CONSTRUCTION OF SITE INDEX EQUATIONS FOR PINUS-SYLVESTRIS L. USING PERMANENT PLOT DATA IN SWEDEN

Three general methods for construction of site index equations (the gu ide curve method, the parameter prediction method and the difference e quation method) and 13 growth functions were tested on data from perma nent plots in 156 Scots pine (Pinus sylvestris) stands. Most stands we re of cultivated origin with known year of establishment. Data consist ed 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 sq uare error (RMSE), meter) from original data. The guide curve method g ave RMSE similar to 0.56 while the difference equation method with the anamorphic formulation gave RMSE similar to 0.46, with appropriate fu nctions. The difference equation method gave a higher RMSE with polymo rphic than with anamorphic formulation. The special formulation of the Hossfeld function for the difference equation method proposed by Cies zeuski and Bella [Cieszewski, C.J., Bella, I.E., 1989. Polymorphic hei ght and site index curves for lodgepole pine in Alberta. Can. J. Res. 19, 1151-1160] gave a simple and straightforward, polymorphic solution with RMSE = 0.40. The parameter prediction method, in combination wit h the deviation method proposed by Tveite [Tveite, B., 1969. A method for construction of site-index curves. Meddelelser fra Det Norske skog forsoksvesen 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 Hos sfeld function. These functions deviate much at higher ages, but too f ew observations in old stands in primary data prevented their separati on. Published data from yield plots in old, natural stands supported t he Hossfeld function. But, indications of a changed growth pattern in later decades still leave open the question of height growth pattern i n old pine stands of cultivated origin. The presented Hossfeld equatio n can only be recommended for use in 10-80-year-old pine stands of cul tivated origin. (C) 1997 Elsevier Science B.V.