ABSTRACT
A. fruticosa (Amorpha fruticosa L.) is widely used for revegetation in semiarid lands that undergo secondary salinization. Understanding A. fruticosa plants response to soil water and salt stress is essential for water irrigation management and proper revegetation practices. In this study, we measured sap flow, stomatal conductance, meteorological and soil characteristics in an A. fruticosa community that recently experienced secondary salinization in northwestern China. Results of our study showed that daytime and nocturnal sap flows averaged 804.37 g·cm-2·day-1 and 46.06 g·cm-2·day-1, respectively, during the growing season. Within individual days, the highest sap flow appeared around noon local time and followed a similar pattern of photosynthetically active radiation (PAR). Despite the significant effect of meteorological factors on the characteristics of sap flow, our study highlighted that the sap flow of A. fruticosa is strongly regulated by the availability of soil relative extractable water (REW). The daytime sap flow, which is predominant compared to nocturnal sap flow, was strongly affected by PAR, air temperature and vapor-pressure deficit. With water stress in the top 40 cm of the soil (REW0-40 cm < 0.4), daytime sap flow displayed a strong relationship with soil water content (SWC) (positive) and soil electrical conductivity (EC) (negative) in the relatively shallow soil profile (up to 40 cm). For the nocturnal sap flow, our results suggest that in the absence of soil water stress (REW0-40 cm > 0.4), the nocturnal sap flow is mainly used to replenish the stem water content and sustain nocturnal transpiration. Under soil water stress, nocturnal sap flow is mainly used to replenish stem water content. The results of our study indicate that it is necessary to shorten the irrigation cycle during the primary growing period (May-July) of A. fruticosa. Moreover, in the absence of soil water stress (REW0-40 cm > 0.4), A. fruticosa can survive well in an saline environment with soil EC < 5 mS·cm-1.
ABSTRACT
As the most serious form of soil erosion, gully erosion can be triggered by individual high-intensity rainfall events. In this study, a total of 369 small catchments in 24 sites were sampled to investigate the relationship between rainfall and gully erosion on hillslopes and to study the impacts of vegetation restoration following heavy rainstorms in the central Loess Plateau, China. A total of 280 newly formed gullies on hillslopes were identified by comparing pre-storm Google Earth images and post-storm unmanned aerial vehicle (UAV) images. The results showed that the dimensions and density of gullies increased significantly with rainfall gradient increasing from the periphery to the storm center. When the rainfall amount exceeded 200 mm, gully volumetric density reached up to 928.39 m3/km2 and the mean gully volume was 15.74 m3, 12.8 times and 2.3 times the mean gully volume for rainfall amounts of 106 and 150 mm, respectively. In the sampled small catchments, where cropland was dominant, the relationships between the gully densities and rainfall amount could be fitted with exponential functions. Vegetation restoration was found to reduce the densities and dimensions of gullies on hillslopes. Compared to those in cropland-dominated catchments, the density of gullies in grassland-dominated catchments was found to be lower by > 60%, while the individual gully volume was found to be 1.6 times higher. In small catchments, no new hillslope gullies were observed when the rainfall amount fell below 106.7 mm. Therefore, the rainfall thresholds for (1) ephemeral-gully initiation on grassland hillslopes, (2) permanent-gully initiation on grassland hillslopes, and (3) permanent-gully initiation on cropland hillslopes are concluded to be not >106.7 mm, not >136.1 mm, and not >110.2 mm, respectively. This suggests that the restoration of cropland to grassland would reduce the rainfall threshold for gully initiation.
