Comparing biomass and nutrient removals of stems and fresh and predried whole trees in thinnings in two Norway spruce experiments

Abstract

In Denmark, thinning trees used for energy purposes are cut and left to dry in the stand before they are removed as whole trees. This practice causes shedding of needles and reduces nutrient removals for the benefit of long-term site fertility. It is uncertain, however, to what extent needles are shed and actual nutrient loss is affected by this practice. To address this question, we compared biomass and nutrient removals in two Norway spruce ( Picea abies (L.) Karst) experiments in western Denmark. Three contemporary thinning harvest intensities were examined: harvesting of fresh whole trees, predried whole trees, and stems only. The whole trees were chipped individually, and samples were removed to determine moisture and nutrient contents, whereas sample discs were removed from harvested stems. The biomass content of the cut whole trees was estimated to decrease 17% during predrying, whereas nutrient contents decreased 35%–60% for N, P, and K and <32% for Ca and Mg. The biomass content of stems was estimated to be 35%–42% lower than that of fresh whole trees. The corresponding differences in nutrient contents were in the range 84%–89% for N, P, and K and 73%–80% for Ca and Mg. Predrying and technological methods to reduce nutrient removals were compared and discussed.