Water Use Rates Research Articles

Groundwater Capture Using Hybrid Poplar Trees: Evaluation of a System in Ogden, Utah

ABSTRACT A phytoremediation system was installed in 1996 in Ogden, Utah, with the objective of controlling groundwater containing petroleum hydrocarbons. Hybrid poplar trees were deeply and densely planted in rows oriented perpendicular to the direction of groundwater flow, and the stand was never irrigated. Piezometers were installed to measure water table elevation and contaminant levels upgradient, within, and down-gradient of the trees. In 1998, an analysis of the root structure of a representative tree indicated that roots had extended down to the saturated zone, approximately 6 ft below ground surface. The rate of water use by the stand during 1998 was estimated from reference evapotranspiration (ETo), leaf area, and a water use multiplication factor ($tH) specific to poplar trees. Sap velocity data were collected to measure actual water use by the stand in late summer of 1998. Estimated and measured values compared favorably, with measured values averaging 2.8 gallons day-1 tree-1 (1.7 mm day-1 tree-1). Water use by the stand in 1999 averaged an estimated 445 gallons day-1 (6.9 mm day-1 for the stand). Although the trees transpired a volume of water equivalent to a 10-ft thickness of the saturated zone, water table elevation data collected in 1999 did not indicate a depression in the water table.

Risk Perception and Bottled Water Use

This paper discusses a study of four communities in Oregon that were divided into two control groups and surveyed according to water contamination problems and how their perceptions of risk affected their use of tap water versus bottled water. Results indicated that risk perception about drinking water appears to be affected by three factors: the consumer's level of awareness of a problem; the presence or absence of drinking water contamination problems; and, the chronicity of the problem. Findings suggest that water contamination problems contribute to increased levels of risk perception with those drinking tap water. Also, long‐term problems that are not easily solved by the water utility may heighten consumers' perception of risk. Higher levels of risk perception may be the driving force behind higher rates of bottled water use in some communities.

Water utilization, plant hydraulic properties and xylem vulnerability in three contrasting coffee (Coffea arabica) cultivars.

Water use, hydraulic properties and xylem vulnerability to cavitation were studied in the coffee (Coffea arabica L.) cultivars San Ramon, Yellow Caturra and Typica growing in the field under similar environmental conditions. The cultivars differed in growth habit, crown morphology and total leaf surface area. Sap flow, stomatal conductance (g(s)), crown conductance (g(c)), apparent hydraulic conductance of the soil-leaf pathway (G(t)), leaf water potential (Psi(L)) and xylem vulnerability to loss of hydraulic conductivity were assessed under well-watered conditions and during a 21-day period when irrigation was withheld. Sap flow, g(c), and G(t) were greatest in Typica both with and without irrigation, lowest in San Ramon, which was relatively unresponsive to the withholding of irrigation, and intermediate in Yellow Caturra. The cultivars had similar g(s) when well watered, but withholding water decreased g(s) more in Typica and Yellow Caturra than in San Ramon. Typica had substantially lower Psi(L) near the end of the unirrigated period than the other cultivars (-2.5 versus -1.8 MPa), consistent with the relatively high sap flow in this cultivar. Xylem vulnerability curves indicated that Typica was less susceptible to loss of hydraulic conductivity than the other cultivars, consistent with the more negative Psi(L) values of Typica in the field during the period of low soil water availability. During soil drying, water use declined linearly with relative conductivity loss predicted from vulnerability curves. However, cultivar-specific relationships between water use and predicted conductivity loss were not observed because of pronounced hysteresis during recovery of water use following soil water recharge. All cultivars shared the same functional relationship between integrated daily sap flow and G(t), but they had different operating ranges. The three cultivars also shared common functional relationships between hydraulic architecture and water use despite consistent differences in water use under irrigated and dry soil conditions. We conclude that hydraulic architectural traits, rate of water use per plant and crown architecture are important determinants of short- and long-term variations in the water balance of Coffea arabica.

Nighttime water use in an irrigated Eucalyptus grandis plantation.

