The long-term impact (30 years) of three contrasting thinning programs (unthinned, moderately thinned, and heavily thinned) on selected forest-floor properties and on the foliar nutrient status of Norway spruce (Picea abies (L.) Karst.) stands (46, 50, and 67 years old) was evaluated at three sites on acid soils in the Belgian Ardennes. Sampling involved needles (current-year, 1-year-old, and recently fallen) and soil organic layers (OL, OF, OH, OA). For all samples, dry mass and element concentrations (C, N, P, Ca, Mg, K, Na, Mn, Al, Fe) were determined. Linear mixed models were used to analyze these data and showed that forest-floor mass was negatively affected by thinning (p = 0.0003) and that the N concentration in the forest floor increased with thinning intensity (p = 0.0008), while its Mn concentration decreased (p < 0.0001). The N, P, and K concentrations in the current-year needles were decreased by thinning (p < 0.05), while the Ca, Mg, and Na concentrations were not affected. We hypothesize that thinning negatively affected N, P, and K nutrition by removing the nutrients contained in the thinned trees and by decreasing the forest-floor thickness, thus reducing its nutrient contents and its ability to support root growth.
