Climate,vegetation, and soil controls on hydraulic redistribution in shallow tree roots

Hydraulic redistribution defined as the translocation of soil moisture by plant root systems in response to water potential gradients is a phenomenon widely documented in different climate, vegetation, and soil conditions. Past research has largely focused on hydraulic redistribution in deep tree roots with access to groundwater and/or winter rainfall, while the case of relatively shallow (i.e., ≈1–2 m deep) tree roots has remained poorly investigated. In fact, it is not clear how hydraulic redistribution in shallow root zones is affected by climate, vegetation, and soil properties. In this study, we developed a model to investigate the climate, vegetation, and soil controls on the net direction and magnitude of hydraulic redistribution in shallow tree root systems at the growing season to yearly timescale. We used the model to evaluate the effect of hydraulic redistribution on the water stress of trees and grasses. We found that hydraulic lift increases with decreasing rainfall frequency, depth of the rooting zone, root density in the deep soil and tree leaf area index; at the same time for a given rainfall frequency, hydraulic lift increases with increasing average rainstorm depth and soil hydraulic conductivity. We propose that water drainage into deeper soil layers can lead to the emergence of vertical water potential gradients sufficient to explain the occurrence of hydraulic lift in shallow tree roots without invoking the presence of a shallow water table or winter precipitation. We also found that hydraulic descent reduces the water stress of trees and hydraulic lift reduces the water stress of grass with important implications on tree–grass interactions.     

An easily implemented agro-hydrological procedure with dynamic root simulation for water transfer in the crop–soil system: Validation and application

Models for water transfer in the crop–soil system are key components of agro-hydrological models for irrigation, fertilizer and pesticide practices. Many of the hydrological models for water transfer in the crop–soil system are either too approximate due to oversimplified algorithms or employ complex numerical schemes. In this paper we developed a simple and sufficiently accurate algorithm which can be easily adopted in agro-hydrological models for the simulation of water dynamics. We used a dual crop coefficient approach proposed by the FAO for estimating potential evaporation and transpiration, and a dynamic model for calculating relative root length distribution on a daily basis. In a small time step of 0.001 d, we implemented algorithms separately for actual evaporation, root water uptake and soil water content redistribution by decoupling these processes. The Richards equation describing soil water movement was solved using an integration strategy over the soil layers instead of complex numerical schemes. This drastically simplified the procedures of modeling soil water and led to much shorter computer codes. The validity of the proposed model was tested against data from field experiments on two contrasting soils cropped with wheat. Good agreement was achieved between measurement and simulation of soil water content in various depths collected at intervals during crop growth. This indicates that the model is satisfactory in simulating water transfer in the crop–soil system, and therefore can reliably be adopted in agro-hydrological models. Finally we demonstrated how the developed model could be used to study the effect of changes in the environment such as lowering the groundwater table caused by the construction of a motorway on crop transpiration.     

Root controls on water redistribution and carbon uptake in the soil–plant system under current and future climate

Understanding photosynthesis and plant water management as a coupled process remains an open scientific problem. Current eco-hydrologic models characteristically describe plant photosynthetic and hydraulic processes through ad hoc empirical parameterizations with no explicit accounting for the main pathways over which carbon and water uptake interact. Here, a soil–plant-atmosphere continuum model is proposed that mechanistically couples photosynthesis and transpiration rates, including the main leaf physiological controls exerted by stomata. The proposed approach links the soil-to-leaf hydraulic transport to stomatal regulation, and closes the coupled photosynthesis–transpiration problem by maximizing leaf carbon gain subject to a water loss constraint. The approach is evaluated against field data from a grass site and is shown to reproduce the main features of soil moisture dynamics and hydraulic redistribution. In particular, it is shown that the differential soil drying produced by diurnal root water uptake drives a significant upward redistribution of moisture both through a conventional Darcian flow and through the root system, consistent with observations. In a numerical soil drying experiment, it is demonstrated that more than 50% of diurnal transpiration is supplied by nocturnal upward water redistribution, and some 12% is provided directly through root hydraulic redistribution. For a prescribed leaf area density, the model is then used to diagnose how elevated atmospheric CO₂ concentration and increased air temperature jointly impact soil moisture, transpiration, photosynthesis, and whole-plant water use efficiency, along with compensatory mechanisms such as hydraulic lift using several canonical forms of root-density distribution.     

