Yield capacity of planted Picea abies in northern Sweden

Norway spruce (Picea abies (L.) Karst.) is the dominant species in the older forests of central northern Sweden. However, spruce has seldom been planted in the area, partly because existing tools for site classification have indicated a low yield capacity for the species. The aim of this study was to examine the yield capacity of spruce on the basis of existing plantations. In total, 91 operational and experimental plantations in the age interval 27-46 yrs were sampled. Stands were located between 62 degrees and 65 degrees N at altitudes 130-620 m a.s.l. Site index was estimated by height growth and site-factor equations previously developed from old-growth stand data. Height developments in the plantations indicate that site index for these stands is on average 4.6 m higher than predicted by site-factor equations. The differences between the two methods are larger on poor sites than on rich sites. No systematic deviations of top height development from the site index curves could be detected on remeasured sample plots. Existing growth models were applied on measured stand data to predict future growth. Calculated mean annual increments were on average 20% lower when site index was predicted by site factors instead of height and age. The bias means that the yield capacity of planted spruce in northern Sweden has been underestimated by about 35%.