The k-nearest-neighbour method for estimating basal-area diameter distribution

The purpose of this study was to examine the use of the non-parametric k-nearest-neighbour method in estimating the basal-area diameter distribution. Distance-weighted nearest-neighbour estimation was based on the similarities of known stand characteristics, e.g. basal area and basal area median diameter, between the target stand and reference stands. The weights for the reference stands were determined using similarity distances. These similarity distance functions were determined by tree species using standardized stand variables. The material of this study consisted of 553 forest stands located in eastern Finland. The accuracy of the k-nearest-neighbour method was compared with the Weibull distribution by tree species, using current stand volume characteristics and those that should exist after IO yrs' simulation time. The Weibull-based method tn;as, in most cases, more accurate than the nearest-neighbour method, but the results were also quite good for the k-nearest-neighbour method. Both methods were negligibly biased across the whole material. However, at the extremes of the volume distribution clear underestimates were obtained when using the XI-nearest-neighbour method.