Sap flow measurements showed that a well-watered four-year-old plantation of Eucalyptus grandis (Hill ex Maiden) at Wagga Wagga, New South Wales, Australia, used 0.8 mm of water between 2100 and 0500 h on the midwinter night of July 30. Sap flow ceased for 2 to 3 h after sunset before recommencing at high rates that reached a maximum of 0.3 mm per h between 0200 and 0300 h. This pattern is inconsistent with the replenishment of tissue water reserves depleted during the day. Moreover, maximum leaf conductance at night was about 20 times maximum cuticular conductance values reported in the literature, which strongly suggests that stomata were partly open and that there was substantial water loss by way of the foliage. In an 8-month period from late winter to mid-autumn, comparable rates of nighttime water use were observed on only one other occasion. However, water use at rates of 0.1 mm per h or more occurred on 24 other nights. Almost 70% of the variation in nighttime sap velocity was explained by nighttime mean vapor pressure deficit and nighttime mean wind speed. Total recorded nighttime water use of the plantation was 48 mm, or 5% of total transpiration during the 8-month study. In view of the insensitivity of heat pulse measurements at low sap flows, this value may be an underestimate of actual nighttime transpiration.

Evaluation and classification of water use rate policy using the Points Credit Index

In order to be able to have clean water piped to our homes, an enormous investment both in money and expertise is required. The typical household water consumption, industrial, commercial, government sectors and agricultural authorities are known to be of several types of country's water consumption slices (stresses) which controls the water industry of any country. A mathematical model called STRESS has been developed, which demonstrates the relationship between various stresses with respect to consumption rate and costs which impact water use economy. The method of a Points Credit Index (PCI) was developed that is based on the following factors: credit, volume of consumption, and tariff systems, where the earlier accounts for both the environmental and the socio-economic norms represented by the PCI system. The model evaluates the various stresses to determine the correct tariff with respect to the PCI concept. This method will enable decision-makers to draw a logical conclusion on how much these stresses can change the water-use rate of a certain country. The significance of this paper is that it is the first attempt to evaluate water consumption by the PCI system, which controls the high consumption rate initiated by various stresses and predicts a suitable consumption tariff for each of the different stresses.

Partitioning of soil water among tree species in a Brazilian Cerrado ecosystem.

Source water used by woody perennials in a Brazilian savanna (Cerrado) was determined by comparing the stable hydrogen isotope composition (deltaD) of xylem sap and soil water at different depths during two consecutive dry seasons (1995 and 1996). Plant water status and rates of water use were also determined and compared with xylem water deltaD values. Overall, soil water deltaD decreased with increasing depth in the soil profile. Mean deltaD values were -35 per thousand for the upper 170 cm of soil and -55 per thousand between 230 and 400 cm depth at the end of the 1995 dry season. Soil water content increased with depth, from 18% near the surface to about 28% at 400 cm. A similar pattern of decreasing soil water deltaD with increasing depth was observed at the end of the 1996 dry season. Patterns consistent with hydraulic lift were observed in soil profiles sampled in 1995 and 1997. Concurrent analyses of xylem and soil water deltaD values indicated a distinct partitioning of water resources among 10 representative woody species (five deciduous and five evergreen). Among these species, four evergreen and one deciduous species acquired water primarily in the upper soil layers (above 200 cm), whereas three deciduous and one evergreen species tapped deep sources of soil water (below 200 cm). One deciduous species exhibited intermediate behavior. Total daily sap flow was negatively correlated with xylem sap deltaD values indicating that species with higher rates of water use during the dry season tended to rely on deeper soil water sources. Among evergreen species, minimum leaf water potentials were also negatively correlated with xylem water deltaD values, suggesting that access to more readily available water at greater depth permitted maintenance of a more favorable plant water status. No significant relationship between xylem water deltaD and plant size was observed in two evergreen species, suggesting a strong selective pressure for small plants to rapidly develop a deep root system. The degree of variation in soil water partitioning, leaf phenology and leaf longevity was consistent with the high diversity of woody species in the Cerrado.

