Climatic water deficit,tree species ranges,and climate change in Yosemite National Park

Aim (1) To calculate annual potential evapotranspiration (PET), actual evapotranspiration (AET) and climatic water deficit (Deficit) with high spatial resolution; (2) to describe distributions for 17 tree species over a 2300‐m elevation gradient in a 3000‐km² landscape relative to AET and Deficit; (3) to examine changes in AET and Deficit between past (c. 1700), present (1971–2000) and future (2020–49) climatological means derived from proxies, observations and projections; and (4) to infer how the magnitude of changing Deficit may contribute to changes in forest structure and composition. Location Yosemite National Park, California, USA. Methods We calculated the water balance within Yosemite National Park using a modified Thornthwaite‐type method and correlated AET and Deficit with tree species distribution. We used input data sets with different spatial resolutions parameterized for variation in latitude, precipitation, temperature, soil water‐holding capacity, slope and aspect. We used climate proxies and climate projections to model AET and Deficit for past and future climate. We compared the modelled future water balance in Yosemite with current species water‐balance ranges in North America. Results We calculated species climatic envelopes over broad ranges of environmental gradients – a range of 310 mm for soil water‐holding capacity, 48.3°C for mean monthly temperature (January minima to July maxima), and 918 mm yr^?1 for annual precipitation. Tree species means were differentiated by AET and Deficit, and at higher levels of Deficit, species means were increasingly differentiated. Modelled Deficit for all species increased by a mean of 5% between past (c. 1700) and present (1971–2000). Projected increases in Deficit between present and future (2020–49) were 23% across all plots. Main conclusions Modelled changes in Deficit between past, present and future climate scenarios suggest that recent past changes in forest structure and composition may accelerate in the future, with species responding individualistically to further declines in water availability. Declining water availability may disproportionately affect Pinus monticola and Tsuga mertensiana. Fine‐scale heterogeneity in soil water‐holding capacity, aspect and slope implies that plant water balance may vary considerably within the grid cells of kilometre‐scale climate models. Sub‐grid‐cell soil and topographical data can partially compensate for the lack of spatial heterogeneity in gridded climate data, potentially improving vegetation‐change projections in mountainous landscapes with heterogeneous topography.     

用Minirhizotrons观测柠条根系生长动态

 Minirhizotrons是一种非破坏性、定点直接观察和研究植物根系的新方法。该文介绍了用Minirhizotrons测定植物根系的方法,并同根钻取原状土样法进行了比较;探讨了根系生长动态同土壤含水量间的关系。试验于2004年植物生长季在沙坡头沙漠试验研究站的水分平衡观测场的人工柠条(Caragana korshinskii)林进行,结果表明：Minirhizotrons 管埋入土壤后需要10个月时间允许柠条根系在其周围定居,其观测图片中的根系代表了管子周围2.6 mm土层的根系。柠条根系生长动态和土壤水分变化相关,含水量的升高导致根系的大量繁殖,而根系吸水及蒸发散又导致含水量的减少;在2004年植物生长季,土壤水分和根系的这种相互作用出现了两次,但根系生长高峰比土壤含水量高峰滞后20 d左右。     

The influence of crop management on the water balance of lupin and wheat crops on a layered soil in a Mediterranean climate

The influence of time of sowing and sowing density on evapotranspiration and drainage loss beneath wheat (Triticum aestivum cv. Spear) and lupin (Lupinus angustifolius cv. Gungurru) crops grown on a layered soil was investigated for three seasons in a Mediterranean climate in Western Australia. The aim of the study was to investigate whether managing crops to maximise their canopy growth would increase their water use and minimise groundwater recharge contributing to dryland salinity. A soil water balance approach was used to estimate evapotranspiration, with changes in soil water content measured with a neutron water meter. The study was carried out on a layered soil typical of agricultural soils in the region with variable depth to clay (0.22–0.38 m) and a marked contrast in hydraulic properties between the topsoil and subsoil. As a result of the low permeability subsoil, a perched water table occurred in the sandy topsoil in each of the three seasons under study during winter when rainfall was high and potential evaporation low. Perched water tables persisted for 2–3 months, with hydraulic gradients consistently downward causing drainage losses to occur. Although crop management had a large influence on shoot and root development, evapotranspiration from the different treatments was generally similar. Drainage losses were not influenced by either crop type, time of sowing or sowing density, because potential evaporation and hence evapotranspiration was low during the period when drainage losses occurred. The total drainage loss measured in each season was different, with losses ranging from 20.1 to 22.2 mm in 1990, from 40.4 to 46.7 mm in 1991 and from 49.4 to 66.6 mm in 1992. The increase in drainage loss from 1990 to 1992 was a result of progressively more seasonal rainfall in 1990, 1991 and 1992. It was concluded that there was little scope to increase water use and decrease deep drainage through crop management for sites with climatic conditions where winter rainfall exceeds potential evaporation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ecohydrology and the Partitioning AET Between Transpiration and Evaporation in a Semiarid Steppe

