|其他摘要||As the dominated species in Tarim River Basin, Populus euphratica and Tamarix ramosissima play an important role in maintaining ecological balance of desert riparian forests, sapcing desert and protect the agricultural in oasis. However, the distribution areas of P. euphratica and T. ramosissima have withered extremely due to hydrological processes change by human in the Tarim River Basin in recent decades. We studied the water uptake of P. euphratica and T. ramosissima, explored the water-use strategy and the adaptive strategy of arid environment under different moisture conditions, which will be important significance to reveal the ecohydrology process of desert riparian forests and to protect and reconstruct the ecological environment in arid regions. A pot experiment with 4 different groundwater depths was conducted. The δD and δ18O compositions of the stem water and potential water sources (soil water and groundwater), and photosynthetic characteristic parameters were measured. By using the IsoSource model and the mean depth of water uptake model, we calculated the contributions of potential water sources to total plant water uptake and the mean soil depth of water uptake by P. euphratica seedlings and T. ramosissima seedlings under different groundwater tables. In the upstream of Tarim River, we also measured the δD and δ18O compositions of the stem water from P. euphratica and T. ramosissima and potential water sources in the riparian zone with different water tables. The results were as follows:
The rates of water uptake for P. euphratica seedlings and T. ramosissima seedlings planting alone were not significant, with a similar δ18O value of stem water. When the groundwater tables reached up to 0.3 m and 0.8 m, the soil water was favorable for their growth, that is, P. euphratica seedlings and T. ramosissima seedlings mainly used groundwater and lower soil water, with the utilization rate over 80%. When the groundwater tables were 1.3 m and 1.8 m, soil water stress was heavy, P. euphratica seedlings and T. ramosissima seedlings used shallow soil water primarily. Populus euphratica seedlings could not use groundwater directly when the groundwater table was 1.8 m, and obtained 79.9% of its water from the soil layer 0-80 cm, Tamarix ramosissima seedlings obtained 12.9% and 62% of its water from groundwater and the soil layer 0-80 cm, respectively.
When P. euphratica seedlings were coexisted with T. ramosissima seedlings, their water-use strategies were changed to reduce the competition for water. T. ramosissima seedlings was mainly used lower soil water and groundwater, obtained 47.3% and 24.5% of its water from the soil layer 60-130 cm and groundwater respectively when the groundwater was 1.3 m. While P. euphratica seedlings was mainly used shallow soil water from the soil layer 0-40 cm, with the utilization rate over 46.6%. These results suggest that T. ramosissima seedlings were much easy to adapt the drought environment than those of P. euphratica seedlings in the arid region.
The root biomass of different orders of P. euphratica seedlings was less than that T. ramosissima seedlings, and the maximum value was 36.07 g when the groundwater table was 0.8 m. While the root biomass of T. ramosissima seedlings in the same groundwater table was the minimum, with a biomass of 27.29 g. When the groundwater tables reached up to 0.3 m and 1.3 m, the root biomass of T. ramosissima seedlings was a maximum, with the biomass of 65.87 g and 75.05 g respectively. The vertical distribution of fine roots of P. euphratica seedlings and T. ramosissima seedlings was deeper as the groundwater depth increased. The deeper the groundwater table was, the greater the proportion of fine root biomass in the deep soil layer accounted. Salt and drought tolerance of P. euphratica seedlings was weaker than that of T. ramosissima seedlings.
The δ18O values of stem water decreased as the distance far away from river increased, and the δ18O value of stem water from T. ramosissima was lower than P. euphratica. At the river bank, P. euphratica and T. ramosissima could use stream water directly, the largest utilization rate of P. euphratica was 14.2%, and 35.3% for T. ramosissima. The contribution of shallow soil water to water uptake was high. Far away from the river bank, the contribution of subsoil water in 100-300 cm layers and utiliazation of groundwater increased, and the contribution of shallow soil water decreased. P. euphratica mainly used the soil water under 120 cm layers and also the groundwater, with the largest utilization rate of groundwater 40%~50%. T. ramosissima mainly used groundwater, with the largest rate 94.5%. P. euphratica could survive at suitable water tables (3.5~4.2 m), T. ramosissima could grow well in shallow groundwater area and the area with groundwater table under 4.5 m. Comparition the water sources between P. euphratica and T. ramosissima, the adaptability of T. ramosissima was superior than that P. euphratica under variable water availabilities.|