Wood density of Norway spruce: Responses to timing and intensity of first commercial thinning and fertilisation

The effect of different fertilisation-thinning treatments on growth and wood density of Norway spruce (Picea abies (L.) Karst.) was studied in two long-term experiments established in the 1970s in central Finland. The experiments included three thinning and three fertilisation treatments in a randomised block design, in which each treatment combination occurred once. Fertilisation treatments included the following levels: (F0) unfertilised, (F1) 150 kg N ha−1 and (F2) 300 kg N ha−1. NPK fertiliser was applied at 5-year intervals. Thinning treatments were (T0) delayed first thinning, (T1) normal first thinning and (T2) intensive first thinning. A total of 85 trees (on average 70 years old) were collected in 2003. Growth rate and wood density were measured from the sampled discs taken at 3–4 m intervals above breast height. Compared with the unfertilised control (F0), the fertilisation treatments (F1) and (F2) increased the growth rate of sample trees by 41% and 38%, respectively; while wood density on both fertilised plots decreased by 7%. Compared with T0, the thinning treatments T1 and T2 enhanced the growth rate of sample trees by 8% and 29%, respectively. For wood density the differences between thinning treatments were not significant. According to these results, the prevailing fertilisation intensities and thinning treatments of Norway spruce stands in Finland have significantly increased radial growth rate of individual trees, but showed no or a slight effect on wood density.