Hydrologic balance,net primary productivity and water use efficiency of the introduced exotic Eucalyptus grandis × Eucalyptus urophylla plantation in south-western China

AimsLand cover changes can disrupt water balance and alter the partitioning of precipitation into surface runoff, evapotranspiration and groundwater recharge. The widely plantedEucalyptustrees in south-western China have the potential to bring about hydrologic impacts. Our research aims to elucidate the hydrologic balance characteristics of the introduced exoticEucalyptus grandis×Eucalyptus urophyllaplantation and to assess whether its high productivity results from high water use efficiency (WUE) or large water consumption.     

Spring warming and carbon dioxide exchange over low Arctic tundra in central Canada

Tundra‐atmosphere exchanges of carbon dioxide (CO₂) and water vapour were measured near Daring Lake, Northwest Territories in the Canadian Low Arctic for 3 years, 2004–2006. The measurement period spanned late‐winter until the end of the growing period. Mean temperatures during the measurement period varied from about 2 °C less than historical average in 2004 and 2005 to 2 °C greater in 2006. Much of the added warmth in 2006 occurred at the beginning of the study, when snow melt occurred 3 weeks earlier than in the other years. Total precipitation in 2006 (163 mm) was more than double that of the driest year, 2004 (71 mm). The tundra was a net sink for CO₂ carbon in all years. Mid‐summer net ecosystem exchange of CO₂ (NEE) achieved maximum values of ?1.3 g C m^?2 day^?1 (2004) to ?1.8 g C m^?2 day^?1 (2006). Accumulated NEE values over the 109‐day period were ?32,?51 and ?61 g C m^?2 in 2004, 2005 and 2006, respectively. The larger CO₂ uptake in 2006 was attributed to the early spring coupled with warmer air and soil conditions. In 2004, CO₂ uptake was limited by the shorter growing season and mid‐summer dryness, which likely reduced ecosystem productivity. Seasonal total evapotranspiration (ET) ranged from 130 mm (2004) to 181 mm (2006) and varied in accordance with the precipitation received and with the timing of snow melt. Maximum daily ET rates ranged from 2.3 to 2.7 mm day^?1, occurring in mid July. Ecosystem water use efficiency (WUE_eco) varied slightly between years, ranging from 2.2 in the driest year to 2.5 in the year with intermediate rainfall amounts. In the wettest year, increased soil evaporation may have contributed to a lower WUE_eco (2.3). We speculate that most, if not all, of the modest growing season CO₂ sink measured at this site could be lost due to fall and winter respiration leading to the tundra being a net CO₂ source or CO₂ neutral on an annual basis. However, this hypothesis is untested as yet.     

A gauge to measure the mass flow rate of water in trees

A gauge that measures the mass flow rate of water in a growing tree is described. The gauge consists of an electric band heater wrapped around a section of the stem, a temperature controller that switches the current to the heater on and off so as to maintain a constant temperature rise across the heated section, and a timer to record the total time for which the heater is switched on. An energy balance shows the mass flow of water to be proportional to the time of operation of the heater. The magnitude of measurement errors is estimated using dimensional analysis and a numerical model. Experimental measurements of the flow rate of water in a tree using the gauge agree well with its recorded loss of weight.     

Diurnal patterns of canopy photosynthesis,evapotranspiration and water use efficiency in chickpea (Cicer arietinum L.) under field conditions

Diurnal changes in net photosynthetic rate (PN), evapotranspiration rate (ET) and water use efficiency (WUE=PN/ET) of field grown chickpea (Cicer arietinum) L. cv. H-355 were studied from the vegetative phase through maturirty at Haryana Agricultural University Farm, Hissar, India. The maximum photosynthetic rate (PN max) increased from the initial vegetative phase to pod formation and declined at a rapid rate from pod filling to maturity. The response of PN to photosynthetic photon flux density (PPFD) (400–700 nm) was temperature-dependent during the day, i.e. on cool days the PN rates were lower for certain quanta of PPFD during the first half than during the second half of day, and vice versa on warm days. ET was affected both by crop cover and evaporative demand up to flowering, but thereafter it was independent of crop cover and followed the course of evaporative demand. ET was related to air temperature during the day while PN was related to PPFD. There was a lag of two to three hours between PN_max (around noon) and ET_max (around 2 p.m.). WUE increased from the vegetative stage through flowering but decreased thereafter to maturity.Abbreviations DAS days after planting - ET evapotranspiration - LAI leaf area index - PAR photosynthetically active radiation (in figures) is equivalent to PPFD (see below) - PN net photosynthetic rate - PPFD photosynthetic photon flux density - WUE water use efficiency (= PN/ET)     

A highly conductive drainage extension to control the lower boundary condition of lysimeters

