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Stem Form and Taper Changes After Thinning and Nitrogen Fertilization in Picea abies and Pinus sylvestris Stands

Stem form and taper changes after thinning and thinning combined with N fertilization were studied in 23 Norway spruce [Picea abies (L.) Karst.] and 46 Scots pine (Pinus sylvestris L.) stands in Sweden observed for 5-28 yrs. Average taper was calculated for the stem section 1.3-6.0 m above ground level and as the ratio between the diameter at breast height and total tree height for periods of 5-10 yrs. At the start of the experiment, before the first thinning, the dominant height was 12-15 m. Once 65% of the basal area had been removed in a single thinning from below, the remaining trees showed a strong increase in taper compared with trees in unthinned stands. Stems in stands treated with heavily recurrent thinnings from below also increased in taper, although the increases were not always statistically significant. Trees in thinned, N-fertilized Scots pine stands in middle and northern Sweden developed a more pronounced taper compared with stems in equally thinned, unfertilized stands.