Plant Water-use Characteristics And Responses To Drought Stress In The Water-wind Erosion Crisscross Region

Extreme droughts are becoming increasingly frequent and severe with the changes in global climate and precipitation distribution patterns,which will seriously threaten the stability and healthy development of terrestrial ecosystems in arid and semi-arid regions.The Loess Plateau is a typical fragile ecological region in China,where large-scale vegetation restoration and increasing water use for production and living have led to an increasingly severe water shortage.Therefore,on the one hand,studying the response of typical plants to drought stress and water use mechanisms can better to understand the plant physiological regulation and adaptation mechanisms under drought stress and,on the other hand,provide a basis for revealing typical plant water use strategies and driving mechanisms.Meanwhile,it can also provide a reference for sustainable vegetation construction under the limited water resources of the Loess Plateau.Summarily,this study was conducted in the water-wind erosion crisscross region of the Loess Plateau.The typical sand fixing plants:Artemisia desterorum Spreng（A.desterorum）,Amygdalus pcdunculata Pall（A.pcdunculata）,Salix psammophila（S.psammophila）,Amorpha fruticosea（A.fruticosea）,Pinus sylvestris var.mongolica（P.sylvestris）were be chosen as research plants,and using field observations,soil column water control tests combined with isotope techniques firstly to systematic study the response of typical sand-fixation plants to drought stress,and secondly analyze water source differences,seasonal and age variation characteristics and its influencing factors of different sand-fixation plants,and finally discusses the water utilization strategies and competition mechanism of different sand-fixation plants under mixed and monoculture.The main results are listed below:（1）Soil texture significantly affected the sensitivity of A.pedunculata and S.psammophila to the water deficiency.Under the slight drought stress,the net photosynthetic rate（P_n）and stomatal conductance（G_s）of A.pedunculata in the loamy soil were significantly higher than those in the sandy soil,while opposite trends were observed for S.psammophila（p<0.05）.A significant difference was observed between the intrinsic water use efficiency（IWUE）of Amygdalus pedunculata pall and S.psammophila in soils with different textures under moderate and severe drought-stress（p<0.05）.The soil water availability threshold of the two studied plants varied slightly with the different indices selected.The water availability in the sandy soil was higher than that in the loamy soil when using the instantaneous gas exchange indices(relative net photosynthetic rate(R_Pn)and relative stomatal conductance(R_Gs))as the evaluation indices,while it showed an opposite trend when using the relative water use efficiency(R_WUE)as the evaluation index.The soil moisture thresholds of instantaneous gas exchange indices(R_Pn and R_Gs)at the transient scale were higher than those for the indices at the daily scale(relative daily average transpiration rate,R_Td).（2）P.sylvestris,A.pcdunculata and S.psammophila derived the majority of water from deep（59.2±9.7%from moist layer and groundwater）,intermediate（57.4±9.8%from stable and moist layers）and shallow sources（54.4±10.5%from active and stable layers）,respectively.Seasonally,three plant species absorbed more than 60%of water from moist layer and groundwater in the early dry season（June）;then they switched water sources（50.1～62.5%）to active and stable layers in the rainy season（July–September）.From October to November in the late dry season,P.sylvestris（54.5～66.2%）and A.pcdunculata（52.9～63.6%）mainly used water from stable and moist layers,while S.psammophila（52.6～70.7%）still extracted water predominantly from active and stable layers.Variations in the soil water profile induced by seasonal fluctuations in precipitation and groundwater level were the main external factors affecting seasonal water-use patterns in the three plant species.Whereas,the discrepancies in plant root distribution and water requirement are main internal factors resulting in differences of water-use patterns among three plant species.（3）Forest ages significantly affected the water use patterns of sand-fixation plants.In the dry season,the main water use depth of A.pcdunculata and P.sylvestris increased as they older.In contrast,the main water use depth of S.psammophila deepened firstly and then shifted upward as tree age increased.In the rainy season,different aged S.psammophila and P.sylvestris extracted water primarily from 0–120 cm soil layer.However,the 4-,10-and 16-yr A.pcdunculata absorbed water mainly from the 0–120 cm,0–120 cm and 40–120 cm soil layers,respectively.Seasonal variations of precipitation and profile soil water content,root biomass and their vertical distribution depth of different aged plants,and their water consumption were the main factors affecting the variation of water sources of different aged sand-fixation plants.（4）There were significant differences in the water sources of sand-fixation plants under mixed and single planting types.A transference of water-use pattern from deep（moist layer and groundwater）to shallow sources（active and stable layers）during the transition period between the dry and rainy seasons was observed for A.fruticosa,and P.sylvestris in both pure and multispecies plantations.However,A.desterorum in two planting types predominately absorbed water from the shallow soil（active and stable layers）across the whole growing season.From the annual contribution rate of different water sources,the three plant species in pure stands extracted water predominantly from the stable layer（37.0%～46.2%）,followed by the active layer（23.5%～37.0%）.In the mixed stand,however,all the plants absorbed water mainly from the active layer（33.0%～39.4%）,followed by the stable layer（30.5%～33.7%）.Multi-species plantation intensified shallow soil water competition,causing severe soil desiccation（SDI<0.5）and a higher rate of groundwater decline(0.29 m yr^-1).This study showed that the response of sand-fixation plants to drought stress was different among species and affected by soil texture in the water-wind erosion crisscross region.The water sources and utilization strategies of sand-fixation plants varied due to differences of plant species and stand age.Seasonal changes in precipitation and groundwater level were external factors that lead to different water sources for sand-fixation plants in the dry and rainy seasons,while plant species,growing age,and root distribution were internal factors for differences in water sources for different sand-fixation plants.Mixed planting intensified the competition for shallow soil water among different sand-fixation plants,leading to soil desiccation and a decline in groundwater levels.This study is beneficial to further understanding the relationship between sand-fixation plants and water.Meanwhile,it can provide a basis for the reasonable construction and management of sand-fixation vegetation under limited water resources in the water-wind erosion crisscross region.