Lysimeters are used to study and monitor water, fertilizers, salts and other contaminants and are particularly valuable in transpiration and evapotranspiration research. Saturation at the soil bottom boundary in a lysimeter is inherent to its design. A drainage extension made of porous media with high hydraulic conductivity and substantial water holding capacity was devised to extend the lysimeter in order to produce soil moisture conditions mimicking those in the field. Design criteria that assure equal discharge in the soil and in the highly conductive drain (HCD) were established and formulated. Desired matric head at the lysimeter base is determined by HCD extension length. Its value can be manipulated and can range between saturation and the soil's field capacity. Conditions where the HCD is not limiting to flow are obtained through selection of the appropriate cross sectional area ratio between the soil in the lysimeter and the HCD. The validity of these criteria was confirmed with 200 l working lysimeters in the field, with and without plants, and with detailed flow tests utilizing smaller (15 l) lysimeters. Comparison of computed and measured matric head and leachate volume indicates that the proposed method can serve to maintain conditions similar to those in the field.     

内蒙古太仆寺旗退耕草地植物种类变化与水分收支

2008年和2009年(均为枯水年), 在半干旱区内蒙古太仆寺旗农田-草地生态系统国家野外站开展观测实验, 通过观测蒸散发(波文比系统)、土壤水分(烘干称重法)、降水量, 以及植被土壤特征调查, 基于水量平衡理论, 对比研究了3块天然草地、3块不同退耕时间草地共6个样地的水分收支, 旨在定量地评估退耕草地的水分收支, 为采取科学措施促进退耕草地尽快向天然草地过渡提供依据。结果表明: 1)随着退耕时间增加, 植被盖度逐渐增加, 但是群落中科、属、种的数量趋于减少, 且优势种从一年生的中旱生草本植物逐渐转变成多年生的旱生草本植物; 2)植被蒸腾是草原植被主要的耗水途径, 随着退耕时间增加, 退耕草地的蒸散发量呈增加趋势, 其最大值在4.5-5.8 mm·d^-1之间; 3)退耕草地土壤含水量平均值为0.09 m³·m^-3, 其水分剧烈变化主要发生在距地 表60 cm内, 且随退耕时间增加土壤含水量减少, 而天然草地土壤含水量平均值为0.06 m³·m^-3, 其水分剧烈变化发生在距地 表20 cm内; 4)随退耕时间增加, 退耕草地与天然草地的土壤水分与蒸散发在数值上差距逐渐缩小; 5)退耕草地水分收支基本平衡, 但在极枯年份(降水量174 mm)的生长季, 降水不能满足蒸散发需求, 呈现水分亏损。退耕草地逐步向天然草地过渡, 但是退耕草地的土壤水分在逐渐减少, 呈现“生境干旱化现象”。今后应加强对草地的封育与监测, 促进植物群落向水分利用效率更高、更适应半干旱环境的方向演替。     

Artificial drainage and associated carbon fluxes (CO2/CH4) in a tundra ecosystem

Ecosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near Cherskii, Far-Eastern Federal District, Russia. Methane (CH₄) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m⁻² (i.e. an ecosystem C loss) and +8 g C m⁻² after draining the study site. When taking CH₄ as another important greenhouse gas into account and considering the global warming potential (GWP) of CH₄ vs. CO₂, the ecosystem had a positive GWP during all summers. However CH₄ emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C-CO₂-e m⁻² before drainage in 2003 vs. 23 ± 26 g C-CO₂-e m⁻² after drainage in 2005).     

Energy and Mass Exchange Over Incomplete Vegetation Cover

Competition for fresh water between agriculture and domestic and industrial uses is increasing worldwide. This is forcing subsistence and commercial agriculture to produce more with less water. Consequently, it is crucial to properly and efficiently manage water resources. This requires accurate determination of crop water loss into the atmosphere, which is greatly influenced by the exchange of energy and mass between the surface and the atmosphere. Measurement of these exchange processes can best be accomplished by micrometeorological methods. However, most micrometeorological methods are very expensive, difficult to set up, require extensive post-data collection corrections and/or involve a high degree of empiricism. This review discusses estimation of evapotranspiration using relatively inexpensive micrometeorological methods in temperature-variance (TV), surface renewal (SR) and mathematical models. The TV and SR methods use high frequency air temperature measurements above a surface to estimate sensible heat flux (H). The latent heat flux (λE), and hence evapotranspiration, is calculated as a residual of the shortened surface energy balance using measured or estimated net radiation and soil heat flux, assuming surface energy balance closure is met. For crops with incomplete cover, the disadvantage of these methods is that they do not allow separation of evapotranspiration into soil evaporation and plant transpiration. The mathematical models (single- and dual-source) involve a combination of radiation and resistance equations to determine evapotranspiration from inputs of automatic weather station observations. Single-source models (Penman-Monteith type equations) are used to determine evapotranspiration over homogeneous surfaces. The dual-source models, basically an extension of single-source models, determine soil evaporation and plant transpiration separately over heterogeneous or sparse vegetation. These mathematical models have also been modified to accommodate inputs of remotely-sensed radiometric surface temperatures that enable estimation of evapotranspiration on a regional and global scale.     