Water availability defines and is the most frequent control on processes in arid and semiarid ecosystems. Despite widespread recognition of the importance of water in dry areas, knowledge about key processes in the water balance is surprisingly limited. How water is partitioned between evaporation and transpiration is an area about which ecosystem ecologists have almost no information. We used a daily time step soil water model and 39 years of data to describe the ecohydrology of a shortgrass steppe and investigate how manipulation of soil and vegetation variables influenced the partitioning of water loss between evaporation and transpiration. Our results emphasize the overwhelming importance of two environmental factors in influencing water balance processes in the semiarid shortgrass steppe; high and relatively constant evaporative demand of the atmosphere and a low and highly variable precipitation regime. These factors explain the temporal dominance of dry soil. Annually and during the growing season 60–80% of the days have soil water potentials less than or equal to −1.5 MPa. In the 0–15 cm layer, evaporation accounts for half of total water loss and at 15–30 cm it accounts for one third of the loss. Annual transpiration/actual evapotranspiration (T/AET) ranged from 0.4–0.75 with a mean of 0.51. The key controls on both T/AET and evaporation/actual evapotranspiration in order of their importance were aboveground biomass, seasonality of biomass, soil texture, and precipitation. High amounts of biomass and late timing of the peak resulted in the highest values of T/AET.     

Patterns and controls of the variability of radiation use efficiency and primary productivity across terrestrial ecosystems

Aim The controls of gross radiation use efficiency (RUE), the ratio between gross primary productivity (GPP) and the radiation intercepted by terrestrial vegetation, and its spatial and temporal variation are not yet fully understood. Our objectives were to analyse and synthesize the spatial variability of GPP and the spatial and temporal variability of RUE and its climatic controls for a wide range of vegetation types. Location A global range of sites from tundra to rain forest. Methods We analysed a global dataset on photosynthetic uptake and climatic variables from 35 eddy covariance (EC) flux sites spanning between 100 and 2200 mm mean annual rainfall and between ?13 and 26°C mean annual temperature. RUE was calculated from the data provided by EC flux sites and remote sensing (MODIS). Results Rainfall and actual evapotranspiration (AET) positively influenced the spatial variation of annual GPP, whereas temperature only influenced the GPP of forests. Annual and maximum RUE were also positively controlled primarily by annual rainfall. The main control parameters of the growth season variation of gross RUE varied for each ecosystem type. Overall, the ratio between actual and potential evapotranspiration and a surrogate for the energy balance explained a greater proportion of the seasonal variation of RUE than the vapour pressure deficit (VPD), AET and precipitation. Temperature was important for determining the intra‐annual variability of the RUE at the coldest energy‐limited sites. Main conclusions Our analysis supports the idea that the annual functioning of vegetation that is adapted to its local environment is more constrained by water availability than by temperature. The spatial variability of annual and maximum RUE can be largely explained by annual precipitation, more than by vegetation type. The intra‐annual variation of RUE was mainly linked to the energy balance and water availability along the climatic gradient. Furthermore, we showed that intra‐annual variation of gross RUE is only weakly influenced by VPD and temperature, contrary to what is frequently assumed. Our results provide a better understanding of the spatial and temporal controls of the RUE and thus could lead to a better estimation of ecosystem carbon fixation and better modelling.     