Hydraulic redistribution by deeply rooted grasses and its ecohydrologic implications in the southern Great Plains of North America

Perennial bioenergy crops with deep (>1 m) rooting systems, such as switchgrass (Panicum virgatum L.), are hypothesized to increase carbon storage in deep soil. Deeply rooted plants may also affect soil hydrology by accessing deep soil water for transpiration, which can affect soil water content and infiltration in deep soil layers, thereby affecting groundwater recharge. Using stable H and O isotope (δ²H and δ¹⁸O) and ³H values, we studied the soil water conditions at 20–30 cm intervals to depths of 2.4–3.6 m in paired fields of switchgrass and shallow rooted crops at three sites in the southern Great Plains of North America. We found that soil under switchgrass had consistently higher soil water content than nearby soil under shallow-rooted annual crops by a margin of 15%–100%. Soil water content and isotopic depth profiles indicated that hydraulic redistribution of deep soil water by switchgrass roots explained these observed soil water differences. To our knowledge, these are the first observations of hydraulic redistribution in deeply rooted grasses, and complement earlier observations of dynamic soil water fluxes under shallow-rooted grasses. Hydraulic redistribution by switchgrass may be a strategy for drought avoidance, wherein the plant may actively prevent water limitation. This raises the possibility that deeply rooted grasses may be used to passively subsidize soil water to more shallow-rooted species in inter-cropping arrangements.     

On quantifying hydraulic conductance at the soil–root interface

This study sought to test a widely used hypothesis in water transport from soil to root: the soil textural effect on soil–root interfacial conductance can be quantified by the degree of soil water saturation. This hypothesis is inferred from basic soil physics and is commonly used in modelling root water uptake. However, it has never been subjected to rigorous experimental test. In the current study, the soil texture effect on the interfacial hydraulic conductance was evaluated on well‐watered pot‐grown cotton plants, with soil particle size ranges of 0–0.05, 0.05–0.1, 0.1–0.2, 0.2–0.3, 0.3–0.45, 0.45–0.6 and 0.6–1 mm in seven treatments, in addition to hydroponics. However, our experimental results showed that the volumetric soil water content, not degree of saturation, is the best explanatory variable for quantifying the interfacial effect on hydraulic conductance. Further analysis indicated that if temporal change of this interfacial effect on hydraulic conductance is the subject of concern, degree of saturation may still be a valid option. If soil textural or spatial variation in root water uptake is the subject of concern, volumetric soil water content is best suited to quantifying the interfacial effect on hydraulic conductance. Copyright © 2013 John Wiley & Sons, Ltd.     

The effect of conservation practices in sloped croplands on soil hydraulic properties and root‐zone moisture dynamics

Rain‐induced erosion and short‐term drought are the two factors that limit the productivity of croplands in the red soil region of subtropical China. The objective of this study was to estimate the effects of conservation practices on hydraulic properties and root‐zone water dynamics of the soil. A 3‐year experiment was performed on a slope at Xianning. Four treatments were evaluated for their ability to reduce soil erosion and improve soil water conditions. Compared with no practices (CK) and living grass strips (GS), the application of polyacrylamide (PAM) significantly reduced soil crust formation during intense rainfall, whereas rice straw mulching (SM) completely abolished soil crust formation. The SM and PAM treatments improved soil water‐stable aggregates, with a redistribution of micro‐aggregates into macro‐aggregates. PAM and SM significantly increased the soil water‐holding capacity. These practices mitigated the degradation of the soil saturated hydraulic conductivity (Ks) during intense rainfalls. These methods increased soil water storage but with limited effects during heavy rainfalls in the wet period. In contrast, during the dry period, SM had the highest soil water storage, followed by PAM and CK. Grass strips had the lowest soil water storage because of the water uptake during the vigorous grass growth. A slight decline in the soil moisture resulted in a significant decrease in the unsaturated hydraulic conductivity (Ku) of the topsoil. Therefore, the hydraulic conductivity in the field is governed by soil moisture, and the remaining soil moisture is more important than improving soil properties to resist short‐term droughts. As a result, SM is the most effective management practice when compared with PAM and GS, although they all protect the soil hydraulic properties during wet periods. These results suggest that mulching is the best strategy for water management in erosion‐threatened and drought‐threatened red soils. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of the roots of Cynodon dactylon and Schefflera heptaphylla on water infiltration rate and soil hydraulic conductivity

