Soil Water Use Strategies Of Dominant Plants In Typical Natural Shrubs In Loess Plateau Under Different Precipitation Gradient

The Loess Plateau has a low and unevenly distributed annual precipitation,which shows a decreasing trend over the years.Water shortage is the main limiting factor for vegetation restoration and ecological environment reconstruction in the Loess Plateau.In recent years,research on vegetation water uptake strategies of plant roots in arid and semi-arid regions has mainly focused on ecological forests and artificial forests.However,there is still a lack of clarity regarding the water sources and dynamic changes in typical naturally restored vegetation communities under different precipitation gradients,differences in root distribution characteristics,and similarities and differences in water use strategies between shrubs and herbaceous plants.This knowledge gap hinders the selection of drought-tolerant plants,optimization of vegetation restoration and afforestation configurations,enhancement of vegetation system stability,and ultimately,the efficient utilization of limited water resources in the Loess Plateau.In this study,we focused on the typical dominant shrub species(Artemisia ordosica,Zizyphus jujuba,Poa sphondylodes,Medicago Sativa and Chenopodium album)in the Loess Plateau to address the aforementioned issues.We controlled different precipitation gradients by reducing natural rainfall by 15% and 30%,and investigated the response characteristics of plant water sources and growth to these varying precipitation gradients.The potential water sources of vegetation communities were divided into shallow,middle,and deep soil water.The water uptake characteristics of plant roots under different precipitation gradients were quantitatively analyzed using the direct comparison method,stable water isotope technique,and Mix SIAR model.The following conclusions were primarily drawn:(1)The dynamic distribution characteristics of soil moisture under different precipitation gradients were quantified.Significant differences in soil water content were observed in both temporal and spatial dimensions among the typical natural shrub plots under different precipitation gradients.Temporally,the soil water content in the plots exhibited a consistent trend of change with time under different precipitation gradients.However,during dry years,the soil water content was significantly lower than that during wet years,and an overall increasing trend in soil water content was observed with increasing precipitation across all precipitation gradients.Spatially,the shallow soil moisture content showed distinct fluctuations over time.The fluctuation amplitude of middle soil moisture content was relatively small,while the deep soil moisture gradient moisture content exhibited a relatively stable trend of change.(2)The influence of different precipitation gradients on the spatial distribution of plant roots was clarified.Medicago sativa and Ziziphus jujuba exhibited welldeveloped root systems,with the majority of fine roots distributed in the surface soil layer(0-80 cm).Artemisia ordosica,Chenopodium album and Poa sphondylodes predominantly had their fine roots distributed in the surface soil layer(0-60 cm).Across all precipitation gradients,the root length density of each plant species decreased with increasing soil depth within the 0-120 cm soil profile.Under drought conditions,plants reduced their root distribution in the 0-40 cm soil layer to maintain their own growth,which promoted the growth of fine roots in the 60-100 cm soil layer.However,extreme drought conditions had detrimental effects on the overall root growth in the soil profile.(3)The response characteristics of plant growth and development to different precipitation gradients were elucidated.The plant leaf water potential in dry years was significantly lower than that in wet years,indicating that plants experienced drought stress during dry years,with the severity of stress following the trend of 135.9 mm <165.0 mm < 194.2 mm of precipitation.There were significant differences in growth parameters and leaf water potential among different plants under different precipitation gradients.In dry years,the order of variation was Artemisia ordosica < Medicago sativa< Chenopodium album,while in wet years,the order of variation was Artemisia ordosica < Medicago sativa < Poa sphondylodes < Ziziphus jujuba.Therefore,it was observed that the drought tolerance of typical dominant shrub species in the Loess Plateau followed the order of Artemisia ordosica > Medicago sativa > Poa sphondylodes > Ziziphus jujuba.(4)The response mechanisms of water utilization strategies of dominant natural shrub species to different precipitation gradients were revealed.Plants exhibited efficient utilization of limited water resources,whereby the vegetation roots could absorb water from deeper soil layers to meet their transpiration demands when the shallow soil moisture content was insufficient.When subjected to drought stress,the four typical dominant shrub species demonstrated flexible shifts in their water sources among different soil layers,and they exhibited complementary effects in terms of water utilization strategies.In summary,the typical dominant shrub species(Artemisia ordosica,Ziziphus jujuba,Poa sphondylodes,Medicago sativa,and Chenopodium album)in the Loess Plateau exhibit strong adaptation to drought and can survive in arid environments.Their drought tolerance and adaptability are ranked as follows: Artemisia ordosica >Medicago sativa > Poa sphondylodes > Ziziphus jujuba.To improve plant survival and conservation rates,drought-tolerant species such as Artemisia ordosica and Medicago sativa should be prioritized in extremely arid regions.In relatively humid areas with high precipitation,Poa sphondylodes and Ziziphus jujuba can be selected as primary choices to fully utilize the plants’ strong resource acquisition capabilities.