Variation in survival and growth strategies for seedlings of broadleaved tree species in response to thinning of larch plantations: Implication for converting pure larch plantations into larch-broadleaved mixed forests

• Shade-tolerant and neutral broadleaved species are planted in thinned larch plantations.
• Neutral species present “slow-growth, high-survival” at 50% thinning intensity.
• Shade-intolerant species present “fast-growth, low-survival” at clear-cut site.
• Different adaptive strategies for the two types responding to thinning intensities.
• Proper broadleaved species are chosen based on light irradiance in larch plantations.

In Northeast China, larch (Larix spp.) plantations exchanged from broadleaved secondary forests to meet the demand of timber consumption are facing some serious problems, including soil fertility decline, acidification of surface runoff and low levels of biodiversity. How to promote the regeneration of broadleaved tree species in larch plantations and convert the pure larch plantations into mixed larch-broadleaved forests would be a top priority to solve some of these issues. The success of this conversion mainly depends on the seedling performances of the selected broadleaved tree species. Seedlings of four dominant broadleaved tree species (intermediate shade tolerant species: Fraxinus mandshurica and Acer mono, shade intolerant species: Juglans mandshurica and Quercus mongolica) from the secondary forests were planted in the thinned pure larch stands (0%, 25%, 50% and 100% thinning) to explore the adaptive strategies (biomass accumulation, leaf structural, nutrient and physiological traits) of two shade-tolerant types of broadleaved tree species responding to the thinning in larch plantations. We found that compared with the intermediate shade tolerant group, the shade intolerant species had higher leaf construction cost (CC), carbon:phosphorus (C:P) ratio, nitrogen:phosphorus (N:P) ratio, and nonstructural carbohydrate concentration (NSC) in stems and roots at all thinning treatments, and had higher leaf ash and specific leaf area (SLA) only in the larch plantations with a canopy openness more than 25 ± 1% (i.e., the 50% and 100% thinning intensity). Across all the thinning treatments, higher survival rates were found for the shade intolerant species relative to the intermediate shade tolerant species, but relative height growth (RHG) did not differ significantly between the groups. When the canopy openness of larch plantations was less than 21% (i.e., 0% and 25% thinning intensity), the intermediate shade tolerant group could utilize essential growth resources more efficiently by investing less energy per unit of RHG, and had a greater photosynthetic capacity at minimal costs relative to the shade intolerant species. However, regression analyses indicated that across all the thinning treatments (and hence all the light irradiance in this study), at a given NSC, there was a higher RHG for the shade intolerant group; also at a common SLA, there were a lower CC and lower area-based N for the shade intolerant group. Ordination using 15 eco-physiological and chemical traits of plants indicated that the major axis of shade intolerant-intermediate shade tolerant dichotomy was primarily driven by leaf NSC, CC and C:P ratio, and was significantly linked with RHG. These results demonstrated that it is a potentially feasible practice for building uneven-aged larch-broadleaved mixed forests by thinning and planting seedlings of native broadleaved tree species in pure larch plantations. Furthermore, the regeneration of the intermediate shade tolerant group and the shade intolerant species could be promoted in larch plantations with the canopy openness less than 21% and more than 25%, respectively. This study may provide an insight into uneven-aged and mixed silviculture for larch plantation forests.