Water infiltration rate and hydraulic conductivity in vegetated soil are two vital hydrological parameters for agriculturists to determine availability of soil moisture for assessing crop growths and yields, and also for engineers to carry out stability calculations of vegetated slopes. However, any effects of roots on these two parameters are not well‐understood. This study aims to quantify the effects of a grass species, Cynodon dactylon, and a tree species, Schefflera heptaphylla, on infiltration rate and hydraulic conductivity in relation to their root characteristics and suction responses. The two selected species are commonly used for ecological restoration and rehabilitation in many parts of the world and South China, respectively. A series of in‐situ double‐ring infiltration tests was conducted during a wet summer, while the responses of soil suction were monitored by tensiometers. When compared to bare soil, the vegetated soil has lower infiltration rate and hydraulic conductivity. This results in at least 50% higher suction retained in the vegetated soil. It is revealed that the effects of root‐water uptake by the selected species on suction were insignificant because of the small evapotranspiration (<0.2 mm) when the tests were conducted under the wet climate. There appears to have no significant difference (less than 10%) of infiltration rates, hydraulic conductivity and suction retained between the grass‐covered and the tree‐covered soil. However, the grass and tree species having deeper root depth and greater Root Area Index (RAI) retained higher suction. Copyright © 2015 John Wiley & Sons, Ltd.     

Uptake and Accumulation of Arsenic in Different Organs of Carrot Irrigated with As‐Rich Water

Irrigation with arsenic (As)‐rich water in agricultural soil may increase high levels of As in crops and cause food chain contamination. In this study, a greenhouse experiment was established using Spanish agricultural soil (Valladolid and Segovia provinces), that are extensively cultivated for carrot plant, to investigate the process of As uptake, bioaccumulation, and translocation of As from root to shoot and leaves in carrot plant. Arsenic concentrations in different organs of carrot plant, rhizosphere soil, and soil solutions were determined by hydride generation atomic absorption spectrometry (AAS). High concentrations of As in irrigation water, and the alkaline and sandy character of this soil enhanced As uptake in carrot plants indicating the potential health risk from consumption of carrots cultivated in these areas. Bioaccumulation of As into the leaves and roots increased with increase of As concentration in irrigation water. Both roots and leaves demonstrated a higher accumulation rate of As at an As concentration of 41 than 131 µg L^?1 in the soil solution. The ratios of As_root/As_leaves showed no statistically significant differences for the different irrigation treatments, and had an average value of 0.36 indicating the high magnitude of As translocation from roots to leaves in carrot plants. The leaves of carrots had a higher affinity for As than roots did. The correlation between As uptake by leaves or roots of carrots and the soluble As in rhizosphere soil did not demonstrate a linear or a plateau curve, indicating a slow but continuous constant As absorption which could be prolonged over time with high potential environmental risks.     