Modeling soil water movement with water uptake by roots

Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

华北落叶松人工林蒸散及产流对叶面积指数变化的响应

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R~20.99,P0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R~20.99,P0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。     

1980—2016年三江源国家公园水源供给及水源涵养功能时空变化研究

为实现三江源国家公园水源供给及涵养功能评估,服务区域生态服务价值估算,基于InVEST模型,利用1980—2016年期间共7期土地利数据,结合气象数据,土壤数据,地形数据等,评估了三江源国家公园水源供给及水源涵养量的时间变化特征与空间分布状况。结果表明:1)1980—2016年三江源国家公园年降水呈不显著增加趋势;潜在蒸散、实际蒸散显著增加。在此影响下,园区产水量及水源涵养量总体呈不显著增加趋势。在不同年代,园区水资源总量经历了骤降-好转-略微降低的变化过程。降水量与实际蒸散量对园区产水量及水源涵养量影响最为显著。2)园区产水量及水源涵养量空间分布趋势一致,呈由北向南先减少后增加的变化趋势。这种空间差异主要由降水差异及地表覆盖特征引起的蒸散差异引起。3)在极端降水条件下,园区产水量及水源涵养量的数量和空间分布差异十分显著。长江源园区生态水源对降水变化的响应最为敏感。     

Assessment of climatic indices limiting rainfed wheat yield

In this study variation of six climatic indices including accumulated precipitation (P), accumulated potential evapotranspiration (PET), accumulated actual evapotranspiration (AET), accumulated crop evapotranspiration (ETC), accumulated water stress (S) and climatic water deficit (D), was investigated. Climatic indices and their variation were calculated during seven growth stages of wheat in five locations in the northeast of Iran from 1983 to 2008. Principal component analysis (PCA) technique was applied to explore major modes of variation in the regional climatic indices during different crop growth stages. The principle component obtained for each region was correlated to the regional winter wheat yield. Finally the regional amount of water and precipitation use efficiency (WUE and PUE) were analyzed in order to assess any possible association with wheat yield. The results showed that the highest precipitation occurred during the tillering stage and spatially decreased from north (Bojnord) to south (Birjand) and from east (Mashhad) to west (Sabzevar). The difference between the highest and the lowest precipitation across all locations was 2.5 of standard value. The variation pattern of AET, compared to other indices, showed more similarity to variation of precipitation at different growth stages and the highest AET (more than 2 of standard value in all locations) occurred during the tillering stage. The PCA indicated that effective components varied in different locations. The most positive and effective components were types of evapotranspiration that are associated with crop (ETC and AET) and precipitation. However none of these effective PCs showed a significant correlation with final yield. The PUE and WUE analysis indicated that PUE provides more information to interpret the relationship between total amounts of precipitation and the final yield.     

Water flux through a semi-deciduous forest grove of the Orinoco savannas

Water relations were analysed in a semi-deciduous forest grove occurring in the oxisols of the Orinoco savannas. This grove has a shallow unconsolidated ironstone cuirass, which is overlaid by a sandy loam layer (0.0–0.5 m) that contains more than 90% of the grove forest root phytomass. Evapotranspiration and through drainage were calculated by using data from the soil profile as related to physical characteristics of the site root zone, hydraulic conductivity, volumetric water content and potential hydraulic gradient. Mean annual evapotranspiration was 783 mm year^–1 and annual through drainage below the root zone was 14% (162 mm year^–1) of the gross rainfall. This drainage recharged the 42% of the annual saturation deficit of the water table. Similar mean annual evapotranspiration (770 mm year^–1) was also calculated by using the water balance components. The mean daily coupling omega factor () between the grove canopy and the surrounding atmosphere indicated that a high degree of coupling (=0.14±0.16) occurs in the grove and evapotranspiration was mainly controlled by surface conductance. As the dry season proceeded, the soil saturation deficit () increased rapidly resulting in a threshold surface conductance (0.030–0.005 m s^–1) for ranging from 0.05 to 0.10. Hypotheses to explain the omnipresence of perennial species in the wide range of physical conditions in neotropical savannas are discussed.     

Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream

Although drought in temperate deciduous forests decreases transpiration rates of many species, stand-level transpiration and total evapotranspiration is often reported to exhibit only minor interannual variability with precipitation. This apparent contradiction was investigated using four years of transpiration estimates from sap flux, interception–evaporation estimates from precipitation and throughfall gauges, modeled soil evaporation and drainage estimates, and eddy covariance data in a mature oak-hickory forest in North Carolina, USA. The study period included one severe drought year and one year of well above-average precipitation. Normalized for atmospheric conditions, transpiration rates of some species were lower in drought than in wet periods whereas others did not respond to drought. However, atmospheric conditions during drought periods are unlike conditions during typical growing season periods. The rainy days that are required to maintain drought-free periods are characterized by low atmospheric vapor pressure deficit, leading to very low transpiration. In contrast, days with low air vapor pressure deficit were practically absent during drought and moderate levels of transpiration were maintained throughout despite the drying soil. Thus, integrated over the growing season, canopy transpiration was not reduced by drought. In addition, high vapor pressure deficit during drought periods sustained appreciable soil evaporation rates. As a result, despite the large interannual variation in precipitation (ranging from 934 to 1346 mm), annual evapotranspiration varied little (610–668 mm), increasing only slightly with precipitation, due to increased canopy rainfall interception. Because forest evapotranspiration shows only modest changes with annual precipitation, lower precipitation translates to decreased replenishment of groundwater and outflow, and thus the supply of water to downstream ecosystems and water bodies.     

Global patterns in root decomposition: comparisons of climate and litter quality effects

Root decomposition represents a significant C flux in terrestrial ecosystems. Roots are exposed to a different decomposition environment than aboveground tissues, and few general principles exist regarding the factors controlling rates of root decay. We use a global dataset to explore the relative importance of climate, environmental variables, and litter quality in regulating rates of root decomposition. The parameters that explained the largest amount of variability in root decay were root Ca concentrations and C:N ratios, with a smaller proportion explained by latitude, mean annual temperature, mean annual precipitation, and actual evapotranspiration (AET). Root chemistry and decay rates varied by plant life form (conifer, broadleaf, or graminoid). Conifer roots had the lowest levels of Ca and N, the highest C:N and lignin:N ratios, and decomposed at the slowest rates. In a stepwise multiple linear regression, AET, root Ca, and C:N ratio accounted for approximately 90% of the variability in root decay rates. Root chemistry appeared to be the primary controller of root decomposition, while climate and environmental factors played secondary roles, in contrast to previously established leaf litter decomposition models.     

Possible impacts of climate change on hydrologic cycle in Slovakia and results of observations in 1951–2007

The hydrologic cycle in Slovakia is determined mainly by three basic components: precipitation (P), evapotranspiration (E) and runoff (R). The mean annual P total was 747 mm, E sum 476 mm and the mean R was 271 mm in Slovakia in 1951–1980 (E ≈ 0.65P and R ≈ 0.35P). T increase in 1.6°C and annual P decrease in 24 mm (3.1%) were detected in the 1881–2007 period. Regimes of potential and actual evapotranspiration, soil moisture and R have been changed mainly in the southern Slovakia. A physical model for the estimation of the energy balance equation components (total radiation balance and its components, potential and actual evapotranspiration, sensible heat flux) has been developed. Input data are the air temperature and humidity, cloudiness, the number of days with snow cover and precipitation, all measured in the network of 31 meteorological stations in Slovakia since 1951. The 20-year period 1988–2007 was by 0.9°C warmer than the normal period mean. The observed increase is at the upper limit of all climate change scenarios projected for Slovakia in 1991–2001. Annual P totals have not changed significantly, but substantial changes have been found in the P regime. The scenarios show significant changes in the hydrological cycle not only at river basins balance but also in case of soil water balance, mainly in the southern Slovakia.     

