Thinning effects on forest evolution in Masson pine (Pinus massoniana Lamb.) conversion from pure plantations into mixed forests

Abstract

Monocultural coniferous plantations have prevailed worldwide in recent decades, which supplied much of the world’s timber, but also exerted some negative effects on local ecologies and environmental systems. Continuous development has increased the various demands of human society for forests and it is necessary to balance concerns for the ecological and economic functions. Ample evidence indicates that mixed forests are an ideal option for providing more diversified ecological services and forest goods. Converting pure forests into mixed forests by introducing broad-leaved hardwood species below coniferous plantations has become an increasingly popular forest management strategy. Yet, there has been not enough research to date on suitable management methods for enhancing forest diversity and resilience in the context of forest conversion. To comprehensively examine how the intensity in Masson pine thinning influences forest evolution, seedlings of two native hardwood species were introduced below unthinned and thinned (varying intensity) Masson pine plantations. The effects of thinning on residual tree growth, seedling survival and growth, and understory vegetation development were analyzed using a generalized additive mixed model (GAMM). Monitoring results over 10 years indicate that thinning is a necessary management measure to accelerate forest succession in the conversion process; thinning exerts significant effects on the growth of residual trees, the survival and growth of seedlings, and the development of understory vegetation. Intense thinning results in more residual tree growth and enhances the richness and diversity of herbaceous species. However, excessive thinning can reduce the likelihood of seedling survival and growth as well as the richness and diversity of shrubs. Optimal thinning intensities appear to fall between 50% and 60% depending on the specific introduced species; light-demanding species may need higher overstory thinning intensity than shade-tolerant species. The biological characteristics of the introduced species must be taken into account to design an effective thinning strategy for pine plantations conversion.