Effects of forest gap remodeling on understory plant diversity and soil physicochemical property in Pinus massoniana plantations

Based on the transformation of the pure Pinus massoniana plantations through making forest gaps and planting precious tree species in the gaps, it is essential to study the effects of different gap sizes on understory plant diversity and the soil physicochemical properties, to reveal the relationship between the understory plant diversity and soil physicochemical properties, aiming to provide foundation for improving the quality of the P. massoniana plantations.

An experiment of forest gap-based transformation were carried out in the 22-year-old pure plantations of P. massoniana, with Dalbergia cochinchinensis seedlings planted in gaps. The experiment involves forest gaps with a series of sizes (100, 400, 900 m²), using clear cutting plots (1 hm²) and untransformed stands as control. The growth of D. cochinchinensis, understory plant diversity and soil physicochemical properties were investigated four years after the experiment began, and the response of understory plant diversity and soil physicochemical properties to forest gap management as well as relationship between these indexes were analyzed.

1) Ground diameter and tree height of D. cochinchinensis were 43.88% and 15.06% higher in medium and large forest gaps (400 m² and 900 m²) than in small forest gaps (100 m²), respectively; 2) The diversities of shrubs and herbs were mostly higher in the forest gaps than in the CK2, the diversity of shrubs and herbs generally declined with increase of gap sizes. Among them, abundance and Shannon diversity index in medium and small forest gaps (400 m² and 100 m²) were 101.50%, 26.09% (shrub) and 64.93%, 12.29% (herb) higher than large forest gaps (900 m²); 3) RDA analysis showed that forest gap management significantly drove the changes in soil organic carbon (SOC) and total potassium (TK) contents, and the contents of SOC and total nitrogen (TN) gradually increased while the content of TK showed a decreasing trend with increase of gap sizes; 4) Contents of SOC, TN, and avaible nitrogen (AN) were in significantly negative correlation with Pielou index and positive correlation with Simpson index of shrubs, Richness, Shannon, Pielou, and Margalef index of herbs were significantly negatively correlated with contents of TK while positively correlated with total porosity (TPO).

Therefore, forest gap management, especially application of medium gaps (400 m²) is beneficial for enhancing plant diversity, improving soil physicochemical properties, promoting the growth of the replanted tree species (D. cochinchinensis) in P. massoniana plantations. The findings will contribute to biodiversity conservation, stand structure optimization and forest quality improvement.