鼎湖山顶级森林生态系统水文要素时空规律

运用连续7a(1993～1999)的水文观测资料,对南亚热带顶级生态系统鼎湖山季风常绿阔叶林集水区水文要素时空规律进行分析,得到如下一些主要结论：(1)鼎湖山多年平均降水量为1910mm,湿季降水量占年降水量80％,干季仅占20％。6月份的降水量最大,1月份最小。(2)季风常绿阔叶林冠层截留率为31．8％,湿季的截留量占全年截留量的66．7％,截留量最大值和最小值所在的月份分别为7和1月份。各月的截留率差异很大,截留量大的月份,截留率较低;截留量小的月份,截留率较高。(3)季风常绿阔叶林集水区多年平均总径流量953．0mm,总径流系数49．9％,其中地表径流量为252．3mm,地表径流系数13．2％;地表径流与降水量之间存在二次抛物线型回归关系,与降水强度的关系不大,这说明季风常绿阔叶林的产流形式是是蓄满产流。(4)季风常绿阔叶林多年平均蒸散948．2mm,占同期降水量的49．7％;蒸散力1031．4mm,年蒸散系数为0．92,蒸散月变化规律较降水量的月变化规律有所滞后。(5)系统贮水量的月变化很大,2～8月份,系统处于蓄水阶段;9月份至翌年1月份,系统处于失水阶段。蓄水和失水的最大值分别出现在湿季和干季的第一个月,即4月份和10月份。(6)集水区多年平均水量总输入2129．9mm,实际输入1910mm(降水量),其中219．9mm的水量输入是由系统贮水量变化而产生。支出的总水量2129．9mm,实际支出1901．3mm(径流和蒸散量),其中228．6mm的水量支出是由系统贮水量变化引起的。     

Precipitation signal in pollen rain from tropical forests, South India

We have analyzed the pollen content of 51 surface soil samples collected in tropical evergreen and deciduous forests from the Western Ghats of South India sampled along a west to east gradient of decreasing rainfall (between 11 degrees 30-13 degrees 20'N and 75 degrees 30-76 degrees 30'E). Values of mean annual precipitation (Pann, mm/yr) have been calculated at each of the 51 sampling sites from a great number of meteorological stations in South India, using a method of data interpolation based on artificial neural network. Interpolated values at the pollen sites of Pann range from 1200 to 5555mm/yr, while mean temperature of the coldest month (MTCO) remains >15 degrees C and humidity factor (AET/PET, the actual evapotranspiration to potential evapotranspiration ratio) remains also included between 65 and 72%.Results are presented in the form of percentage pollen diagrams where samples are arranged according to increasing values of annual precipitation. They indicate that the climatic signal of rainfall is clearly evidenced by distinct pollen associations. Numerical analyses show that annual precipitation is an important parameter explaining the modern distribution of pollen taxa in this region. Pollen taxa markers of high rainfall (Pann >2500mm/yr) are Mallotus type, Elaeocarpus, Syzygium type, Olea dioica, Gnetum ula, and Hopea type, associated with Ixora type and Caryota. Pollen taxa markers of low rainfall (Pann <2500mm/yr) are Melastomataceae/Combretaceae, Maytenus type, Lagerstroemia and Grewia. The proportions of evergreen taxa and of arboreal taxa vary according to rainfall values. Indeed, when rainfall is <2500mm/yr, percentage of arboreal pollen (AP) is <50% and proportion of evergreen taxa is <20%. When rainfall exceeds 2500mm/yr, AP values average 70%, and proportion of evergreen taxa increases from 60 to 90%. Moreover, a good correlation between precipitation and proportion of evergreen taxa (0.85) presumes that precipitation can be estimated from pollen data.     

Root biomass,root distribution and the fine-root growth dynamics of Quercus coccifera L. in the garrigue of southern France

Quercus coccifera L., the characteristic scrub oak of the garrigue, covers more than 100,000 ha in southern France alone. Precipitation in this area averages 900 mm/year and summer rains are not rare. A total belowground biomass of 7.2 kg/m², including rhizomes and lignotubers, was harvested. Roots were concentrated in the uppermost 50 cm of the soil. It was hypothesized that low winter temperatures inhibit active fine-root growth. This hypothesis was tested by means of fine-root extractions of soil samples from 0–50 cm depth from November 1987 to June 1988. Although the fine-root analysis could not be extended into late summer and fall, the data supported the hypothesis. Ratios of live/dead fine roots reached their minimum at 0.2–0.3 from December to April. They increased to 1.0–1.2 during late spring and early summer. Initiation of fine-root growth in early April was synchronous with bud break. Starch contents of roots, rhizomes, and lignotubers fluctuated from 4.3% in January to 8.3% in April. The starch stored in belowground organs of Q. coccifera in a closed canopy stand amounted to about 500 g/m² in April. This amount declined to 400 g with bud burst and fine-root growth initiation.     

