Analyzing forest effects on runoff and sediment production using leaf area index

Quantifying the effects of forests on water and soil conservation helps further understanding of ecological functions and improving vegetation reconstruction in water-eroded areas. Studies on the effects of vegetation on water and soil conservation have generally focused on vegetation types or vegetation horizontal distribution densities. However, only a few studies have used indicators that consider the vegetation vertical distribution. This study used the leaf area index (LAI) to investigate the relationship between forests and water and soil conservation in experimental plots. From 2007 to 2010, rainfall characteristics, LAI, and water and soil loss in 144 natural erosive rainfall events were measured from five pure tree plots (Pinus massoniana). These tree plots were located in Hetian Town, Changting County, Fujian Province, which is a typical water-eroded area in Southern China. Quadratic polynomial regression models for LAI and water/soil conservation effects (RE/SE) were established for each plot. The RE and SE corresponded to the ratios of the runoff depth (RD) and the soil loss (SL) of each pure tree plot to those of the control plot under each rainfall event. The transformation LAIs of the LAI-RE and LAI-SE curves, as well as the rainfall characteristics for the different water/soil conservation effects, were computed. The increasing LAI resulted in descending, descending-ascending, ascending-descending, and ascending trends in the LAI-RE and LAI-SE curves. The rainfall frequencies corresponding to each trend of LAI-RE and LAI-SE were different, and the rainfall distributions were not uniform per year. The effects of soil conservation in the plots were superior to those of water conservation. Most of the RE and SE values presented a positive effect on water and soil conservation. The main factor that caused different effects was rainfall intensity. During heavy rains (e.g., rainfall erosivity R = 145 MJ·mm/ha·h and maximum 30 min intensity I₃₀ = 13 mm/h), the main effects were positive, whereas light rains (e.g., R = 70 MJ·mm/ha·h and I₃₀ = 8 mm/h) generally led to negative effects. When the rainfall erosivity was lower than that of the positive or the negative effects to a threshold and the tree LAI reached a transformation value, the relationships between LAI and RE or SE notably transformed. Results showed that the plot-transformation LAIs for water and soil conservation during rainfall events were both approximately 1.0 in our study. These results could be used to come up with a more efficient way to alleviate water and soil loss in water-eroded areas.