Controls on evapotranspiration from jack pine forests in the Boreal Plains Ecozone

The exchanges of water, energy and carbon between the land surface and the atmosphere are tightly coupled, so that errors in simulating evapotranspiration lead to errors in simulating both the water and carbon balances. Areas with seasonally frozen soils present a particular challenge due to the snowmelt-dominated hydrology and the impact of soil freezing on the soil hydraulic properties and plant root water uptake. Land surface schemes that have been applied in high latitudes often have reported problems with simulating the snowpack and runoff. Models applied at the Boreal Ecosystem Research and Monitoring Sites in central Saskatchewan have consistently over-predicted evapotranspiration as compared with flux tower estimates. We assessed the performance of two Canadian land surface schemes (CLASS and CLASS-CTEM) for simulating point-scale evapotranspiration at an instrumented jack pine sandy upland site in the southern edge of the boreal forest in Saskatchewan, Canada. Consistent with past reported results, these models over-predicted evapotranspiration, as compared with flux tower observations, but only in the spring period. Looking systematically at soil properties and vegetation characteristics, we found that the dominant control on evapotranspiration within these models was the canopy conductance. However, the problem of excessive spring ET could not be solved satisfactorily by changing the soil or vegetation parameters. The model overestimation of spring ET coincided with the overestimation of spring soil liquid water content. Improved algorithms for the infiltration of snowmelt into frozen soils and plant-water uptake during the snowmelt and soil thaw periods may be key to addressing the biases in spring ET.     

Modelling the effect of changing precipitation inputs on deep soil water utilization

Forests in the Southeastern United States are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change‐induced alterations could increase drought and lower soil water availability. Drought could alter rooting patterns and increase the importance of deep roots that access subsurface water resources. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall reduction experiment in a loblolly pine plantation of the Southeastern United States to calibrate and validate a hydrological model. The model was accurately calibrated against field measured soil moisture data under ambient rainfall and validated using 30% throughfall reduction data. Using this model, we then tested these scenarios: (a) evenly reduced precipitation; (b) less precipitation in summer, more in winter; (c) same total amount of precipitation with less frequent but heavier storms; and (d) shallower rooting depth under the above 3 scenarios. When less precipitation was received, drainage decreased proportionally much faster than evapotranspiration implying plants will acquire water first to the detriment of drainage. When precipitation was reduced by more than 30%, plants relied on stored soil water to satisfy evapotranspiration suggesting 30% may be a threshold that if sustained over the long term would deplete plant available soil water. Under the third scenario, evapotranspiration and drainage decreased, whereas surface run‐off increased. Changes in root biomass measured before and 4 years after the throughfall reduction experiment were not detected among treatments. Model simulations, however, indicated gains in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios but not when precipitation frequency was adjusted. Deep soil and deep rooting can provide an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand of forests. How this buffering capacity will persist in the face of changing precipitation inputs, however, will depend less on seasonal redistribution than on the magnitude of reductions and changes in rainfall frequency.     

Modelling investigation of water partitioning at a semiarid ponderosa pine hillslope

The effects of vegetation root distribution on near‐surface water partitioning can be two‐fold. On the one hand, the roots facilitate deep percolation by root‐induced macropore flow; on the other hand, they reduce the potential for deep percolation by root‐water‐uptake processes. Whether the roots impede or facilitate deep percolation depends on various conditions, including climate, soil, and vegetation characteristics. This paper examines the effects of root distribution on deep percolation into the underlying permeable bedrock for a given soil profile and climate condition using HYDRUS modelling. The simulations were based on previously field experiments on a semiarid ponderosa pine (Pinus ponderosa) hillslope. An equivalent single continuum model for simulating root macropore flow on hillslopes is presented, with root macropore hydraulic parameterization estimated based on observed root distribution. The sensitivity analysis results indicate that the root macropore effect dominates saturated soil water flow in low conductivity soils (K_matrix below 10^?7 m/s), while it is insignificant in soils with a K_matrix larger than 10^?5 m/s, consistent with observations in this and other studies. At the ponderosa pine site, the model with simple root‐macropore parameterization reasonably well reproduces soil moisture distribution and some major runoff events. The results indicate that the clay‐rich soil layer without root‐induced macropores acts as an impeding layer for potential groundwater recharge. This impeding layer results in a bedrock percolation of less than 1% of the annual precipitation. Without this impeding layer, percolation into the underlying permeable bedrock could be as much as 20% of the annual precipitation. This suggests that at a surface with low‐permeability soil overlying permeable bedrock, the root penetration depth in the soil is critical condition for whether or not significant percolation occurs. Copyright © 2010 John Wiley & Sons, Ltd.     

