Construction of site index equations for Pinus sylvestris L. using permanent plot data in Sweden

Abstract

Three general methods for construction of site index equations (the guide curve method, the parameter prediction method and the difference equation method) and 13 growth functions were tested on data from permanent plots in 156 Scots pine ( Pinus sylvestris) stands. Most stands were of cultivated origin with known year of establishment. Data consisted of measured top height and total age at the start and at the end of observation periods of different lengths. Functions and construction methods were compared with respect to residual variation (root mean square error (RMSE), meter) from original data. The guide curve method gave RMSE ∼ 0.56 while the difference equation method with the anamorphic formulation gave RMSE ∼ 0.46, with appropriate functions. The difference equation method gave a higher RMSE with polymorphic than with anamorphic formulation. The special formulation of the Hossfeld function for the difference equation method proposed by Cieszewski and Bella [Cieszewski, C.J., Bella, I.E., 1989. Polymorphic height and site index curves for lodgepole pine in Alberta. Can. J. Res. 19, 1151–1160] gave a simple and straigtforward, polymorphic solution with RMSE = 0.40. The parameter prediction method, in combination with the deviation method proposed by Tveite [Tveite, B., 1969. A method for construction of site-index curves. Meddelelser fra Det Norske skogforsoksvesen 27, 134–159] and a four-parameter growth model, gave the lowest RMSE, but this solution had patterns of overfitting. Parameter prediction with the Mitscherlich function gave an equation with almost the same accuracy as the polymorphic difference equation with the Hossfeld function. These functions deviate much at higher ages, but too few observations in old stands in primary data prevented their separation. Published data from yield plots in old, natural stands supported the Hossfeld function. But, indications of a changed growth pattern in later decades still leave open the question of height growth pattern in old pine stands of cultivated origin. The presented Hossfeld equation can only be recommended for use in 10–80-year-old pine stands of cultivated origin.