STEM NUMBER ESTIMATION BY KERNEL SMOOTHING OF AERIAL PHOTOS

A digitized aerial panchromatic photo of a thinning experiment in pure even-aged Norway spruce (Picea abies (L.) Karst.) is smoothed by a tw o-dimensional isotropic Gaussian kernel. The number of stems per hecta re is estimated from the number of maxima above a certain level of the smoothed image. For the crucial kernel bandwidth estimation problem w e suggest a two-step procedure where the first step consists of comput ing (for each homogeneous stand (or subplot)) an ''internal curve'' co rresponding to the number of maxima at a series or bandwidths. The est imated stem number and the optimal bandwidth is then obtained as the c rossing of the internal curve with an ''external curve,'' here assumed to be of a simple parametric form. If a series of stands (or subplots ) with different known stem numbers is available, the external curve c an be estimated by use of a nonlinear regression method. An experiment with 48-year-old trees and six subplots with varying thinning treatme nts is analyzed. The stem number estimation method, checked by cross v alidation, performs satisfactorily for all thinning grades except the unthinned control.