民勤绿洲-荒漠过渡区水量平衡规律研究

以甘肃省民勤县刘家地村外缘的绿洲 -荒漠过渡地带为定位研究区域 ,利用长期调查和水文学研究方法从生态系统的角度对绿洲 -荒漠过渡区的水量平衡特别是土壤水分和蒸散量的时空格局与动态特征进行了研究。结果显示 :1沿绿洲至流动沙丘这一过渡带 ,随着与绿洲距离的逐渐拉大 ,土壤含水量减小 ,0～ 80 cm土层的月平均土壤含水量的大小顺序为红柳沙包(5 .6 1% ) >白刺沙包 (5 .4 7% ) >流动沙丘 (5 .2 2 % ) ;土壤水分垂直变化规律是由表层到深层依次递增。 2在蒸散日进程中 ,红柳沙包和白刺沙包的蒸散率峰值到来时间比流动沙丘早 1～ 2 h,且日最大蒸散速率大小顺序为红柳沙包 (0 .2 4 mm / h) >白刺沙包 (0 .2 0 m m/ h) >流动沙丘 (0 .18mm/ h) ;影响绿洲荒漠 -过渡区蒸散的主要气象因子为日照百分率、气温、空气饱和差以及风速等 ,它们与日蒸散量的斜率关联度分别为 0 .717、0 .6 4 3、0 .6 4 9和 0 .70 5。3生态系统总的水量平衡特点是 ,水分输入主要靠降水和土壤水分的水平运动补给 ,而土壤和植被蒸发散是系统的主要输出项。     

Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study

Climate‐induced changes in regional precipitation could have important implications for the carbon, water, and nutrient cycles of forest ecosystems. However, few studies have examined the response of deciduous forests to increases or decreases in precipitation. Therefore, the throughfall displacement experiment (TDE) was established in 1993 near Oak Ridge, Tennessee to examine the sensitivity of an upland oak (Quercus spp.) forest to ambient, wet (+33%), and dry (?33%) precipitation regimes. Sap flux measurements on co‐occurring tree species were scaled using species‐specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration (E_C) from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily E_C in the dry plot, and occasionally during extended droughts in the ambient and wet plots, declined as water potential in the upper 0.35 m soil profile approached ‐3.0 MPa. Seasonal patterns of soil water potential and treatment‐specific differences in E_C were dependent on precipitation frequency and intensity. Supplemental precipitation added to the wet plot increased seasonal E_C on average by 9% (range ?1% to 19%), whereas extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce seasonal E_C by 26–30% compared with the ambient plot. There was a strong correlation between seasonal E_C and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in seasonal E_C on the order of 40% were possible given TDE‐imposed reductions in soil water potential. Application of this equation to all years of the TDE (1994–2003) revealed considerable interannual and treatment‐specific variation in canopy transpiration. In general, a 33% removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23–32% reduction in seasonal E_C compared with the ambient plot. While droughts in deciduous forests are often limited in duration and tend to occur late in the growing season, soil water deficits of the magnitude observed in this study have the potential to impact local and regional forest water budgets.     

基于树干液流及涡动相关技术的葡萄冠层蒸腾及蒸散发特征研究

针对西北干旱区绿洲经济作物葡萄树冠层蒸腾及蒸散发特征的相关问题,在甘肃省敦煌市南湖绿洲开展无核白葡萄树液流速率及蒸散发观测试验,采用基于热平衡原理的包裹式茎流计,详细分析了典型生长季7—9月份葡萄树蒸腾耗水规律,使用\"单位叶面积上的平均液流速率SF×叶面积指数LAI\"的方法,实现了从单株到林分冠层蒸腾的尺度扩展,并通过与涡动相关技术所测蒸散发数据对比,详细研究了葡萄地冠层蒸腾及蒸散发规律。结果表明:典型生长季中葡萄树液流速率日变化为单峰型曲线,日均耗水量从2.76 kg到10 kg不等,胸径越大的葡萄树日均耗水量越大;冠层蒸腾及蒸散发日变化曲线亦为单峰型,白天8:00—12:00与17:00—20:00期间,葡萄冠层蒸腾与蒸散发曲线均比较吻合,该时间段葡萄地蒸散发绝大部分来源于葡萄冠层蒸腾,而12:00—17:00之间由于午后太阳辐射强烈土壤蒸发量增加,葡萄蒸散发大于冠层蒸腾;典型生长季3个月中,葡萄冠层蒸腾量的变化范围在1.88—8.12 mm/d之间,日均冠层蒸腾量为6.12 mm/d,蒸散发在1.74 mm/d至10.78 mm/d之间,日均蒸散发量为7.13 mm/d;日均土壤蒸发量约为1.01 mm/d,只占总蒸散发量的14.2%,日均冠层蒸腾占日均蒸散发的比重达到85.8%,说明该生长阶段冠层蒸散发以作物蒸腾为主。