Estimating the effect of Pinus massoniana Lamb plots on soil and water conservation during rainfall events using vegetation fractional coverage

Determining the effects of the critical structure of forests on soil and water conservation aids in understanding forest functions and scientific decision-making in management practices. Quadratic polynomial regression models were established for five pure tree (Pinus massoniana Lamb) plots between vegetation fractional coverage (VFC) and soil/water conservation effects (SCE/WCE) for 144 erosive rainfall events from 2007 to 2010 in Hetian Town, Changting County, Fujian Province, a typical water-eroded area in southern China. The SCE/WCE corresponds to the ratios of soil loss/runoff depth of each tree plot to those of the control plot under each rainfall event. The results show that increasing VFC exhibits descending (DS), descending–ascending (DA), ascending–descending (AD), and ascending (AS) trends in the VFC–SCE and VFC–WCE curves, which indicate positive or negative effects of plot trees on soil and water conservation. In general, the effects on soil conservation on the plots were more notable than those on water conservation, and most of the SCE and WCE values reflect the positive effect of trees on soil and water conservation. Most determination coefficients of both the VFC–SCE and VFC–WCE models exceeded 0.6, and varied with the year and the effect. The positive effects of the pure trees occurred mainly during rainfall events with relatively higher rainfall depth, stronger intensity, and shorter duration, whereas the negative effects occurred mainly during rainfall events with lower rainfall depth, weaker intensity, and longer duration. The transition effects DA and AD appeared largely during rainfall events with moderate rainfall depth, intensity, and duration. The critical VFCs for soil and water conservation during rainfall events were 0.4 and 0.5, respectively, which provided a reference for the evaluation and planning of forest reconstruction in this and similar water-eroded areas.