Water uptake by trees in a riparian hardwood forest (Rhine floodplain,France)

Water flow in the soil–root–stem system was studied in a flooded riparian hardwood forest in the upper Rhine floodplain. The study was undertaken to identify the vertical distribution of water uptake by trees in a system where the groundwater is at a depth of less than 1 m. The three dominant ligneous species (Quercus robur, Fraxinus excelsior and Populus alba) were investigated for root structure (vertical extension of root systems), leaf and soil water potential (Ψ_m), isotopic signal (¹⁸O) of soil water and xylem sap. The root density of oak and poplar was maximal at a depth of 20 to 60 cm, whereas the roots of the ash explored the surface horizon between 0 and 30 cm, which suggests a complementary tree root distribution in the hardwood forest. The flow density of oak and poplar was much lower than that of the ash. However, in the three cases the depth of soil explored by the roots reached 1·2 m, i.e. just above a bed of gravel. The oak roots had a large lateral distribution up to a distance of 15 m from the trunk. The water potential of the soil measured at 1 m from the trunk showed a zone of strong water potential between 20 and 60 cm deep. The vertical profile of soil water content varied from 0·40 to 0·50 cm³ cm^?3 close to the water table, and 0·20 to 0·30 cm³ cm^?3 in the rooting zone. The isotopic signal of stem water was constant over the whole 24‐h cycle, which suggested that the uptake of water by trees occurred at a relatively constant depth. By comparing the isotopic composition of water between soil and plant, it was concluded that the water uptake occurred at a depth of 20 to 60 cm, which was in good agreement with the root and soil water potential distributions. The riparian forest therefore did not take water directly from the water table but from the unsaturated zone through the effect of capillarity. Copyright © 2007 John Wiley & Sons, Ltd.     

The effects of partial rootzone drying on root, trunk sap flow and water balance in an irrigated pear (Pyrus communis L.) orchard

Partial rootzone drying (PRD) means that part of the root system is watered as in full irrigation while the rest is exposed to soil drying. This practice is predicted to influence field hydrological circle. We studied the effect of this practice on soil water distribution, root and trunk sap flow, water consumption of pear trees, and capillary contribution from ground water table and water balance for three months in an irrigated orchard with a shallow ground water table. The irrigation treatments included: (a) conventional flooded irrigation (CFI), (b) fixed partial rootzone drying (FPRD), and (c) alternate partial rootzone drying (APRD). Root and trunk sap flows were monitored using a heat-pulse sap flow meter. The results showed that there were significant differences of soil water content in both sides of rootzone under partial drying. The capillary contribution from ground water table was significantly increased in APRD and FPRD when compared with CFI. More significantly, the total irrigation amount was greatly reduced, by 43.64 and 45.84%, respectively, for APRD and FPRD. The two PRD treatments used more soil-stored water while CFI had more drainage. The root sap flow on the wet side was substantially enhanced as a result of PRD, and was greater than that from same side in CFI. The trunk sap flow in FPRD and APRD was smaller than that in CFI. On average, both APRD and FPRD reduced plant daily water consumption by about 9.96 and 17.97%, respectively, when compared to CFI during the PRD period. Daily root water flow was a significant function of the reference evapotranspiration. The daily trunk water flow was also related to the reference evapotranspiration but the CFI carried more water than APRD and FPRD under the same evaporation demand, suggesting a restriction of transpirational water loss in the PRD trees. CFI needed a higher soil water content to carry the same amount of trunk flow than the PRD trees, suggesting the hydraulic conductance of roots in PRD trees enhanced, and the roots had a greater water uptake capacity than in CFI when the average soil water content in the rootzone was the same.     

