Aims Land cover changes can disrupt water balance and alter the partitioning of precipitation into surface runoff,evapotranspiration and groundwater recharge.The widely planted Eucalyptus trees in south-western China have the potential to bring about hydrologic impacts.Our research aims to elucidate the hydrologic balance characteristics of the introduced exotic Eucalyptus grandis×Eucalyptus urophylla plantation and to assess whether its high productivity results from high water use efficiency(WUE)or large water consumption.Methods A 400-m^(2) experimental plot was established in an E.grandis×E.urophylla plantation in south-western China.Water balance components,including stand transpiration(T_(r)),evapotranspiration(E_(t))and runoff(R)were obtained as follows:T_(r) was estimated based on sap flow measurements,E_(t) was estimated as the average of surface transpiration and evaporation weighted by the fractional green vegetation cover using a modeling approach,and R was collected using the installed metal frame.Net primary productivity(NPP)was obtained from allometric equation and annual diameter at breast height(DBH)increment determination.Important Findings Annual E_(t) and T_(r) were 430±31 and 239±17 mm,respectively.Annual T_(r) accounts for 56±8%of total evapotranspiration on average.WUE(NPP/T_(r))of the E.grandis×E.urophylla was estimated to be 3.3–3.9 mmol·mol^(−1).Based on the comparative analysis of T_(r) and WUE,E.grandis×E.urophylla had a high productivity due to its high WUE without exhibiting prodigal water use.Meteorological factors including vapor pressure deficit and global solar radiation(R_(s))were key factors regulating E_(t) and T_(r) in our research site.Annual surface runoff,E_(t) and canopy interception occupied 7%,27–30%and 16%of total precipitation,while the remaining 46–50%of precipitation was used for sustaining groundwater recharge and altering soil water storage.The higher runoff coefficient(7.1%)indicated the weaker capability of E.grandis×E.urophylla to reserve water
Aims Exotic fast-growing tree species have been commonly planted as pioneer species to facilitate ecological restoration in South China.Their growth and resource utilization behavior related to intrinsic physiology and structural properties have profound influences on forest ecosystem.However,the contrastive research focusing on water utilization features along with xylem anatomical properties between native and exotic species is scarce in South China.The objective of this study is to investigate the sapwood anatomical characteristics and water utilization conditions of native and exotic fast-growing species,and to elucidate the relationship between sap-flux density and conduit features.Methods We measured sap-flux density,conduit length,diameter and density of four native species(Schima superba,Michelia mac-clurei,Castanopsis hystrix and Castanopsis fissa)and four exotic species(Eucalyptus citriodora,Eucalyptus urophylla×grandis,Acacia auriculaeformis and Acacia mangium).Sap flux density was measured based on the Granier’s thermal dissipation probe method.The whole-tree water transport was quantified by mul-tiplying sap-flux density by sapwood area.The measurements of conduit characteristics were conducted by using segregation and slice method.Important Findings Sapwood area increased with the growing diameter at breast height(DBH)as a power function.Native species had a larger water-conducting tissue area than exotic species at the same DBH value when trees grew to a size with a certain value of DBH.The con-duit diameter of exotic species was significantly larger than that of native species.Conversely,native species,such as S.superba and M.macclurei,had longer conduit length and higher conduit den-sity than other tree species.Based on a physiological interpretation of the measured conduit characteristics,native tree species devel-oped a safe water transport system while exotic fast-growing tree species come into being an efficient system instead.Water trans-port increased with the growing DBH as a power func
Water storage has important significance for understanding water cycles of global and local domains and for monitoring climate and environmental changes. As a key variable in hydrology, water storage change represents the sum of precipitation, evaporation, surface runoff, soil water and groundwater exchanges. Water storage change data during the period of 2003-2008 for the source region of the Yellow River were collected from Gravity Recovery and Climate Experiment （GRACE） satellite data. The monthly actual evaporation was estimated according to the water balance equation. The simulated actual evaporation was significantly consistent and correlative with not only the observed pan （20 cm） data, but also the simulated results of the version 2 of Simple Biosphere model. The average annual evaporation of the Tangnaihai Basin was 506.4 mm, where evaporation in spring, summer, autumn and winter was 130.9 mm, 275.2 mm, 74.3 mm and 26.1 mm, and accounted for 25.8%, 54.3%, 14.7% and 5.2% of the average annual evaporation, respectively, The precipitation increased slightly and the actual evaporation showed an obvious decrease. The water storage change of the source region of the Yellow River displayed an increase of 0.51 mm per month from 2003 to 2008, which indicated that the storage capacity has significantly increased, probably caused by the degradation of permafrost and the increase of the thickness of active layers. The decline of actual evaporation and the increase of water storage capacity resulted in the increase of river runoff.
Min XUBaiSheng YEQiuDong ZHAOShiQing ZHANGJiang WANG
Agriculture could suffer the water stress induced by climate change. Because climate warming affects global hydrological cycles, it is vital to explore the effect of tree transpiration, as an important component of terrestrial evapotranspiration, on the environment. Thermal dissipation probes were used to measure xylem sap flux density of a Schima superba plantation in the urban area of Guangzhou City, South China. Stand transpiration was calculated by mean sap flux density times total sapwood area. The occurrence of the maximum sap flux density on the daily scale was later in wet season than in dry season. The peak of daily sap flux density was the highest of 59 g m-2 s^-1 in July and August, and the lowest of 28 g m-2 s-1 in December. In the two periods （November 2007-October 2008 and November 2008-October 2009）, the stand transpiration reached 263.2 and 291.6 ram, respectively. During our study period, stand transpiration in wet season （from April to September） could account for about 58.5 and 53.8% of the annual transpiration, respectively. Heat energy absorbed by tree transpiration averaged 1.4×10s and 1.6×10s kJ per month in this Schima superba plantation with the area of 2 885 m2, and temperature was reduced by 4.3 and 4.7℃ s^-1 per 10 m3 air.
Aims Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night.This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China.Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance.Methods Granier’s thermal dissipation method was used to determine the nighttime sap flux of A.mangium.Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees.Important Findings Nighttimesap flowwas substantial and showed seasonal variation similar to the patterns of daytime sap flowin A.mangium.Mean nighttime sap flow was higher in the less precipitation year of 2004(1122.4 mm)than in the more precipitation year of 2005(1342.5 mm)since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow.Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow,they could only explain a small fraction of the variance in nighttime sap flow.The total accumulated water loss(E_(L))by transpiration of canopy leaves was only;2.6–8.5%of the total nighttime sap flow(E_(t))during the nights of July 17–18 and 18–19,2006.Therefore,it is likely that the nighttime sap flow was mainly used for refillingwater in the trunk.The stem diameter at breast height,basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features,such as tree height,stem length below the branch,and canopy size.The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3%depending on different DBH class and was considerably higher in the dry season compared to the wet season.
