Ecological groundwater level of main drought-tolerant plants

The main drought-tolerant plants in the arid area of northwest China are Tamarix ramosissima (Tamarix ramosissima), Populus euphratica, Haloxylon ammodendron, Alhagi, Phragmites communis, etc. They are widely distributed in northwest China and have good sand prevention and fixation ability, and most of them belong to excellent sand fixation trees.

1. Tamarix chinensis population

Tamarix chinensis is a widely distributed species in the arid areas of northwest China. It is a drought-tolerant and salt-tolerant plant with a well-developed root system. It is reported that the root system of Tamarix chinensis can be as long as 40m, which is suitable for living in different environments. In the lower reaches of Heihe River, the groundwater depth of 3 ~ 5m is most suitable for the growth of Tamarix chinensis, which often constitutes the dominant plant in the vegetation community. When the local groundwater depth is 1 ~ 3m, it often becomes an associated population in the vegetation community, and when the local groundwater depth is more than 5m, the Tamarix community is obviously degraded. In sandy land, Tamarix ramosissima has obvious sand fixation effect, and often forms tufted Tamarix ramosissima dunes. From the Tamarix ramosissima sand dunes stripped by wind erosion in the lower reaches of Heihe River, it can be observed that the roots of Tamarix ramosissima are intertwined all over the sand dunes, which has good sand fixation effect. In these areas, Tamarix ramosissima can grow well at the top of sand dunes, although the groundwater depth is more than 5 m. Due to the difference of precipitation, lithology and vadose zone structure, the buried depth of groundwater suitable for Tamarix chinensis growth is also different. In Tarim River Basin, Xinjiang, the buried depth of groundwater is 1.0 ~ 8m, and the optimum buried depth of water level is 1.5 ~ 3m. In the lower reaches of Shiyang River in Hexi Corridor, the suitable water level for Tamarix chinensis growth is 1 ~ 7m, and the most suitable water level is 5 ~ 7m.

2. Populus euphratica population

Populus euphratica is a tall tree widely distributed in northwest China, with developed horizontal roots. In Ejina Banner, it is observed that the horizontal root system extends over 60m, which has certain salt tolerance and can grow normally in areas with salinity of1~ 3g/L. Because of its large canopy, large transpiration and high water demand, it is usually distributed along rivers or ancient rivers with shallow groundwater. The most suitable groundwater depth for Populus euphratica growth is 1 ~ 3m. In the area where the groundwater level is above 3 m, Populus euphratica grows badly and degrades seriously. Populus euphratica is sensitive to the change of water environment. The change of river course has changed the water environment of the original river course. Degraded and dead Populus euphratica can be seen everywhere on both sides of the dry river, often forming a "strange forest", which has become a common landscape in Ejina Banner in the lower reaches of Heihe River and is favored by photographers. In the lower reaches of Shiyang River, in 1950s and 1960s, the groundwater depth was1~ 3m; Populus euphratica grows well. By the 1970s, the groundwater level had dropped to 3 ~ 5m, and the Populus euphratica forest was seriously degraded. After 1980s, the groundwater depth dropped below 5m, and Populus euphratica population basically disappeared.

3. Haloxylon ammodendron population

Haloxylon ammodendron is the main tree species of desert vegetation in China, and it is an excellent sand-fixing tree species with drought tolerance. It is mainly distributed in arid desert areas with precipitation below130 mm. There are still a large number of primary Haloxylon ammodendron woodlands in desert areas in China, mainly distributed in Manas Lake Basin in Junggar Basin, and the adult plants are often as high as 4 ~ 5m m. The annual precipitation in Ejina Banner is only 40mm, and there is also a large area of primary Haloxylon ammodendron forest in the sandy land around Haloxylon ammodendron head and Guaizi Lake in the west of Gulinai. Haloxylon ammodendron is tall, with a plant height of 3 ~ 4m, a community coverage rate of 20% ~ 40% and a groundwater depth of 2 ~ 6m. In the eastern part of Gurinai, due to the decrease of groundwater recharge, the water level dropped below 6m, resulting in the large-scale death of the original Haloxylon ammodendron forest. When the precipitation in the middle reaches of Heihe River is above 100mm, Haloxylon ammodendron's developed roots can absorb water from the vadose zone. The growth of artificial Haloxylon ammodendron forest mainly depends on precipitation, not on groundwater level. In extremely arid areas, the development of Haloxylon ammodendron community is highly dependent on groundwater, and the buried depth of ecological water level should be less than 6 m. The development of new plants depends on the melting of snow in winter or occasional heavy rain.

4. Reed population

Reed is a kind of hidden plant, which is widely distributed. It is the main constructive species of wetland vegetation and has strong adaptability to environmental changes. In the arid area of northwest China, reeds are mainly distributed on the banks of rivers and lakes formed by wetlands and groundwater discharge areas, as well as deserts and sandy lands formed by the degradation of wetlands and lakes, and often become the main species and companion species.

The characteristics of reed population are closely related to the depth of groundwater table. Zhao (2002) studied the riverbed and wetland reeds in Linze County, Hexi Corridor. The results showed that the population density of Phragmites communis in wetland with better water conditions was higher. The population density of Phragmites communis gradually decreased with the increase of groundwater depth, the growth advantage gradually shifted from underground to above ground, and the population height increased with the increase of groundwater depth. When the local groundwater depth is greater than 1m, the population height increases rapidly.

Gulinai wetland in the lower reaches of Heihe River is degraded from Taihu Lake. Due to the decrease of groundwater discharge, Gulinai Lake has dried up, and the southern part is a dense reed grassland, which belongs to a single reed population. The population height is 20 ~ 50 cm, the coverage rate is 40% ~ 70%, the groundwater depth is 1 ~ 2 m, the vadose zone near the spring is full of water, and the reed height is over 3m, which is very dense and overgrown. The development height of reed community is related to the water condition of vadose zone. The area with a groundwater level of 2 ~ 3m is a sparse reed grassland with Haloxylon ammodendron or Tamarix chinensis. The community density of Phragmites communis is relatively small, with community coverage of 20% ~ 30% and community height of 10 ~ 20 cm. Reed has strong adaptability to the environment. The ecological water level suitable for growth is less than 5m, and the optimum water level is 1.5 ~ 3m.

5. Alhagi community

Alhagi is a widely distributed tree species in arid areas of northwest China, with developed roots, and it is a shrub with good drought resistance and sand fixation performance. In the desert area in the middle reaches of Heihe River, the precipitation is more than 150mm, although the groundwater level is more than 20m, the water held by Alhagi can still grow, which is related to the precipitation and the lithologic structure of vadose zone. In extremely arid areas, the camel thorn is highly dependent on groundwater, and the buried depth of groundwater is 2 ~ 3.5 m, which is most suitable for the growth of camel thorn.