黄土丘陵沟壑区土壤水分环境及植被生长响应——以燕沟流域为例

调查了黄土丘陵沟壑区燕沟流域刺槐(Robinia pseudoacacia)林地、辽东栎(Quercus liaotungensis)林地、荒草地、农地等不同植被类型条件下7种地类的土壤水分环境,分析不同植被类型对水分环境的生长响应.结果认为,各地类均存在一定程度的水分亏缺,亏缺量由大到小依次为:阳坡刺槐林地991.57mm、阳坡荒草地941.21mm、阴坡刺槐林地866.53mm、阳坡辽东栎林地815.89mm、阴坡荒草地790.27mm、阴坡辽东栎林地745.20mm、农地325.55mm.土壤水分的交换深度农地达320cm,阴坡荒草地为240cm,阴坡辽东栎林地为200cm,阴坡刺槐林地和阳坡辽东栎林地均为160cm,阳坡荒草地为140cm,阳坡刺槐林地为120cm.试验期间,林地、荒草地和农地分别约有10%、14%、30%的降水储存于土壤中,林地、荒草地600cm深土壤水库可利用水量62.6～309.0mm,与农地728.6mm相比土壤水库的调节能力很有限.受林木耗水量和土壤供水能力的双重影响,阳坡刺槐林枯梢现象严重,有整株枯死林木;阴坡刺槐林有明显的枯梢,但没有整株枯死的林木;辽东栎林也存在枯梢现象,但较刺槐林轻微,林木生长仍然十分旺盛.人工林地植被较高的截留和蒸腾耗水是造成土壤干燥化的主要原因,在植被建设中应遵循区域植被的演替规律,以水定植,尽量选择低耗水的适生乡土树种,采取自然修复为主、人工栽植为辅的措施,同时实施好水土保持措施.黄土丘陵区天然辽东栎林是当地植被演替的顶级群落,林地土壤的干燥化是黄土高原气候整体趋于旱化造成的,并不是人为干扰导致植被过度耗水造成的,这种土壤干燥化不宜归属于干层的范畴.判别土壤干层应以当地稳定天然植被群落的生物量水平和土壤水分状况为基准.

Soil water environment and vegetation growth in the hilly and gully region of the Loess Plateau: a case study of Yangou Catchment

Soil water environments were surveyed in acacia woodland, Quercus liaotungensis woodland, grassland and farmland in the Yangou Catchment of the hilly and gully region of the Loess Plateau, and vegetation growth feedback on soil water conditions was discussed. We found that soil water deficits decreased in the order: acacia woodland on south-facing slopes (991.57 mm), grassland on south-facing slopes (941.21 mm), acacia woodland on north-facing slopes (866.53 mm), Quercus liaotungensis woodland on south-facing slopes (815.89 mm), grassland on north-facing slopes (790.27 mm), Quercus liaotungensis woodland on north-facing slopes (745.20 mm), and farmland (325.55 mm). The exchange depths of soil water decreased in the order: farmland and grassland on north-facing slopes (320 cm), Quercus liaotungensis woodland on north-facing slopes (240 cm), acacia woodland on north-facing slopes (200 cm), Quercus liaotungensis woodland on south-facing slopes (160 cm), grassland on south-facing slopes (140 cm), and acacia woodland on south-facing slopes (120 cm). During the experimental period, about 10% of precipitation was stored in woodland soil, about 14% was stored in grassland soil, and about 30% was stored in farmland. The available water in the 600 cm deep soil reservoir in woodland and grassland was only 62.63-309.00 mm, much less than that present in farmland, which was 728.65 mm, and thus the soil reservoirs in woodland and grassland had less regulative capacity than that in farmland. As a consequence of both the water consumption by vegetation and the limited water supply from the soil, the proportion of tree branches dying appeared to be serious, even resulting in some entire trees dying, in acacia woodland on south-facing slopes. There were also dying branches in acacia woodland on north-facing slopes, but no entirely dead trees. There were few dying branches in Quercus liaotungensis woodland, but overall the trees grew exuberantly. Higher rainfall interception and transpiration rates were the main causes leading to soil desiccation in artificial woodland. Therefore, native tree species that consume less water should be selected according to local natural vegetation succession and measures used to restore vegetation under natural conditions should be the main focus, while artificial planting should be supplemental, in the establishment of vegetation in the hilly region of the Loess Plateau. Quercus liaotungensis in the hilly region is the climax community and it was the overall drought climate on the Loess Plateau that led to its soil desiccation rather than excessive soil water consumption by the vegetation, and thus it is not suitable to simply ascribe soil desiccation to dried soil layers. Distinguishing dried soil layers should be based on the biomass and soil water content of local, stable, natural vegetation communities.