Where no comparable studies are available, the growth of dominant trees is a useful lower threshold. Theoretically the diameter of an open-grown tree should be approximately twice as large as that of a mean stem at maximum density (Sterba, 1975). This is confirmed by comparisons between open-grown trees and stand-grown dominant trees (Lässig, 1991). For spruce on good sites (SI = 38 m), open-grown tree dbh of 68 cm, 99 cm, 107 cm, and 245 cm were simulated with Silva, Prognaus, Moses and BWIN, respectively. Lässig (1991) reported a dbh of 91 cm for a reference open-grown spruce tree (constructed
from stem analysis on 12 open-grown trees on 5 sites) at the age of 100. However, individual-tree diameters from stem analysis varied as much as 20 cm at the same age and site index. Gerecke selleck screening library (1991) investigated dominant trees on good sites IDH inhibition (SI = 36 m). At a breast height age of 90 years (corresponding approximately to 100 years), he reported an average dbh of 58 cm. Thus, the simulated values for open-grown trees are all higher than observed values
for dominant trees. Furthermore, the simulated diameters of Silva, Prognaus and Moses seem to be in good agreement with the results from Lässig (1991). BWIN clearly overestimates open-grown spruce growth. Open-grown trees on an alpine site were investigated by Rossi et al. (2008). He reported the average age, dbh, height, and standard deviation of his 5 sample trees. At an average age of 300 years, dbh was 81 cm, and average height was 23 m. The diameters observed compare surprisingly well to a 300-year simulation of a 14 m site index with Prognaus, Moses and Silva; predicted dbh was 86 cm, 98 cm, and 107 cm, respectively. In contrast, BWIN overestimates the dbh of open-grown spruce and predicts a dbh of 216 cm. The heights predicted by the growth models were 16, 28, 32, and 36 m for Silva, Prognaus, BWIN, and Moses, respectively. The height growth of Silva is lowest, because of a strongly curved site-index function for poor sites. The other
growth models seem to over-predict the height growth, with values ifenprodil obtained from Moses being clearly too high. For pine, Thren (1986) reported an open-grown tree diameter of 57 cm for a site index of 22 m. The diameters simulated by all growth models are lower, but do not deviate more than 15 cm from Thren’s (1986) results. Thus, open-grown pine growth is reasonably well predicted by all four growth models. Again, site has a different weight in the four models: differences in diameter between poor and good sites vary from 7 to 62 cm. All models predict an increase in height:diameter ratios with increasing stand density, which corresponds to results from growth and yield experiments. The observed effects of density are both overestimated and underestimated in Arnoldstein, depending on the growth simulator. The magnitude of the discrepancy was within a reasonable range. Schmid et al.