How can the shade intolerant Korean pine survive under dense deciduous canopy?

Abstract

The regeneration of Korean pine (Pinus koraiensis Sieb. et Zucc) is a core issue for restoration from secondary to primary forests. There is a lack of Korean pine regeneration in primary forests, compared to an abundance of regeneration in secondary forests. However, the mechanisms behind this mysterious phenomenon have not been studied. This study investigated seasonal understory light availability in addition to carbon (C) gain of understory Korean pine, nonstructural carbohydrate (NSC) storage of Korean pine in secondary forests, and the growth and survival of seedlings under manipulated light regimes. Understory light intensities in early spring and late autumn in secondary forests were significantly higher than in primary forests. The C gain of understory Korean pine occurred mostly in early spring and late autumn, whereas it was negative in the growing season (summer). Total NSC concentrations of Korean pine saplings increased between growing seasons, and decreased during the growing season. In particular, the NSC was dominated by a progressive increase in starch in early spring and by a peak in soluble sugar in late autumn. In this study, the biomass, NSC concentration and pool sizes, as well as survival rates, were positively correlated with light availability, revealing that the species is shade intolerant. The accumulated survival rate, examined in the autumn, was less correlated with biomass, but positively correlated with the carbohydrate storage in autumn. High light availability and positive C gain in early spring and late autumn are key factors affecting the growth and survival of understory seedlings. These two periods in secondary forests are crucial for the survival of seedlings by ensuring sufficient energy to live through the long-lasting dark summer. In primary forests, however, light availability was relatively stable across all seasons, especially in the leafless period, and insufficient through-canopy radiation, along with dense lateral closure, caused seedlings mortality.