590 Growth and Physiological Characteristics of Diploid and Tetraploid Citrus Rootstock Seedlings Grown at Elevated CO2

We grew three diploid (2X) Citrus rootstock seedlings and their autotetraploids (4X) at elevated CO2 to obtain insights into limitations on growth and net gas exchange that have been associated with tetraploidy. Well-nourished Volkamer lemon (Volk), Troyer citrange (Troy), and Cleopatra mandarin (Cleo) were grown in greenhouses at ambient or twice ambient CO2 for 3 months. We measured plant growth, water relations, mineral nutrition, and net gas exchange characteristics of leaves. Overall, tetraploid roots were thicker as 4X had lower root length: dry weight ratio or specific root length (SRL) than 2X roots. Tetraploid plants were smaller and had higher root/shoot ratios, shorter fibrous roots, and lower whole plant transpiration than 2X. Tetraploids also had lower leaf N and P concentrations on a dry weight basis. Since 4X leaves had thicker leaves (more dry weight per area) than 2X leaves, these nutrient differences disappeared when expressed on an leaf area basis. Elevated CO2 increased plant growth but decreased leaf N, P, and K apparently by a growth dilution effect. Elevated CO2 also increased fibrous root thickness, leaf thickness, and net assimilation of CO2 (ACO2) but decreased stomatal conductance and transpiration such that leaf water use efficiency increased. There was no effect of ploidy level on ACO2 but 4X Volk and Troy had lower rates of ACO2 than their diploids at elevated CO2. Hydraulic conductivity of intact root systems (measured in a pressure pot) was correlated to total plant growth but variability obscured effects of CO2 or ploidy on root conductivity. The low SRL of tetraploids were correlated with lower rates of water use and lower leaf nutrient concentrations, which may be operative in determining the growth characteristics associated with tetraploidy.

281 Water Use in Herbaceous Landscape Plants

Water conservation in a landscape is an important issue because periodic water shortages are common in many regions of the world. This increases the importance of specifying landscape plants that require less water and matching the plant to site microclimates. Our objectives were to establish water-use rates for three herbaceous landscape plants and to determine the level of water reduction these plants can tolerate while maintaining both visual and landscape quality. Water use rates were determined for Schizachyrium scoparium (Little bluestem), Hosta spp. (Hosta) and Festuca cinerea `Dwarf' (Dwarf blue fescue) in studies using pot lysimeters at the Univ. of Nebraska Horticulture Research Greenhouse facility. Each lysimeter was watered to saturation, allowed to drain to field capacity, and weighed. The lysimeters were weighed again 24 h later, and the process was repeated to determine daily evapotranspiration. Results indicated that hosta used less water than dwarf blue fescue and little bluestem. In a subsequent study to compare the relative effects of withholding irrigation among these species, seven groups of five replicates of each species were grown in 1 peat: 0.33 vermiculite: 0.66 soil: 1 sand (by volume) in 7.6-L containers. Each container was watered to saturation, allowed to drain for 24 h to reach field capacity, and allowed to dry down in 10-day increments. Results of the dry-down study indicated that little bluestem maintained the best visual quality for the longest duration of drought, followed by dwarf blue fescue and hosta in decreasing order of visual quality.

Characterizing traits that enhance the competitiveness of winter wheat (Triticum aestivum) against jointed goatgrass (Aegilops cylindrica)

Our objective was to identify traits in winter wheat important to competitiveness against jointed goatgrass, measured as increased wheat yields and reduced jointed goatgrass seed production. Jointed goatgrass is an important winter annual grass weed that cannot be controlled selectively in winter wheat. Seven cultivars of soft white winter wheat were grown with and without competition from jointed goatgrass over two growing seasons. Measurements of numerous traits of winter wheat and jointed goatgrass were recorded throughout each growing season. The data were analyzed using path analysis with latent variables to determine which traits most enhanced competitiveness. In a drier year, increased rate of height development was important in maintaining wheat yields when wheat was growing in competition with jointed goatgrass. Increased rate of height development also was an important trait in reducing jointed goatgrass seed production. In a wet year compared to a dry year, the number of wheat heads per plant, the rate of water use, and weight gain were positively correlated to maintaining winter wheat yields. Jointed goatgrass seed production in the wet year was reduced overall compared to the dry year, but from the cultivars tested, there were no traits identified that were critical in enhancing this loss of seed production. This study suggests that cultivars with greater height development rates will be more competitive when growing in fields infested with jointed goatgrass.

Survival, growth and water use of a range of tree species irrigated with saline drainage water

Summary We irrigated 36 species and provenances of mainly Australian native trees with saline drainage water (EC 2 dS/m and boron 2.4 mg/L) in a trial sited at Loxton, in semi-arid south eastern Australia. Twelve months after planting, the survival rates amongst species were similar with a site mean of 96%. Three years after planting, the tallest trees were E. gomphocephala and the provenances/clones of E. occidentalis and E. camaldulensis, with heights ranging from 5.3 to 6.7 m; these trees also had the greatest girth with basal areas (measured at 0.3 m above ground level) ranging from 16 m2/ha to 24 m2/ha. The basal area and height of E. camaldulensis was highly variable ranging from 1 m2/ha and 2.1 m for the CML42 clone, to 16 m2/ha and 6.2 m for the Alcoa clone 20. Provenance also had a significant effect on the values of these parameters in E. occidentalis. In the third year, water use rates were determined for 12 of the better growing species. Over summer, the mean daily water use rates ranged from 49 L/tree for the Alcoa clone 20 of E. camaldulensis, to 12 L/tree for E. kondininensis. The Alcoa clone 20 of E. camaldulensis had the higher daily rate of water use per unit area of sapwood, 4.7 kL/m2. The hourly rates of water use per unit area of sapwood rose with increasing vapour pressure deficit up to a value of about 17 hPa and then remained constant as VPD rose to 55 hPa. Variation amongst tree species in the relationships between hourly rates of water use per unit area of sapwood and VPD was highly significant.