Temporal stability of soil water content as affected by climate and soil hydraulic properties: a simulation study

Temporal stability of soil water content (TS SWC) is an often‐observed phenomenon, which characterization finds multiple applications. Climate and variability in soil properties are usually mentioned as factors of TS SWC, but their effects are far from clear. The objective of this work was to use SWC modeling to evaluate the effects of climate and soil hydraulic properties on the TS of soil water at different measurement schedules. We selected four representative climates found in USA and simulated the multiyear SWC dynamics for sandy loam, loam, and silty clay loam soils, all having the lognormal spatial distribution of the saturated hydraulic conductivity. The CLIMGEN and the HYDRUS6 codes were used to generate weather patterns and to simulate SWC, respectively. Four different methods were applied to select the representative location (RL). The low probability of having the same variability of mean relative differences of soil water under different climates was found in most of the cases. The probability that the variance of mean relative differences depended on sampling frequency was generally higher than 91% for the three soils. The interannual difference in mean relative differences variation from short and intensive summer campaigns was highly probable for all climates and soils. The RLs changed as climate and measurement scheduling changed, and they were less pronounced for coarse‐textured soils. The RL selection methods based solely on bias provided more consistency as compared with other methods. The TS appears to be the result of the interplay between climate, soil properties, and survey protocols. One implication of this factor interaction effect on TS SWC is that a simulation study can be useful to decide on the feasibility of including a search for TS‐based RLs for a specific site. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulating daily, monthly and annual water balances in a land surface model using alternative root water uptake schemes

Hydrological simulations at multi-temporal time scales by a widely used land surface model (LSM) are investigated under contrasting vegetation and meteorological conditions. Our investigation focuses particularly on the effects of two different representations of root water uptake and root profile on simulated evapotranspiration (ET) and soil moisture by the Integrated BIosphere Simulator (IBIS). For this purpose, multi-year eddy covariance measurements, collected at four flux-tower sites across North America, were used to gauge IBIS simulations with: (a) its standard version (IBIS2.1), in which static root water uptake (RWU) and root profile schemes are incorporated; and (b) a modified version in which dynamic RWU and root profile schemes replaces the static schemes used in the standard version. Overall, our results suggest that the modified version of the model performs more realistically than the standard version, particularly when high atmospheric demand for evaporation is combined with high atmospheric vapour pressure deficit and low soil water availability. The overall correlation between simulated and measured monthly ET rates at the simulated sites reached 0.87 and 0.91 for the standard and the modified versions, respectively. Our results also show that the incorporation of the dynamic RWU in IBIS yields improved simulations of ET under very dry conditions, when soil moisture falls down to very low levels. This suggests that adequate representations of vegetation responses to drought are needed in LSMs as many state of the art climate models projections of future climate indicate more frequent and/or more intense drought events occurring in some regions of the globe. Our analysis also highlighted the urgent need for adequate methodologies to correct field measurements that exhibit energy imbalances in order to provide rigorous assessments of land surface model simulations of heat and mass exchanges between the land surface and the atmosphere.     

Impacts of hydraulic redistribution on eco-hydrological cycles: A case study over the Amazon basin

Hydraulic redistribution(HR)refers to the process of soil water transport through the low-resistance pathway provided by plant roots.It has been observed in field studies and proposed to be one of the processes that enable plants to resist water limitations.However,most land-surface models(LSMs)currently do not include this underground root process.In this study,a HR scheme was incorporated into the Community Land Model version 4.5(CLM4.5)to investigate the effect of HR on the eco-hydrological cycle.Two paired numerical simulations(with and without the new HR scheme)were conducted for the Tapajos National Forest km83(BRSa3)site and the Amazon.Simulations for the BRSa3 site in the Amazon showed that HR during the wet season was small,0.1 mm day~(–1),transferring water from shallow wet layers to deep dry layers at night;however,HR in the dry season was more obvious,up to 0.3 mm day~(–1),transferring water from deep wet layers to shallow dry layers at night.By incorporating HR into CLM4.5,the new model increased gross primary production(GPP)and evapotranspiration(ET)by 10%and 15%,respectively,at the BRSa3 site,partly overcoming the underestimation.For the Amazon,regional analysis also revealed that vegetation responses(including GPP and ET)to seasonal drought and the severe drought of 2005 were better captured with the HR scheme incorporated.     

