A test of the accuracy of two individual tree growth functions for Picea abies

Abstract

The accuracy of two distant‐independent diameter growth functions for individual trees of Picea abies L. (Karst.) are tested on plots older than 55 years b.h., and with relatively few trees per hectare. Both functions (I1 and I2) have site index, stand basal area, and individual tree basal area as explanatory variables; in addition age at breast height is used in Function I1. Relative to mean measured increment, Functions I1 and I2 underestimate the growth of the test plots by 4.4% and 10.9% respectively; Function I1 shows a root mean squared deviation of 29.2% for single plot predictions, and 5.4% for mean of all plots. Function II predicts the growth as well as some stand functions frequently in use in Norway. Function I2 is unacceptable because age is not an independent variable.

Key words:

• Picea abies
• individual tree diameter growth functions
• distance‐independent tree growth