The role of brash in augmenting forest site carbon capital and maintaining site nutrition in a Sitka spruce forest in Ireland

Abstract

The decomposition of harvest residues (brash) in managed forests has an important influence on the carbon (C) and nitrogen (N) stocks of these ecosystems. The brash input from thinning events in a 25-year-old Sitka spruce plantation was determined. A litter-bag method was used to determine the mass loss and decomposition rate of brash left on the forest floor. The changes in C and N concentrations and the C:N ratio of the needles and branches were also monitored as decomposition progressed for 2.5 years. Using the decomposition rate (k b) and estimated brash inputs, we then determined the total cumulative stock of C that the brash could supply to the deadwood pool over a 41-year rotation period. The three thinning events resulted in the addition of 37.99 t C ha−1 and 0.61 t N ha−1 to the forest floor. A significant mass loss of 44 % was recorded from brash decomposition bags after 2.5 years, with a rapid loss of 35 % in the first year, after which the rate of decomposition slowed. The k b-value and residence time (95 % decomposition) were 0.311 year−1 and 9.6 years, respectively. There was a 69 % increase in the N concentration of needles after 1.5 years, while an increase of 185 % in the N concentration of branches was recorded after 2.5 years. The C concentration (48.55 ± 0.20 %) did not differ significantly between the needles and branches over time. The accumulated C stock from decomposing brash at clearfell was estimated at 18.51 t C ha−1.