PREDICTING JUVENILE TREE HEIGHT GROWTH IN UNEVEN-AGED MIXED-SPECIES STANDS IN AUSTRIA

The recruitment of trees following the death or cutting of an overstor y tree is crucial for simulation models to achieve a consistent simula tion output. To enhance the prediction of ingrowth uneven-aged mixed s pecies stands, we developed a height-growth model for juvenile trees ( less than or equal to 1.3 m). The model predicts the actual periodical 5-year height increment by adjusting the corresponding potential heig ht increment using: (1) the competition of the remaining overstory, (2 ) the intra-and interspecies competition among the regeneration itself , and (3:) a modifier for the edge effected incidence of light. Data f or this study came from 240 sample plots located in uneven-aged mixed species stands consisting of Norway spruce (Picea abies L. Karst), fir (Abies alba Mill.), and common beech (Fagus silvatica L.). The number of trees on each plot was recorded by species and height class. One ty pical tree was selected to determine the tree height and the height in crement of the past five years of growth for each species and height c lass. The sample consists of 265 spruce: 139 fir, and 286 beech trees. Additionally, we measured the distance to the stand edge along with t he maximum possible angle of incidence for sunlight. For all trees in the overstory (height > 1.3 m), we recorded the diameter at breast hei ght, the tree height, and the height to the crown base. The results de monstrate two things: first, the two competition measures and the modi fier for the edge effected incidence of light were important for spruc e and beech regeneration, while the competition among the regeneration for fir had no significant impact. Second, except for the smallest he ight class of fir and beech (less than or equal to 20 cm), which showe d a slight tendency to overestimate 5-year height increment rates, the resulting models produce unbiased predictions. (C) 1997 Elsevier Scie nce B.V.