| As the world’s largest area of exposed carbonate rock,the ecosystem of the Southwestern China’s karst region is extremely fragile,and rocky desertification has intensified,and the issue of vegetation restoration has become the primary issue restricting local development.Future climate predictions indicate that the frequency and severity of extreme droughts in the world will be more serious,which will destroy the global water supply,thereby changing the water balance.Intense evaporation and drought will cause a serious reduction in soil moisture and affect the growth and restoration of vegetation.Studying the water use characteristics of woody plants in different habitats in karst areas of Southwest China is important for understanding plant growth habits.It helps to alleviate competition between species in karst areas with uneven water resources in time and space,thereby achieving symbiosis,which is important for promoting rocky desertification,regional ecological restoration is of great significance.The first choice for ecosystem restoration in rocky desertification control is vegetation restoration.In the future,under the extreme arid climate,water shortages will seriously threaten the survival of vegetation,understanding the water use characteristics of various vegetation species in karst areas is an urgent problem to be solved in ecological restoration in this area.In the Guizhou plateau mountainous area,which represents the overall structure of the southern karst rocky desertification ecological environment,the Bijie Sala Stream,Guanling-Zhenfeng Huajiang River,and Shibing Karst are selected as the study areas.From 2020 to 2022,use field instruments to continuously monitor the branches and leaves and water sources of selected vegetation species.Using methods such as field investigations,indoor experiments and data analysis,the collected samples are tested for stable carbon,hydrogen and oxygen isotopes.The Mix SIAR model was used to quantitatively study the water sources of different habitats,different vegetation and different seasons,distinguish the contribution ratio of different sources to vegetation,and clarify the water use characteristics of each vegetation species in the basin.It focuses on understanding whether there are differences in water use strategies of different life types of vegetation in different habitats,revealing whether there is interspecific competition and hydrological niche separation among different habitats,response of forest vegetation to hydrological conditions in a catchment,so as to provide a demonstration for suitable species collocation in karst areas and provide references for ecosystem restoration in karst areas.(1)It is found that the distribution and composition characteristics of hydrogen and oxygen isotopes of plant species and potential water sources in karst area are affected by temperature and precipitation in the basin,and the regional characteristics show obvious seasonal variation trend.The slope and intercept of the local atmospheric precipitation lines(LMWL)of the three small catchments are generally smaller than that of the global atmospheric precipitation lines.Most of the isotope values are plotted on the right side of the LMWL,which means that soil water comes from rainwater and has undergone certain evaporation.The isotopic composition of soil water varies with soil depth and plant species.During the sampling period,the isotopic composition of soil water became more and more negative with the increase of soil depth,and the difference of isotopic values and changes in shallow soil water(0-30cm)was higher than that in deep soil water(30-60cm).The isotopic composition of plant xylem was generally close to that of soil water,indicating that the main source of plant water was soil water.From2021.9 to 2022.7,the isotopic composition of xylem water and potential water sources of all plants showed a trend of first enrichment and then depletion,reaching the highest value in January.By comparing the isotopic composition of each plant species with the potential water source,we can understand the specific water source of plants and provide a scientific method for clarifying the water use pattern of plants in the basin.(2)The water use of different types of vegetation and the same type of vegetation have obvious seasonal variation characteristics in the three study areas.Some tree species generally use deep soil water,supplemented by shallow soil water,fissure water,groundwater,etc.,with little change between seasons.For example,walnut and betula bright-leaf,walnut in the rainy season in July and September mainly rely on 30-60 cm deep water(41.8±6.86%)and crack water(32.5±4.21%),0-30 cm surface water(22.5±2.86%)accounted for a small proportion of water,the rainy season is not much different,does not change with the change of precipitation.Always maintain a more stable water absorption mode;On the other hand,shrub species can use both shallow and deep soil water,and can change between seasons,showing strong survival ability.This fully reflects that different plant communities in different or the same habitats adapt to the seasonal variation pattern and instability of precipitation with their own water use strategies,so as to avoid or overcome the water stress crisis in a more effective way.(3)In the three study areas discussed in this paper,due to the diverse niches in karst areas,different water use strategies appear in different vegetation communities,so that multiple species living in the same habitat may have different water use patterns,which makes it possible for species to coexist,which is called hydrological niche separation.Deep root tree species,Juglans regia and Betula mainly depended on deep water source and fissure water,and there was obvious separation of ecohydrology niche from shrub species,such as Lonicera japonica 、Pyracantha fortuneana,which mainly depended on shallow water source.The tree species with two-form roots that can convert water between shallow layer and deep layer have seasonal ecohydrology niche separation from shrub species.Hydrological niche separation of vegetation does not always occur in different types of vegetation,but also occurs at different stages of the growth of a certain tree species.For example,the younger plant species will give priority to the use of shallow soil water and rainwater,and the growing species will gradually use the potential water sources such as deep soil water,fissure water or groundwater.ecohydrology niche separation avoids water competition among plants,which is of great significance for species continuity within habitats.Based on this,plant species with hydrologic niche separation can symbiosis within their common life without water competition,which is an important mechanism affecting plant community structure and maintaining plant coexistence in an ecosystem with limited water resources.(4)Water plays a crucial role in plant productivity and species diversity,and determines the distribution and ecological function of vegetation.Different plants have different degrees of adaptation strategies to water factors,mainly because different plant species have different degrees of development of the root system,different physiological structure,so water use strategies have different degrees of plasticity.Tree species Lonicera japonica and Pyracantha fortuneana bright leaf belong to deep roots of straight root plants,developed roots,can enable plants to continue to obtain deep stable water,reduce water competition with other plants.Some trees have two-form roots,which can switch between shallow and deep water sources with seasonal changes.This conversion ability reflects the ecological plasticity of plants.The greater the ecological plasticity,the higher the level of adaptation to the environment.Secondly,tree age also affects plant root development.The main body of young trees presents the characteristics of shallow layer.When the adult,the roots gradually dig into the deep soil cracks to grow.In order to adapt to the changes of environmental factors,plant roots and their distribution characteristics change correspondingly with soil water content,which can be considered as the result of water-tropism and geotropism of roots.Due to the water tropism of roots,many plant species have a dimorphic and functional root system.In an environment with limited water,plants with dimorphic roots are more competitive than plants that rely solely on surface soil water. |
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