Wood production, litter fall and humus accumulation in a Czech thinning experiment in Norway spruce ( Picea abies (L.) Karst.)

Abstract

This paper reports results from a thinning experiment in even-aged Norway spruce, located on former agricultural land at Vitkov in the Czech Republic. The experiment is part of a European-wide series, established under the auspices of IUFRO. Site productivity at Vitkov is high. The experiment includes five treatments with four (pseudo) replications in two blocks. The treatments comprise four thinning regimes and an unthinned control. The thinning regimes were defined based on two levels of simulated mechanization, two different selection principles (thinning pre-dominantly from below or crown thinning), three different sequences of thinning operations (early, late or depending on available assortments), and two different rack widths (3.5 or 5.0 m). All treatments, except the unthinned control, include an early, heavy thinning and aim for a final crop of 400 pre-selected trees per ha. In the unthinned control, stem number is being reduced only through natural mortality. After 29 years of observations, from age 8 to 37 years, the experiment demonstrates a reduction of 14–23% in stand volume growth due to thinning. Following an initial, heavy thinning, subsequent thinnings result in a larger reduction in volume growth if they are carried out shortly after the first thinning. The reduction in volume growth increases slightly with increasing rack width. Assortment thinnings (crown thinning) lead to the largest reduction in stand volume growth, but this pattern seems to depend on site or other local conditions. At an age of 39 years, litter fall in an unthinned control and a thinned plot ranged from 8500 to 8700 kg ha −1 (dry weight), with foliar litter accounting for 7100–7800 kg ha −1. The dry weight of biomass accumulated in the holorganic L-, F-, and H-horizons ranged from 80,000 to 100,000 kg ha −1. The organic horizons accumulated 720–950 kg ha −1 of nitrogen, with an annual return of nitrogen through litter fall at 75 kg ha −1. The biomass in the organic horizons contained 20,000 kg ha −1 of carbon. There were no differences between litter fall in the unthinned control and the thinned plot, but a lower content of biomass and nutrients (except calcium) in the thinned plot. This supports the theory of faster decomposition and cycling of nutrients in organic horizons of thinned stands, as compared to unthinned stands of even-aged Norway spruce.