In the north-east of France 2 open-pollinated half-sib progenies (GANN 7 and MORZ 4) of Norway spruce (Picea abies Karst.) were planted in pure and mixed stands with a special experimental design: clinally varying stand density (200-10 000 stems/ha) and mixture rate (0-100%). The GANN 7 progeny was much faster growing than MORZ 4. Fourteen years after planting, at age 18 (mean height = 5 to 8 m), girth at breast height was strongly reduced by density in both progenies. In pure stands, for girth, the statistical spacing- genotype interaction, if any, was not important enough to be noticed. An abolute value of reduction in girth with increasing density was the same for the two progenies. In mixed stands and for a given density level, the growth of individual trees, whatever their genotype, increased with an increasing proportion of surrounding trees of MORZ 4. Therefore MORZ 4 can be considered as less competitive than GANN 7, which is in accordance with the difference in vigour of the two genotypes. The observed trend was emphasized by increasing stand density. It was shown that the fast growth of individual GANN 7 trees surrounded mainly by MORZ 4 trees was only due to the rather weak competition exerted by MORZ 4 and could not be attributed to some kind of cooperation. An estimate of basal area for stands of simulated mixtures of the two genotypes showed clearly that the pure stand of the most vigorous genotype (GANN 7) was superior for all mixed stands. The spacing-genotype interaction in a pure stand and the inter-genotype competition in mixed stands must be taken into account when establishing growth curves, especially for genotypes of very low genetic variability.
