Site-specific height growth models for six common tree species in Denmark

Site-specific height growth models were developed for European beech, pedunculate and sessile oak, Norway spruce, Sitka spruce, European silver fir and Douglas fir in Denmark. The models were derived based on base-age invariant methods to fit the three-dimensional surface of stand age, stand top height and site index in a manner that directly estimated site indices as parameters from the fitting procedure. The estimated base-age invariant models represented the observed age-height patterns well across the range of site conditions. For all species the model explained more than 98% of the observed variation in stand height development and exhibited no apparent bias across the range of predicted site indices. Compared to historical site index curves, the growth patterns expressed by the site-specific height growth models were considerably different, indicating that the historical site index curves were biased. The pattern of deviation depended on tree species and site conditions.