Converting larch plantations to mixed stands: Effects of canopy treatment on the survival and growth of planted seedlings with contrasting shade tolerance

Larch (Larix spp.) plantations are important for timber production in Northeast China, but this monoculture practice has led to problems such as decreased soil fertility and water-holding capacity. To examine the possibility of gradually converting pure larch plantations to mixed stands by small-scale canopy regulation, we planted seedlings of two species with contrasting shade tolerance, light-demanding Manchurian walnut (Juglans mandshurica Maxim.) and shade-tolerant Korean spruce (Picea koraiensis Nakai), in larch plantations with four different canopy retention intensities (larger gap, 160 m2; smaller gap, 45 m2; thinning, 25% intensity based on basal area; and control, forest understory). After two growing seasons, we found that both species had higher survival rates and growth rates in larger gaps than in forest understories, but the detailed responses to treatments differed between species. Manchurian walnut responded strongly to larger gaps but insensitively to other treatments, especially with respect to biomass accumulation. In contrast, Korean spruce responded gradually with increasing canopy openness. However, canopy treatments had almost no effect on non-structural carbohydrate (NSC) concentration, biomass allocation, and NSC pool allocation, which only differed between species. Our findings indicated that the two species of contrasting shade-tolerance were able to survive and grow in larch plantations, and a small-scale canopy treatment, especially creating gaps of ∼160 m2 in size, could significantly improve seedling survival and growth during the first two years. Therefore, enrichment planting in conjunction with a low-intensity canopy regulation may play an effective role in converting larch plantations to mixed stands while maintaining continuous stand functions during the conversion process.