Water and solute movement in a coarse-textured water-repellent field soil

Unstable water flow in water-repellent unsaturated soils can significantly affect the processes of infiltration and soil water redistribution. A field experiment was carried out to study the effect of water-repellency on water and bromide movement in a coarse-textured soil in the southwestern part of The Netherlands. The field data were analyzed using a relatively simple numerical model based on the standard Richards' equation for unsaturated water flow and the Fickian-based convection-dispersion equation for solute transport. Water-repellency was accounted for by multiplying the water content and the unsaturated hydraulic conductivity of the soil with F, a factor equal to the volumetric fraction of soil occupied by preferential flow paths resulting from the unstable flow process. The good comparison of simulated and measured bromide concentrations suggests that the model provides a viable method for simulating unstable water flow in water-repellent soils.     

Isotopic analysis of water sources of mountainous plant uptake in a karst plateau of southwest China

Ecosystem in the karst region of southwest China is very fragile due to a very limited amount of water storage for plant uptake in the thin and rocky soils underlain by rock fractures. Plants in these karst regions are thought to take water from the soils and shallow fractured rock zone (subcutaneous zone) as well. However, the role of subcutaneous water in maintaining karst vegetation remains unclear, and proportions of the water sources for plant uptake in different environment conditions are unknown. In this study, five typical species of plants at two sites were selected in a karst plateau of Qingzhen, central Guizhou Province of China. Proportions of the possible water sources contributed for the plant uptake from two soil layers and subcutaneous zone were determined on the basis of δD and δ¹⁸O values of plant stem water, soil water and subcutaneous water. The analysis reveals that most plants take water from the soil layers and the subcutaneous zone as well, but proportions of these water contributions for plant uptake vary seasonally and depend on site‐specific conditions and plant species. Plant uptake of the subcutaneous water for all species averages less than 30% of the total monthly amount in June and September, compared with more than 60% in dry December. Plants tend to take a larger proportion of water from the upper soil layer at the bush site than at the forest site in June and September (63 vs 28% in July; 66 vs 54% in September for all species in average). In December, however, 98% of water is taken from the subcutaneous zone at the bush site which is much greater than 68% at the forest site. Compared to deciduous arbor, evergreen shrub takes a greater proportion of subcutaneous water in the December drought. Copyright © 2011 John Wiley & Sons, Ltd.     

Responses of canopy transpiration and canopy conductance of peach (Prunus persica) trees to alternate partial root zone drip irrigation

We investigated canopy transpiration and canopy conductance of peach trees under three irrigation patterns: fixed 1/2 partial root zone drip irrigation (FPRDI), alternate 1/2 partial root zone drip irrigation (APRDI) and full root zone drip irrigation (FDI). Canopy transpiration was measured using heat pulse sensors, and canopy conductance was calculated using the Jarvis model and the inversion of the Penman–Monteith equation. Results showed that the transpiration rate and canopy conductance in FPRDI and APRDI were smaller than those in FDI. More significantly, the total irrigation amount was greatly reduced, by 34·7% and 39·6%, respectively for APRDI and FPRDI in the PRDI (partial root zone drip irrigation) treatment period. The daily transpiration was linearly related to the reference evapotranspiration in the three treatments, but daily transpiration of FDI is more than that of APRDI and FPRDI under the same evaporation demand, suggesting a restriction of transpiration water loss in the APRDI and FPRDI trees. FDI needed a higher soil water content to carry the same amount of transpiration as the APRDI and FPRDI trees, suggesting the hydraulic conductance of roots of APRDI and FPRDI trees was enhanced, and the roots had a greater water uptake than in FDI when the average soil water content in the root zone was the same. By a comparison between the transpiration rates predicted by the Penman–Monteith equation and the measured canopy transpiration rates for 60 days during the experimental period, an excellent correlation along the 1:1 line was found for all the treatments (R² > 0·80), proving the reliability of the methodology. Copyright © 2005 John Wiley & Sons, Ltd.