Study On The Ecohydrological Processes Of Black Locust Forest Land On The Loess Plateau

Significant conversions of farmland to woodland and grassland were found to have resulted in increased land coverage,enhanced soil and water conservation and improved ecological environment in the Loess Plateau region of China.However,larger scale afforestation in water limited area has also resulted in some negative effects,such as soil desiccation and forest declination,which may be related to forest evapotranspiration.Excessive evapotranspiration after afforestation may result in soil water imbalance and soil desiccation,which will affect forest growth and may cause forest decline during extreme drought years.Therefore,it is necessary to study the ecohydrological processes of afforested forest in the Loess Plateau region,which could reveal the water consumption characteristics of representative vegetation in this region,and also of significant importance to understand the interactive relationships between forest and water,and helpful for ecological rehabilitation and sustainable development of ecosystem establishment.This thesis clustered full-fledged field investigation and laboratory measurements.Along with the field and laboratory work,this thesis represents the study of vascular traits of black locust and its relationships to environmental factors;then it represent the laboratory work of recalibration of the Granier's empirical equation to calculate sap flux density by thermal dissipation method.Following,the thesis analyzes the characteristics of black locust transpiration,rainfall partition and evapotranspiration and its relationships to environmental factors.Finally,this thesis represents the modeling work of Hydrus1-D model to reveal the dynamics of soil water content and evapotranspiration and give a full analysis of the characteristics of soil water balance.The main conclusions are congregated as the following:1.Our results showed marked differences?p=0.032?in vascular traits between actively growing?15-year-old?and declining?35-year-old?trees.Compared with actively growing trees,conduit diameters of declining trees narrowed,their frequency increased with positive skew in distribution and cumulative theoretical hydraulic conductivity declined.In all the tree samples,hydraulically weighted mean conduit diameter tapered acropetally from breast height to the apex of the stem.The degree of conduit tapering in actively growing trees?0.244,95%CI 0.201–0.287?was in agreement with hydraulic optimality model prediction?optimal value of 0.20?,while trees in declining status had markedly lower conduit tapering?0.175,95%CI0.146–0.198?.This indicated that hydraulic resistance in declining trees increased with tree height,which will result in lower stomatal conductance and assimilation rate in leaves at tree top.Differences in hydraulic properties were mainly directly attributed to variations in tree height,rather than variations in soil water content and stem diameter.For trees in both actively growing and declining statuses,correlation between conduit diameter and soil water content was an indirect effect of variation in tree height,rather than a direct response to drought stress.2.In order to determine the relationship between sap flux density and flux index K for black locust,measured sap flux density by gravimetric measurements and calculated K with thermal dissipation probe measurements were used.In this study,the regression relationship between measured sap flux density and K was a power function?similar to Granier's original equation?,with coefficient??0.051?significant?p<0.0001?larger than the original value of0.0119,and coefficient??1.18?similar?p>0.05?to the proposed value of 1.231.Actually,the empirical equation could result in about 80%reduction in sap flux density compared with gravimetric measurements,while the discrepancy between thermal dissipation measurements and gravimetric measurement reduced to 3.5%underestimation via our lab calibrated equation.Sap flux density can be underestimated if part of the sensor probes is in contact with inactive xylem,therefore the Clearwater's correction method for inactive xylem should be used to correct the measured values.Furthermore,no physical basis exists for the thermal dissipation technique.Therefore the technique should be calibrated for each new species on which it is used.There were significant time lags between the diurnal courses of sap flux density?SFD?and the diurnal course of environmental factors.In this study,the variation of SFD lagged behind photosynthetically active radiation?Rn?by a factor of¹ hour,while the variation in vapor pressure deficit?VPD?and air temperature?T?lagged that in Rn by a factor of about 2hours.During experimental periods in 2015 and 