Syntaxonomy and Synecology of <Emphasis Type="Italic">Quercus coccifera</Emphasis> Mediterranean Shrublands in Greece

Quercus coccifera (kermes oak) is the most common species of the Mediterranean maquis with a wide distribution across the Mediterranean Basin. This paper presents a syntaxonomic overview of the Q. coccifera plant communities in the Mediterranean zone of Greece (Quercetea ilicis) based on the classification of 221 relevés from 34 (17 continental and 17 insular) mountainous areas throughout Greece. Two associations and eight sub-associations are described and presented in a synoptic constancy table. Querco cocciferae–Pistacietum lentisci is the most widespread, is found in the entire continental Greece and most islands, and is further subdivided into five sub-associations reflecting primarily local peculiarities in the disturbance regime and the influence of local floristic elements. Rhamno lycioidis–Cocciferetum (Rivas Goday & Rivas-Martínez 1954), on the other hand, is geographically confined on the island of Crete and is further subdivided into three sub-associations, reflecting differences in the annual precipitation, and they are characterized by the presence of many phryganic and grazing-resistant species. Climate and the anthropogenic pressure have been identified to be the most important factors determining the structure and the floristic composition of Q. coccifera Mediterranean shrublands of Greece.     

Using Soil Water Depletion to Measure Spatial Distribution of Root Activity in Oil Palm (Elaeis guineensis Jacq.) Plantations

Knowledge of where roots are active is crucial for efficient management of nutrients in tree crops but measurement of root activity is problematic. Measurement using soil water depletion is an approach that has not been tested in a humid climate. We hypothesised that the three dimensional distribution of root activity of a tree crop in the humid tropics (a) can be determined by measuring soil water depletion during rain-free periods, and (b) is influenced by environment (soil type and climate) and surface management. A field study was carried out in which soil water content was measured and water uptake calculated (by difference between soil water content at beginning and end of rain-free periods) for different surface management zones and depths (0.1 m intervals to 1.6 m depth) under oil palm (Elaeis guineensis Jacq.) at a loam–clay site and a sandy site. Significant differences were measured between sites and between surface management zones at each site. At both sites water uptake was highest under the weeded zone close to the palm stem, slightly lower under the zone where pruned fronds are placed, and lowest under the path used for removing harvested fruit. Vertical distribution of root activity differed between the sites, with higher activity near the surface at the finer textured site. Total water uptake values were lower than estimates of evapotranspiration made using climate data. The difference was probably largely due to water uptake from deeper than 1.6 m. This study showed that the spatial distribution of tree root activity in a humid climate could be quantified using a relatively simple method.     

Contrasting physiological effects of partial root zone drying in field-grown grapevine (Vitis vinifera L. cv. Monastrell) according to total soil water availability

Different spatial distributions of soil moisture were imposed on field-grown grapevines by applying the same irrigation volumes to the entire (DI; deficit irrigation) or part of the (PRD; partial root zone drying) root zone. Five treatments were applied: controls irrigated at 60% ETc (crop evapotranspiration) for the whole season (308 mm year(-1)); DI-1 and PRD-1 that received the same irrigation as controls before fruit set, 30% ETc from fruit set to harvest and 45% ETc post-harvest (192 mm year(-1)); and DI-2 and PRD-2 that were the same, except that 15% ETc was applied from fruit set to harvest (142 mm year(-1)). Compared with DI-1, PRD-1 maintained higher leaf area post-veraison and increased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, but decreased intrinsic gas exchange efficiency without causing differences in leaf xylem abscisic acid (ABA) concentration. Compared with DI-2, PRD-2 increased leaf xylem ABA concentration and decreased root water uptake, whole-plant hydraulic conductance, leaf transpiration, stomatal conductance, and photosynthesis, mainly at the beginning of PRD cycles. Distinctive PRD effects (e.g. greater stomatal closure) depended on the volumetric soil water content of the wet root zone, as predicted from a model of root-to-shoot ABA signalling.     

黄土区土壤水分循环特征及其对陆地水分循环的影响

黄河中游黄土地区一般系指长城以南、秦岭以北、太行山以西、西宁以东地表覆盖有厚层黄土的地区。包括整个黄土高原及南部海拔低于800米的黄土台源及高阶地区。由于这一地区土壤均发育于黄土母质之上,又同属季风区,所以在土壤水分的性质与循环特征上存在很多相似规律性。过去把土壤水分作为一个肥力因素,研究它的状况、移动及其和作物生长