The morphology and stability of concave surface of the straw checkerboard barriers are the fundamental guiding principles of exploring the mechanism of erosion and deposition, evaluating effectiveness and life period, and optimizing the physical structures of the sand barriers. Especially, in alpine sandy land, characteristics of erosion (deposition) and capacity for anti-erosion and sand burial of straw checkerboard barriers are significantly different from the arid and semi-arid desert regions. Erosion (deposition) measurements and wind-sand observations for different specifications (1 m 9 1 m, 1.5 m 9 1.5 m and 2 m 9 2 m) and slope positions (toe, middle and top of the windward areas) of wheat straw checkerboard barriers were adopted in the eastern shore of the Qinghai Lake study area. The different sizes of straw checkerboards at different windward areas have distinctly erosive and depositional stability and intensity. Including the checkerboards with 1.5 m 9 1.5 m (medium) size at the middle and top, 1 m 9 1 m (small) size at the top and 2 m 9 2 m (large) size at the toe, all the erosion (deposition) coefficients are between 0.09 and 0.11, while their intensities of accumulation are relatively steady (70-90 kg m −2 ), which are the easiest to form stable concaves, and the heights of the barriers change least. Nevertheless, the concaves with small size at the toe are seriously buried, but eroded in the center of some checkerboards with large size at the top, which lead to a short protective period within 3 years and an unbalance between erosion and deposition. Moreover, the transects of erosion (deposition) dominated by southwesterly and northwesterly winds reflect the different intensities of erosion (deposition) at various orientations. On the transect of the NW-SE orientation, at the dune section, each square in the NW direction is strongly accumulated, and the center-SE azimuth is weakly eroded. Usually, deeper accumulation in the center of transects happen in those checkerboards with smaller size and lower terrain slope, which is mainly caused by an obviously positive correlation between the northwest and southwest wind velocity and the erosive depth, and the same is true with the wind frequency (all correlation coefficients are between 0.85 and 0.95). Taking the characteristics of erosion (deposition), sand protection benefits and costs of all types into account, large size at the toe and medium size at the middle of windward slope are the most practical combinations, while small size is suitable to play an emergency treatment role in some extremely serious hazard areas in alpine sandy land.
The planting of sand-binding vegetation in the Qinghai Lake watershed at the northeastern edge of the Qinghai–Tibet Plateau began in 1980. For this paper, we took the desert on the eastern shore of Qinghai Lake as the study area. We analyzed a variety of aged Hippophae rhamnoides communities and aeolian activities, and we discuss the relationship between them. The main conclusions are as follows: (1) With an increasing number of binding years, the species composition became more abundant, natural vegetation began to recover, and biodiversity increased year by year. At the same time, plant height, canopy width, and community coverage increased, but H. rhamnoides coverage was reduced to 36.70% as coverage of Artemisia desertorum increased to 25.67% after 10 years of fixing. The biomass of H. rhamnoides increased significantly, especially the underground biomass. For example, the biomass of area 15a was about 10 to 30 times that of area 1a. (2) Plants are a useful obstacle to aeolian activity. The presence of plants reduced the wind flow in the upper parts of the plants, but it did not have obvious regular characteristics. The longer the fixation term, the lower the surface sediment transport. It is significant that the sediment transport amount in winter was four times that in the summer. After 15 years of binding, H. rhamnoides grows well, and the community is still stable in the study area.
Land desertification and aeolian activity are currently the greatest threats to alpine ecological environments and are also the primary challenges of desertification control and ecological restoration projects. Afforestation of sandy lands around the Qinghai Lake in China has effectively controlled the desertification of this watershed. However, certain issues remain which challenge its overall success, including lack of diverse biological species and poor theoretical understanding of aeolian processes, such as controlling wind-sand flow in relation to complex alpine ecological factors. Therefore, to help improving afforestation techniques, this research focused on Hippophae rhamnoides, Salix cheilophila, Pinus sylvestris, Populus simonii and Artemisia desertorum vegetation implanted in the mobile dunes on the eastern shore of Qinghai Lake. Aeolian transport characteristics and annual changes to community ecological factors from 2010-2016 were monitored in comparison with uncontrolled sand dunes. Based on simultaneous observations using gradient anemometers and sand samplers, it was found that the aeolian activities exhibited the following features: 1) In re-vegetated lands, the logarithmic growth of wind speed was disrupted by the wind speed amplification in the middle and high layers and wind speed reduction in the low layers, while vegetation had significant wind-breaking (> 37%) and sand-fixing (> 85%) effects in 2016. 2) Wind speeds in re-vegetated lands and mobile dunes showed a linear correlation, especially in lower layers of H. rhamnoides and S. cheilophila, while sand transport in re-vegetated land increased linearly or exponentially with increasing wind speed. 3) The four artificial shrubs and forests had greater sand deposition with intensities of 280-860 t/(ha•yr), largely concentrated during winter and spring which accounted for 60%-85% of the annual cycle, while A. desertorum experienced significant root undercutting; and 4) Intensity of aeolian activity in re-vegetated lands, except for A. desertorum, was significantly negative with respect to plant growth structure, community cover, topsoil moisture, and regional precipitation. Overall, these five sand-binding species produced optimistic wind-sand protection effects for the alpine sandy lands, which relied on the plants' physical disturbance of wind-sand flow during the early stages of community development. In comparison, H. rhamnoides and S. cheilophila individually maintained stable wind-sand protection effects, while P. sylvestris and P. simonii were better in mixing with other shrubs and herbs to achieve a comprehensive ecological system for future control of aeolian activity.
The research on wind regimes and the wind protection mechanism of sand-fixing plants has mainly relied on wind tunnel experiments; few observations have been made in the field. At the same time, airflow around individual standing vegetation elements and communities is relatively lacking in alpine semi-arid deserts. Therefore, this paper selected 10-year-old Hippophae rhamnoides (sea buckthorn) on sandy land on the eastern shore of Qinghai Lake as the study object. Based on spatial and temporal changes of wind regime in the afforestation forest, a structural simulation of airflow near the plant and at different layers above the ground, and the annual changes in wind protection, we studied the wind protection mechanisms of H. rhamnoides as single elements or communities. The results were as follows: the effective protection length of the sublayer of H. rhamnoides was 1.0 to 1.8 m. The higher the layer, the smaller the decrease in wind velocity behind elements, and the smaller the effective protection length. Wind velocity downwind of H. rhamnoides increased, with height increasing where the airflow decreases rate (R) decreased in the sublayer, and increasing in the middle layer as plant height increased. Meanwhile, the airflow decreases rate (R) was negative in the upper layer because it decreased as the plant height increased. The airflow movement between elements had various directions because the upper layer was prone to fluctuations due to the swinging of the crown and branches, and turbulence was seen at the sublayers owing to the mechanical resistance of the elements. When the wind speed at the standard point was 8.5 m/s and the wind direction was east (E), the increase of airflow velocity at the side and center in the upper layer was more significant, and there was a strong wind zone in the azimuth of NW‒N‒NE‒E‒SE, while the S‒SW‒W azimuth zone was weaker. The sand-fixing shrub H. rhamnoides had a significant windproof function, and the 1.5 m square interval density of H. rhamnoides was suitable for alpine desert control projects.
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