2016,the diurnal patterns of SFD and diurnal time lags between SFD and environmental factors were similar,but with larger hysteresis loops in 2015 than that in 2016.On daily basis,potential evapotranspiration?ET0?and transpiration index?VT?were the two dominant factors of SFD,followed by Rn,T and VPD.However,there were no clear relationship between SFD and soil water content?SWC?.With the combination of environmental factor ET0 and the physiological factor leaf area index,an simple empirical regression equation?SFD?28?21.89ET₀?LAI?10?59.84?was established,which could explain 75%of the variation in SFD of black locust.On hourly basis,Rn was the dominant factor of SFD,followed by VPD and T.In this study,there exists no clear relationship between SFD and SWC.VPD had a threshold control on sap flow at threshold values of 1.9 kPa for 2015 and 1.6 kPa for 2016.3.During experimental periods in 2015 and 2016,measured throughfall,stemflow and calculated interception loss of black locust forest accounted for 82.1%,1.6%and 16.3%of incident precipitation,respectively.Corresponding values for Chinese pine were 75.0%,1.0%and 24.0%,respectively.Significant differences?p<0.05?in precipitation partitioning were detected between black locust and Chinese pine.Compared with Chinese pine,Black locust has a higher efficiency in stemflow generation and a lower canopy interception loss,indicating that black locust has an advantage in precipitation partitioning?receiving more net precipitation?than Chinese pine.The threshold values of precipitation for throughfall and stemflow generation were 1.1 mm and 2.3 mm for black locust,respectively.Corresponding values were 1.6 mm and 5.0 mm,respectively,for Chinese pine.The larger differences in precipitation partitioning between these two tree species may largely attribute to their morphological differences.The depth?mm?of precipitation partitioning can be accurately predicted by precipitation characteristics,while other influencing factors?e.g.meteorological variables?other than only precipitation characteristics should be included to get a more precise prediction of the percentage?%?of precipitation partitioning.4.During the growth season in 2016,the degree of energy balance closure measured by eddy covariance system was 74%,which was moderately acceptable as it fell within the closure order of 60-90%,indicating that the data measured in this study can be used with confidence.The diurnal course of average latent heat?LE?during growth season exhibits a unimodal trend,with the smallest value occurred in night and the largest value in late noon.LE accounts for 61%of net solar radiation?Rs?,indicating that the majority of accepted energy was used for water evaporation and black locust transpiration.During experimental period in this study,the controlling factors for black locust forest evapotranspiration were potential evapotranspiration?ET0?,net solar radiation?Rs?,vapor pressure deficit?VPD?,air temperature?T?and leaf area index?LAI?,which could explain 83%?81%?76%?63%and 46%of the variation in LE,respectively.In contrast,soil water content shows no clear relationship with LE on daily basis.The maximum monthly LE occurred in June with a value of 282.6 MJ m^-2month^-1,which is equivalent to 115 mm of water depth;while the minimum monthly LE occurred in October with a value of 128.9 MJ m^-2month^-1,which is equivalent to 52.5 mm of water depth.The total measured latent heat during growing season was1262.2 MJ m^-2,which is equivalent to 515 mm of water depth and accounts for 112%of precipitation?461 mm?during that time period.This indicates that soil water was excessively consumed by black locust forest and was in a deficit state during growing season.5.In this study,the Hydrus1-D model calibrated with soil moisture data and growing season transpiration measured in 2015 accurately simulated the dynamics of soil water content,forest transpiration and evapotranspiration in 2016.Based on modeling results in2015 and 2016,black locust forest evapotranspiration accounts for 114.4%of annual precipitation in this study area,indicating that soil moisture was excessively consumed by black locust.This may result in soil water imbalance and soil desiccation,which will slow down vegetation growth rate and cause forest decline.In this study,black locust transpiration was the main component of evapotranspiration,followed by soil evaporation and canopy interception,which accounts for 62.6%,26.8%and 10.6%of evapotranspiration,respectively.The ratio of actual forest transpiration to potential forest transpiration?WSI?decreased with the increase of soil water content.Moreover,soil water content had a threshold control on WSI in this study,with the threshold value of 0.31 cm³ cm^-3 in 2015 and 0.29 cm³ cm^-3 in2016.