Characteristics Of Stable Hydrogen And Oxygen Isotopes In The Typical Forest Hydrological Processes Of Mt.dinghu

Mt.Dinghu National Nature Reserve is located in the interaction zone of tropics and subtropics in South China and provides a unique and important opportunity for the study of global climate change.Many scientists have conducted studies on the establishment of hydrological models,precipitation,and the hydrochemical characteristics of surface runoff in Mt.Dinghu forest ecosystem using conventional hydrological methods.However,there is little information available about comprehensive and in-depth understanding of the water cycle in typical forest ecosystems?Pinus massoniana coniferous forest,P.massoniana coniferous and broad-leaved mixed forest,monsoon evergreen broad-leaved forest?located in Mt.Dinghu.Besides,quantitative and systematic studies on water sources and water use efficiency of dominant plants as well as hydrological processes of forest ecosystems were stillneeded.The inherently stable isotopes of hydrogen??D?and oxygen(?¹⁸O)in water molecules are good tracers in the study of effects of forest vegetation on hydrological processes.Stable isotope technique can be used to study different hydrological processes in forest ecosystem including precipitation,surface water?stream?,soil water,groundwater,and plant water together to clarify those processes and mechanisms.Stable isotope technique also overcomes the disadvantages of conventional hydrological methods comprehensively reflects the retention ability of vegetation and soil to precipitation,and thus can be used to quantify interception effect on water from canopy,soil moisture,and plant absorption.In this study,the composition and transformation of ?D and ?¹⁸O in precipitation,soil water,plant water,surface water?stream?,and groundwater in three typical forests,P.massoniana coniferous forest,P.massoniana coniferous and broad-leaved mixed forest,and monsoon evergreen broad-leaved forest being located in Mt.Dinghu were examined using the method of stable isotope technique,positioning observation and mathematical models.Also,the migration process of rainfall with different levels in the soil profile,the water utilization rate,and water use strategy of different plants which were dominated in arbor,shrub and grass layers,respectively,were studied in order to reveal the adaptation mechanism of forest vegetation to the precipitation pattern changes,and provide theoretical suggestions for the rational utilization and scientific management of regional water resources.The main conclusions are as follows:The meteoric water line equation was ?D = 7.875?¹⁸O + 9.412?R2 = 0.982,n = 120?.?D values averaged ?36.35 ‰,varying from ?1¹⁸.26 ‰ to 15.52 ‰ and ?¹⁸O was?5.81 ‰,varying from ?16.05 ‰ to 2.25 ‰.Atmospheric precipitation excess deuterium?d?had a seasonal pattern with low values in winter and high values in summer.Dry air mass mainly came from local evaporation,North China,and the cold and dry season of the Eurasian continent;Wet air mass came from the Western Pacific,South China Sea,and Indian Ocean.Precipitation and shallow groundwater were the sources of soil water in the forest of Mt.Dinghu National Nature Reserve.The ?D of surface soil water?0-10 cm?was mainly controlled by rainfall ?D.The faster the rainwater penetration from the soil surface to the deep soil,the greater the rainfall contribution to the soil water at all levels that increased the precipitation intensity.The rainfall contribution rate to litter water was the highest?0-47.7%?,and to the soil layer of 40-100 cm was the lowest,near to zero in 5 days in the light rainfall event of 5-10 mm.The rainfall contribution rate to surface soil?0-10 cm?was the highest?29.6 – 83.1%?,and to the soil layer of 80-100 cm was the lowest?14.3-55.5%?in the moderate rainfall event of 10-20 mm.The precipitation infiltrated the deep soil?>80 cm?and the rainfall contribution to the soil layer of 80-100 cm in the heavy rainfall of >30 mm was 16.1%-70.7% on the same day as precipitation occurred,regardless of in the wet or dry season.Deep soil water?80-100 cm??D value was basically kept stable in the light rainfall event of ?20 mm,which suggests that vegetation and soil structure of the forest in Mt.Dinghu National Nature Reserve played a significant role in the regulation of the precipitation infiltration in the soil profile.Precipitation and pre-rainfall soil water were the main water sources of plant water in Castanopsis chinensis,Schima superba,Psychotria asiatica,Blechnum orientale,and P.massonianain in Mt.Dinghu National Nature Reserve.In the light rainfall event of 5-20 mm,the rainfall utilization rate of plants within 5 days after precipitation was 0 – 80.1% for B.orientale and 0 – 37.5% for S.superba in mixed P.massoniana/ broad-leaved forest;The rainfall utilization rate of plants within 5 days after precipitation was 6.9 – 59.4% for S.superba and 0 – 22.7% for C.chinensis in the monsoon evergreen broad-leaved forest.In the heavy rainfall event of >30 mm either in the wet or dry season,B.orientale within 5 days after the precipitation had the highest rainfall utilization rate?31.6 – 91.5%?,while the rainfall utilization rate of S.superba was the lowest?4.7 – 26.5%?.Plant water uptake in mixed P.massoniana broad-leaved forest and the P.massoniana coniferous forest was,in a decreasing order,B.orientale,C.chinensis,P.massoniana,and S.superba.In the monsoon evergreen broad-leaved forest,the plant water uptake in the rainfall event of 5 – 20 mm was,in a decreasing order,S.superba,B.orientale,P.asiatica,and C.chinensis;the plant water uptake in the rainfall event of >30 mm was,in a decreasing order,B.orientale,P.asiatica,C.chinensis,and S.superba.The water uptake strategies of these different plant functional types were related to the root distribution of plants.Different water uptake strategies among the different plant functional types in the forest of Mt.Dinghu help maintain the stability of the vegetation community.In the light rainfall event of 5 – 10 mm,rainfall had a weak effect on surface water?stream water?,and the rise or fall of rainfall ?D(?¹⁸O)caused the same change of stream water ?D(?¹⁸O),which occurred in four days after rainfall.The change curve of surface water ?D(?¹⁸O)changed with rainfall ?D(?¹⁸O),which occurred in three days after the moderate rainfall event of 10 – 20 mm and the first day after the heavy rainfall event of >30 mm.This illustrated that the vegetation structure of Mt.Dinghu National Nature Reserve played a significant role in the regulation of surface water?stream?.