Analysis of time scale influences on water and soil conservation effects for trees on experimental plots using vegetation fractional coverage

Abstract

Five plots representing a gradient of vegetative cover with trees (Pinus massoniana Lamb.) and one control plot were built in 2007 in Changting County of Fujian Province, a typical water-eroded area in southern China. From 2007 to 2010, the rainfall characteristics, vegetation fractional coverage (VFC), runoff depth, and soil loss were measured and/or calculated for the plots after each rainfall event. To explore time scale influences on water and soil conservation effects at (rainfall) event, month, season, and year scales, quadratic polynomial regression models were established between VFC and water (soil) conservation effects [WCE (SCE)]. The WCE (SCE) corresponds to the ratio of runoff (soil loss) of experimental plots with trees to that of the control plot. The results show that, with increasing plot VFC, VFC-WCE andVFC-SCE models presented descending and/or ascending trends, showing linear and nonlinear (transition) effects (positive and/or negative function of the vegetation).This indicates that more environmental factors in addition to vegetation should be taken into account in the conservation management. Observed water conservation effects are not obviously influenced by the time scale, whereas observed soil conservation effects are better (smaller SCEs) at longer time scales, indicating that caution must be taken in the comparison of vegetation effects on water and soil conservation at different time scales. At all time scales, water conservation effects transform when VFC was between 0.5 and 0.6, and soil conservation effects transform when VFC was between 0.3 and 0.7, where “transform” means mutual transformation between positive and negative effects. For both water and soil conservation, the differences in rainfall erosivity among the effect types substantially increased with a longer time scale. This result implies that longer time scales may help differentiate rainfall characteristics for water and soil conservation effects.