Research On The Interception Process Of Near-Surface Water Vapor By Typical Vegetation In Urban Ecosystem

Dew is an crucial water input item in urban ecosystems.The condensation and evaporation of dew play an important role in water cycle processes as well as the plant growth.However,most of the studies do not distinguish the condensation and evaporation stages of dew,or only study the condensation process of dew,and have limited knowledge of the evaporation process of dew.The water transport processes during the evaporation stage of dew are still not well studied.In this study,the urban greenbelt areas in Northeast China were selected as the experimental area.A dew evaporation monitoring method was constructed to monitor the condensation and evaporation of dew on plants.Elucidated the variation patterns of dew condensation and dew evaporation in urban ecosystems on time and spatial scales,analyzed the meteorological factors affecting dew evaporation.And used the hydrogen and oxygen stable isotope tracing technique to quantitatively analyze the water vapor utilization by plant leaves during the evaporation process.The conclusions were as following:1.Poplar woodstick is a suitable dew evaporation monitor in greenbelt areas of urban ecosystems.In the urban ecosystem of northeast China,dew begins to condense from half an hour after sunset,and ends to condense and turns to evaporation half an hour before sunrise,until the end of evaporation three hours after sunrise.2.In 2021,the daily dew intensity in greenbelt areas of urban ecosystems in Northeast China is 0.042±0.025mm,the annual dew amount is 29.411mm.Dew intensity reaches its highest in September,with an daily condensation intensity of0.082±0.037mm,which is significantly higher than other months of the year（P<0.05）.The daily dew evaporation intensity is 0.026±0.016 mm,and the total annual evaporation amount is 17.08mm.The dew evaporation amount in July,August and September was significantly higher than the average evaporation amount in other months（P<0.05）,reaching 0.407-0.480mm.Both dew condensation intensity and dew evaporation intensity have obvious seasonal characteristics,and the trends are same.The leaf area index（LAI）of plants is an vital factor in the variation of dew condensation and evaporation.Spatial differences in dew condensation and evaporation were not significant.Just in August,dew evaporation amount was higher in the upper layer of the canopy（about 50cm above the ground level）than in the bottom layer of the plant（P<0.05）.The lower air resistance and higher solar radiation in the upper layer of the canopy were responsible for this phenomenon.Compared with other plants in the experimental area,Buxus sinica showed higher dew condensation and evaporation amount,with an average daily condensation amount of0.464±0.367mm and an average daily evaporation amount of 0.284±0.266mm.The higher LAI of Buxus sinica resulted in a larger condensation area and the ability to receive more radiant energy,leading to higher condensation and evaporation amount.3.Several meteorological factors combine to affect the dew evaporation process.Temperature,relative humidity（RH）,wind speed and the wind direction were the main meteorological factors affecting dew evaporation.Temperature was negatively correlated with dew evaporation intensity,while relative humidity,wind speed and the wind direction were positively correlated with dew evaporation intensity（P<0.05）.Temperature had a significant effect on dew evaporation intensity in spring and summer.The average temperature at night increased by 17.058±6.780℃,correspondingly,the dew evaporation intensity increased by 0.0161±0.014mm.On an annual scale,the effect of wind speed on the evaporation intensity of dew outweighed that of temperature.The nighttime wind speed in August was 0.324±0.121m·s^-1and the evaporation intensity was only 0.032±0.016mm.The nighttime wind speed in September increased to 0.378±0.214m·s^-1,and the corresponding evaporation intensity increased to 0.047±0.018mm,which was significantly higher than that in August（P<0.05）.Wind speed affects water evaporation by altering the rate of vapor diffusion.High wind speed accelerates the air convection and thus the evaporation of dew.Therefore,wind speed is the main meteorological factor that determines the intensity of dew evaporation in urban areas of northeastern China.In addition,47.9%of dew evaporation occurs in the 200°-240°wind direction range.The main wind direction in this zone is southwest.The southwestern monsoon enhanced convection and higher temperatures,which promotes increased evaporation.4.The proportion of different water vapor in the process of dew evaporation varies due to plant factors and different seasons.Combining the monitoring of dew evaporation process on different plants and the analysis of isotope tracing techniques,it was found that leaf morphology and microstructure are important factors affecting the amount of dew absorbed by leaves during evaporation.Trichomes,waxes,stomata and folded affect the contribution of dew to the leaves.All the plants selected in the experiment could directly absorb water through the leaf surface.The leaves of Crassula with a high water content had a higher water absorption capacity,and the leaves of Crassula arborescens absorbed 50.18±8.51%of the dew,which reflects this plant’s survival strategy.The stomatal area per unit index（SPI）is another crucial factor that affects leaf water absorption.The higher the SPI,the stronger the water absorption capacity.Chlorophytum comosum,with the highest SPI,absorbed19.29±1.75%of the dew.Leaves with three-dimensional wax generally have low wettability and less water absorption.For example,the leaves of Oxalis corniculata with a three-dimensional star shape absorbed only 1.02±0.69%of the dew.In winter,the lower temperature hinders the evaporation of dew.Therefore plants absorb more dew,88.4±6.01%of dew is absorbed by plants.In summer and autumn,when types of moisture sources of leaves are more,plants preferentially use groundwater or rainwater and absorb less dew.65.61±25.9%,64.78±19.8%and 60.52±13.26%of dew evaporates back to the atmosphere in July,August and September,respectively.Therefore,in arid and semi-arid areas or in dry seasons,dew can play a greater role in the plant or water cycle.