Study On Hydrological Process Of Freshwater Wetland Forests In Tangpu Reservoir Area Based On Stable Hydrogen And Oxygen Isotope Techniques

The relationship between forest and water is a research hotspot in forest hydrology and ecology.In the context of global change,the change of precipitation pattern most directly and strongly impacts the hydrological processes in freshwater wetland forests.Freshwater wetland forests play an important role in regulating climate,flood storage and drought prevention,purifying water quality,and maintaining ecological integrity.Therefore,studying the hydrological processes of freshwater wetlands and their responsive mechanisms to changes of precipitation pattern could provide a scientific theoretical basis for the conservation and utilization of wetland forests and for the restoration and reconstruction of degraded wetland forests.Stable isotope techniques have high sensitivity and accuracy.Based on analysis of hydrogen and oxygen isotope characteristics of atmospheric precipitation,surface water,groundwater,soil water,and plant water?“five waters”?in freshwater wetland forests,some key scientific problems in hydrological processes of ecosystems can be examined.Both the urbanization process of the Yangtze River Delta and global changes affect the regional hydrology and ecosystem stability.In this study,three different types of freshwater wetland forests?broad-leaved,mixed coniferous and broad-leaved,and coniferous wetland forests?were selected in Tangpu Reservoir Area of Shaoxing City in Zhejiang Province.By using hydrogen-oxygen stable isotope techniques,we systematically quantified the variations of hydrogen and oxygen stable isotopes in the“five waters”of the hydrological processes,the contribution of different magnitudes of rainfall to soil water of each layer in the soil profile,and the water use strategy of dominant tree species in freshwater wetland forests.The results of the regulation of freshwater wetland forests on hydrological processes lay the foundation for quantifying hydrological cycle in freshwater wetland forests in the Yangtze River Delta.Primary conclusions are as follows:?1?The atmospheric precipitation line of Tangpu Reservoir Area was?D=8.36?¹⁸O+14.92.?D values in precipitation were between-147.5‰and 2.7‰,?¹⁸O values varied between from-19.05‰to 1.17‰.The?D and?¹⁸O values had obvious precipitation effect.The water vapor of precipitation occured in dry-season was mainly derived from relatively dry inland areas.By comparison,the water vapor of precipitation in wet-season mainly originated from the western Pacific and Indian Ocean with high humidity.?2?Soil water in wetland forests in the Tangpu Reservoir Area mainly originated from atmospheric precipitation and shallow groundwater.Within 9 days after different magnitudes of precipitation,the response of?D values of litter water to precipitation was most significant.The contribution of light rain to the litter in each forest was between 4.2%and 46.4%,and the contribution to surface soil water?0-20 cm?was from 6.1%to 42.2%.Precipitation event?different magnitudes of rainfall?was the main factor affecting the infiltration process in the soil profile of freshwater wetland forests in the Tangpu Reservoir Area?p<0.001?.The difference in the precipitation infiltration in the soil profile among the three wetland forests was significant?p=0.003?.Within 9 days after light rainfall?precipitation?10 mm?,the rainfall had the longest retention time in the soil of coniferous and broad-leaved mixed forest?6 days?,followed by broad-leaved forest?5 days?,and the shortest retention time in coniferous forest?3 days?.In the 9 days after moderate rainfall?10 mm<precipitation?25 mm?,the precipitation had the longest retention time in the soil of coniferous and broad-leaved mixed forest?8 days?,followed by broad-leaved forest?7 days?,and the shortest the coniferous forest?6 days?.?3?The dominant plant water was mainly derived from atmospheric precipitation,pre-rainfall soil water,and shallow groundwater in freshwater wetland forests.Within 9 days after different magnitudes of precipitation,different rainfall events were the main factor affecting plant water?D and the precipitation utilization of different tree species.The utilization rate of dominant plants in each forest for rainfall was:broad-leaved forest?8.6%-56.4%?>coniferous and broad-leaved mixed forest?7.1%-52.8%?>coniferous forest?13.0%-32.5%?.There were differences in the water use of dominant tree species in forests.Species with rich roots in surface soil layer?0-20 cm?like Liquidambar styraciflua had more utilization for light rainfall with the rate of 45.2%-1.5%.The species with deep root distribution like Populus deltoides exploited moderate and heavy rainfall.The peak utilization rate for moderate rainfall and for heavy rainfall was 64.7%and 88.0%,respectively.?4?A exchangeable relationship existed among rainfall,surface water,groundwater,soil water,and plant water in the different freshwater wetland forests.After different magnitudes of rainfall,rainfall,pre-rainfall soil water and shallow groundwater can be converted into plant water of dominant trees in the forests.Soil water was replenished by atmospheric precipitation and groundwater,and hydrogen and oxygen stable isotope fractionation occurred during recharging period.Rainfall replenished soil water and groundwater through infiltration and percolation.A conversion relationship?y=0.91x-3.61,R2=0.598,p<0.05?between surface water and groundwater in the study area closely related to local atmospheric precipitation.?5?Freshwater wetland forests in the Tangpu Reservoir Area had a regulation effect on the hydrological processes of ecosystems.The different hydrological effects of litter and soil layers between the different forests indicated that the regulation of hydrological processes was better in mixed forest than that in broad-leaved and coniferous pure forests.Within 9 days after light and moderate rainfall,the soil water?D of 80-100 cm layer varied from-34.1‰to-34.9‰and from-31.6‰to-25.1‰.The soil water?D was relatively stable,showed the freshwater wetland forest had a certain regulation effect on the rainfall?precipitation?20 mm?infiltration process in soil profile,and coniferous and broad-leaved mixed forest have better ability to regulate the infiltration of soil profiles than broad-leaved and coniferous pure forests.It is suggested that coniferous and broad-leaved tree species with different root distribution should be considered for mixing when the vegetation of degraded freshwater wetland forests is restored and freshwater wetland forests are reconstructed in the Yangtze River Delta.For example,Taxodium ascendens can be mixed with broadleaf species such as L.styraciflua and Betula nigra with shallow root distribution and P.deltoides with deep root distribution.The optimal and reasonable allocation of multiple tree species can maintain the stability of the community and enhance the adaptability of freshwater wetland forests in response to changes in global precipitation patterns.