Soil Available Phosphorus Loss Caused by Periodical Understory Management Reduce Understory Plant Diversity in a Northern Subtropical Pinus massoniana Plantation Chronosequence

Abstract

Clearing of understory plants is a common management method in plantation forests, but its long-term impact on soil properties and understory plant diversity is still poorly understood. In order to uncover the potential relationship between understory diversity and soil properties, we categorized understory plants into herbs and shrubs, and took soil depth into consideration. We measured the soil variables and investigated the understory plant diversity in four stand age-classes (9-year-old for young, 18-year-old for intermediate, 28-year-old for near-mature, and 48-year-old for mature) in a Pinus massoniana plantation. We aimed to examine how the diversity of herbs and shrubs changed with stand succession and to determine which of the three soil depths (0–10 cm, 10–20 cm, 20–40 cm) had the strongest explanation for the understory plant diversity. Furthermore, structural equation modeling (SEM) was performed to assess the direct and indirect effect of understory clearing and stand age on understory diversity. We found that understory clearing influenced the trend of diversity of herbs and shrubs with stand age, and understory diversity showed a strong correlation with soil physical properties in all three soil layers. The soil properties in the 10–20 cm soil layer related with the diversity of herbs and shrubs most, while the 20–40 cm soil layer properties related with them the least. Understory clearing reduced soil available phosphorus (AP). Understory clearing and stand age were found to benefit understory plant diversity directly and decreased the understory diversity indirectly via AP. Consequently, to improve our understanding of the impact of understory clearing and stand age on biodiversity, we should take into account its direct and indirect effects.