| With the development of the social economy,more and more tunnels have been excavated,which has caused a series of problems such as tunnel water inflow and water leakage.In the karst area,the groundwater circulation system is more complicated and fragile due to the shallow soil layer and underground karst fissures.Tunnel water inflow and water leakage caused by tunnel excavation may have a serious impact on the fragile karst groundwater circulation system.Thus,the upper surface water will dry up,may affecting the growth and development of the overlying vegetation.Zhongliang Mountain is located in the west of Chongqing and is a typical karst trough valley.Since 1999,Zhongliang Mountain has successively built a number of tunnels.Among them,three tunnels were built in Longfeng karst trough valley in the north of Zhongliang Mountain?Tunnel affected valley?.In the Longche karst trough valley in the south of Zhongliang Mountain,no tunnel was built?Tunnel unaffected valley?.The tunnel water leakage caused by tunnel excavation caused the spring water inside the Longfeng karst trough valley to dry up,and the paddy field turned into dry land.The local residents need to pump water from the mountain,which seriously affected the local production and life.In Zhongliang Mountain karst trough valley,surface water caused by groundwater exploitation and tunnel excavation may cause serious impact on plant growth.Plants water use strategy?plants water uptake patterns and plants water use efficiency?are important manifestations of a region's plant adaptation to environment.Therefore,systematic research on plants water use strategy in karst trough valley has important theoretical and practical significance for ecological environment protection and rock desertification control in karst trough valley.In this study,Fraxinus chinensis Roxb,Citrus reticulate,Viburnum chinshanense Graebn,Berchemia polyphylla were selected in the Zhongliang Mountain karst trough valley of Chongqing.The plants water use strategies of tunnel affected valley and tunnel unaffected valley were analyzed,and the impact of tunnel construction on plant water use strategies was explored.The main results were as follows:?1?The soil moisture contents of different soil depths in the tunnel affected valley and the tunnel unaffected valley showed obvious changes with the change of precipitation,and the change trend of soil moisture contents was consistent with the precipitation.Although the soil moisture contents show similar changes in the tunnel affected valley and the tunnel unaffected valley,at the same depth,the soil moisture contents of the tunnel affected valley is significantly lower than that of the tunnel unaffected valley?at the same time,the soil moisture contents in the tunnel affected valley is 4%lower than that in the tunnel unaffected valley?.And there is a significant difference between the soil moisture content in the tunnel affected area and the soil moisture content in the tunnel-affected area?P<0.01?.Such obvious difference in soil moisture contents between two valleys,showing lower soil moisture contents of both soil layers in the tunneling affected valley,indicated that tunnels excavation could dry soil moisture contents in karst areas.?2?The difference in?2H and?18O values of xylem water of each plant was not significant,indicating that each plant has a similar source of water.However,there are significant seasonal differences in the?2H and?18O values of the xylem water of each plant during the year?P<0.01?,indicating that the plant water source has obvious seasonal changes.At the same time,the?2H and?18O values of the xylem water of the plant in tunnel affected valley are more positive than the?2H and?18O values of the same plant xylem water in the tunnel unaffected valley.The?2H and?18O values of the plant xylem water are in the tunnel affected valley and tunnel unaffected valley.This difference indicates that tunnel excavation may have altered the source of plants water uptake patterns in the tunnel affected valley.The IsoSource model was used to calculate the plants water uptake patterns.The results showed that there were differences in plants water uptake patterns in different seasons?the proportion of groundwater used by plants is 46%-69%in winter and spring,and is reduced to 20%-44%in summer and autumn?and tunnel excavation not only reduced the soil moisture contents,but also changed the proportion of plants water uptake patterns in karst trough valley:Tunnel construction makes the proportion of groundwater used by plants in the tunnel affected valley higher than that of the tunnel unaffected valley?in winter and spring,plants uptake groundwater at the tunnel affected valley is 9%higher than that plants uptake groundwater at tunnel unaffected valley,plants uptake groundwater at the tunnel affected valley in the summer and autumn is 5%higher than that plants uptake groundwater at tunnel unaffected valley?.The calculation results also show that the proportion of groundwater used by arbors in the same season is higher than that of shrubs?in the same season,arbor uptake 5%more groundwater than shrubs?.?3?The?13C value of plant leaves showed the most positive in spring,the most negative in summer,and the?13C value of plant leaves in autumn and winter is more positive.This indicates that plants have the highest water use efficiency in spring,while plants water use efficiency is low in summer.In the same season,the?13C values of plant leaves in the tunnel affected valley were higher than those in the tunnel unaffected valley.This difference indicates that the plants in the tunnel affected valley have higher water use efficiency than the plants in the tunnel unaffected valley.The?13C values of different life-type plants showed higher arbors than shrubs in the same season,this indicates that arbors have higher water use efficiency than shrubs.This is confirmed by the calculation of plants water use efficiency?WUE?:The water use efficiency of the same plants in the tunnel affected valley is higher than that of the tunnel unaffected valley?the water use efficiency of plants in the tunnel affected valley is higher than that of the tunnel unaffected valley by 0.07mmol/mol,the water use efficiency of plants in the tunnel affected valley increased from 0.45mmol/mol in summer to 1.98mmol/mol in spring,while the water use efficiency of plants in tunnel unaffected valley increased from 0.41mmol/mol in summer to 1.89mmol/mol in spring?,and in the same season,arbors have higher water use efficiency than shrubs?the water use efficiency of arbors is 0.02mmol/mol higher than that of shrubs?,indicating that arbors use a more conservative water use strategy than shrubs.?4?Correlation analysis between soil moisture contents and plants water use efficiency showed that soil moisture contents and plant water use efficiency were negatively correlated.The correlation between plants water use efficiency and soil moisture contents in the tunnel affected valley?R2=0.73,P<0.01?is stronger than that of tunnel unaffected valley water use efficiency and soil moisture contents?R2=0.64,P<0.01?.This indicates that plants are more sensitive to changes in soil moisture contents in tunnel affected valley.Correlation analysis between water use efficiency and soil moisture contents of arbors and shrubs showed that the water use efficiency of arbors and shrubs was significantly negatively correlated with soil moisture contents in different seasons.Among them,shrubs are more sensitive to changes in soil moisture contents?Arbor R2=0.64,P<0.01;shrub R2=0.80,P<0.01?,this may be due to the fact that shrubs are shallow roots compared to trees,so shrubs are more sensitive to changes in soil moisture contents. |
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