| Water is a vital limiting factor for ecological processes and functions in arid and semi-arid ecosystems and plays an important role in plant growth and distribution. Many Mediterranean relict plants including Tetraena mongolica, Ammopiptanthus mongolicus, Potaninia mongolica and Helianthemum songaricum in West Ordos Desert of Inner Mongolia are endangered species. Water use strategies of rare and endangered plants are determined by their long-term evolution and adaptation characteristics. The response of desert plants to climate change, especially changes in rainfall patterns, is affected by their own water use strategies. Thus, better understanding the water use of rare and endangered plants has important ecological implication for protecting rare and endangered plants, conserving species diversity, and maintaining ecological balance in this region.We conducted a systematic and quantitative study on the water source, water use rate, and water use efficiency of T. mongolica, A. mongolicus, P. mongolica, H. songaricum and non-rare endangered plant Sarcozygium xanthoxylum following different rainfall events during the growing season. We explored their water use strategy and dynamics using hydrogen, oxygen, and carbon stable isotope tracer technology, with a multivariate linear mixed model. We also investigated the plant community structure, soil structure, plant eco-physiological characteristics, environmental factors(i.e., precipitation and temperature) in the study area, and further established relationships of different sources(i.e. precipitation, soil water, groundwater, evaporated soil water and plant water) and their transformations. The results of this study are summerized as follows.The local meteoric water line in the West Ordos Desert during summer was δD = 7.287δ18O + 1.170(R2 = 0.937, n=38), indicating local precipitation experienced evaporation effect to some extent. The rain amount effect significantly existed, but the temperature effect was not obvious. The water vapor from continental northwesterly, oceanic monsoons southeasterly and southwesterly and local evaporation have interacted on precipitation in summer in the West Ordos Desert.Precipitation and groundwater were the sources of soil water in the West Ordos Desert. Small rainfall events(0~10 mm) affected the soil moisture andδD value of surface soil(0~10 cm), and above 30% of rainwater was kept in surface soil; medium rainfall events(10~20 mm) influenced the soil moisture andδD value at soil depth of 0~40 cm, while large(20~30 mm) and even more rainfall events(>30 mm)significantly influenced the water content andδD value of each soil layer, except for a layer of 100~150 cm. TheδD in surface soil water was directly affected byδD of rainwater, so theδD value of surface soil water largely changed. With the increase of soil depth, the contribution rate of precipitation to soil water decreased, indicating the impact by precipitation gradually decreased. The soil moisture andδD under 40 cm did not significantly change. Piston flow existed in the process of precipitation infiltration following small rainfall events in the West Ordos Desert. Preferential flow occurred when precipitation increased. Soil water in deep layers or the ground water could be replenished.T. mongolica, A. mongolicus, and S. xanthoxylum primarily relied on water in deep soil, whereas P. mongolica and H. songaricum depended on rainwater. When the precipitation was small, T. mongolica, A. mongolicus, and S. xanthoxylum tend to use stable water in deep soil. However, following a large rainfall event, they turned to use water from the upper soil layer(0~40 cm) that was supplied by rainwater. Plant utilization of different intensity rainfall was, in a decreasing order, P. mongolica, H. songaricum, A. mongolicus, S. xanthoxylum, and T. mongolica. Large rainfall events favored the growth of these desert plants. T. mongolica, A. mongolicus, and S. xanthoxylum had competition for soil moisture following small rainfall events.The average δ13C of plant stem water was A. mongolicus(-23.3‰) >T. mongolica(-25.2‰) >S. xanthoxylum(-25.8‰) >P. mongolica(-26.6‰) >H. songaricum(-26.8‰). The average water use efficiency(WUE) of plants was A. mongolicus(5.4 mmol CO2/mol H2O) >T. mongolica(4.8 mmol CO2/mol H2O) >S. xanthoxylum(4.6 mmol CO2/mol H2O) >P. mongolican(4.3 mmol CO2/mol H2O) >H. songaricum(4.2 mmol CO2/mol H2O). The photosynthetic rate(Pn) of plants was P. mongolica>H. songaricum>S. xanthoxylum>T. mongolica>A. mongolicus. The photosynthetic rate of T. Mongolica, A. mongolicus, P. mongolica and S. xanthoxylum leaves showed diurnal variation of “unimodal” curve and “bimodal” curve. The photosynthetic rate of H. songaricum leaves showed diurnal variation of “unimodal” curve, “bimodal” curve, and “multimodal” curve. The order of transpiration rate(Tr) of plants was P. mongolica>H. songaricum>S. xanthoxylum and T. mongolica>A. mongolicus. The transpiration rate of T. mongolica, A. mongolicus, P. mongolica and S. xanthoxylum leaves showed diurnal variation of “unimodal” curve and “bimodal” curve. The transpiration rate of H. songaricum leaves showed diurnal variation of “unimodal” curve.T. mongolica, A. mongolicus and S. xanthoxylum with deep roots had low Pn, low Tr, and high WUE, indicating that these three desert plants may have a drought tolerance strategy. P. mongolica and H. songaricum with shallow roots had high Pn, high Tr, and low WUE, indicating that they may have a drought avoidance strategy. Compare to non-endangered plant S. xanthoxylum, rare and endangered T. mongolica had a higher WUE but lower Pn and similar Tr, suggesting that T. mongolica may have a strong ability for survival. However, because of their similar water use strategy, they had a competitive relationship in absorption of soil moisture.The evaporated soil water, soil water and plant xylem water were under the local meteoric water line because of strong evaporation. Plant xylem water mainly comes from rainwater and soil water. Hydrogen and oxygen isotopes value in groundwater did not significantly change and kept stable. Groundwater recharged from precipitation by “piston flow” and “preferential flow” in the form of infiltration to groundwater. |
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