Converting larch monocultures to mixed-species forests: Ten-year impacts of canopy manipulation on understory regeneration

Abstract

Larch (Larix spp.) monoculture plantations are widely used for timber production but often lead to reduced biodiversity and ecosystem functioning. Therefore, exploring strategies to transition these plantations into mixed-species forests while maintaining forest ecosystem sustainability is crucial. This study examines the ten-year impacts (2015–2024) of small-scale, low-intensity canopy manipulation on the growth performance of planted Manchurian walnut (Juglans mandshurica Maxim.) and Korean spruce (Picea koraiensis Nakai). Four canopy treatments were applied: control (no treatment), light thinning (25 % basal area reduction), smaller gaps (45 m²), and larger gaps (160 m²). We also compared artificial planting with existing natural regeneration under these treatments. Results indicated that both species primarily survived in gaps, with nearly all individuals in control and thinning treatments dying after 10 years. For Manchurian walnut, stems exhibited significantly greater height and root collar diameter (RCD) at gap centers, particularly in larger gaps, where height was 190 % greater than at gap edges. For Korean spruce, while RCD did not differ significantly between gap sizes, it was 16–21 % higher at gap centers compared to edges. Ecophysiological traits (i.e., specific leaf area, chlorophyll content, and non-structural carbohydrate concentration) were largely unaffected by gap size or within-gap position. Furthermore, the proportion of dominant species was higher in artificial planting plots (69 %) than in natural regeneration plots (57 %), while natural regeneration plots supported more common species (10 vs. 7). These findings highlight the efficacy of combining gap creation with artificial planting as a silvicultural approach to gradually transitioning larch monocultures into mixed-species forests.