Litterfall nutrient return in thinned young stands with Douglas fir

The use of Douglas fir (DF) is on the increase in the Czech Republic. This tree species shows a good production and also a beneficial impact on soil to some extent. We studied both amount and properties of litterfall in 18 to 20-year-old stands with DF in 2011. Two experimental plots were DF-dominated and one showed ca. 20 – 30% share of Scots pine (SP) at the beginning of observation. The experiments consisted of two treatments such as unthinned control and 50 – 62% trees’ density reduction accounting for 43 – 59% basal area reduction. Litterfall was collected using litter traps of 0.25 m² area in 3 – 4 traps per treatment. Forest floor L and F were taken in 2011 and 2018 to investigate the development of their amount after thinning. Both whole-period sum and mean annual litterfall were reduced after thinning compared to controls. The mean annual litterfall totaled ca. 3 t ha⁻¹ in 20-year-old DF–dominated stands. This amount represents an annual nutrient return of 30 – 40 kg N, 1 – 3 kg P, 3 – 5 kg K, 12 – 30 kg Ca and 1 – 2 kg Mg per one hectare. The reduction of the annual litterfall was more pronounced in DF–dominated stands. All thinned plots showed increased decomposition rates reflected in lower total L+F amounts in both DF–dominated plots whereas unthinned plots accumulated more L+F at the end. The mixed DF–SP plot showed reduced L and increased F layer amounts in both unthinned and thinned treatments with only minor change to L+F sum between 2011 and 2018.