of an increment model. The resulting increment model explained from 20 to 63% of the variation for all nine species and from 33 to 63% of the variation if the minor species Pinus cembra is excluded. These results compared quite closely with those of Wykoff for mixed conifer stands in the Northern Rocky Mountains. In the Austrian model, size variables (breast height diameter and crown length) accounted for 14–47% of the variation in basal area increment, depending on tree species. The best competition measure was the basal area of larger trees, which provides a tree-specific measure of competition without requiring spatial information; crown competition factor provided only minor improvement. Competition variables accounted for 9% of the variation on average, and up to 15% for some species. Topographic factors (elevation, slope, aspect) explained up to 3% of the variation, as did soil factors. Remaining site factors; such as vegetation type and growth district accounted for a maximum of 3% of the variation in increment. In total, site factors explained from 2 to 6% of the variation. Even though site factors account for a small percentage of the variation, they are not only significant, but serve to localize a particular prediction. These species-specific interrelationships between basal area increment and the various size, competition, and site varibles correspond quite well with ecological expectations and silvicultural understanding of these species in Austria. Because the sample base is so strong, the resulting growth models can be recommended not only for all of Austria but for surrounding regions with similar growth conditions.