Partitioning of soil water among canopy trees in a seasonally dry tropical forest.

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θv), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about -15‰ at 0.1 m to -50 to -55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of -60‰. θv increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θv dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θv remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season.

Losses of nitrogen from an effluent-irrigated plantation

Effluent-irrigated tree plantations are becoming a popular alternative to the direct discharge of sewerage effluent into inland rivers. However, when irrigated at the water-use rate of the plantation, nitrogen (N) applied in the effluent often exceeds the demand of the trees both in the early stages of growth and after canopy closure when much of the N is supplied from the decomposition of litter. With the ample supply of water and large loadings of N, conditions are conducive to significant N losses. The principal potential N loss mechanisms are leaching of nitrate-N to groundwater and biological denitrification. In 4 separate experiments between winter 1992 and summer 1994, 15N-labelled N (total application of 2 g N/m2; 30 atom%) was applied to 1-m2 subplots or small plastic (PVC) cylinders. In the first 2 experiments, using 1-m2 subplots, the conservative tracer Br was applied as KBr with the 15N at a total application of 12·8 g Br-/m2. In Expt 1, the high recovery of the 15N-labelled N, and the comparable distribution of 15N and Br- in the soil profile, implied that there was little loss of N by denitrification, even though the soil was sufficiently wet for leaching of both tracers. Although less conclusive, the other experiments also showed a high potential for leaching and minimal loss of applied 15N-labelled N by denitrification. The small contribution to N loss of denitrification, despite conditions being otherwise suitable for it, is attributed to the free-draining nature of the soil, which meant that aeration was rarely sufficiently limited to encourage denitrification. Similar results are expected at other effluent irrigation sites on freely draining soils.

A review of whole-plant water use studies in tree.

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

Effects of climatic variation on field metabolism and water relations of desert tortoises.

We used the doubly labeled water method to measure the field metabolic rates (FMRs, in kJ kg-1 day-1) and water flux rates (WIRs, in ml H2O kg-1 day-1) of adult desert tortoises (Gopherus agassizii) in three parts of the Mojave Desert in California over a 3.5-year period, in order to develop insights into the physiological responses of this threatened species to climate variation among sites and years. FMR, WIR, and the water economy index (WEI, in ml H2O kJ-1, an indicator of drinking of free water) differed extensively among seasons, among study sites, between sexes, and among years. In high-rainfall years, males had higher FMRs than females. Average daily rates of energy and water use by desert tortoises were extraordinarily variable: 28-fold differences in FMR and 237-fold differences in WIR were measured. Some of this variation was due to seasonal conditions, with rates being low during cold winter months and higher in the warm seasons. However, much of the variation was due to responses to year-to-year variation in rainfall. Annual spring peaks in FMR and WIR were higher in wet years than in drought years. Site differences in seasonal patterns were apparently due to geographic differences in rainfall patterns (more summer rain at eastern Mojave sites). In spring 1992, during an El Niño (ENSO) event, the WEI was greater than the maximal value obtainable from consuming succulent vegetation, indicating copious drinking of rainwater at that time. The physiological and behavioral flexibility of desert tortoises, evident in individuals living at all three study sites, appears central to their ability to survive droughts and benefit from periods of resource abundance. The strong effects of the El Niño (ENSO) weather pattern on tortoise physiology, reproduction, and survival elucidated in this and other studies suggest that local manifestations of global climate events could have a long-term influence on the tortoise populations in the Mojave Desert.

Growth and Water Use of Red Spruce (Picea Rubens Sarg.) Exposed to Ozone and Simulated Acidic Precipitation for Four Growing Seasons

Abstract Red spruce (Picea rubens Sarg.) saplings from a pristine site in Maine were transplanted in native soil to 360 liter pots and relocated to Ithaca, New York. After a 1 yr establishment period, 90 saplings were chosen for use, assigned to treatments, placed in large open-top chambers, and exposed to ozone (0.5 to 2.0 x ambient concentration) and simulated acidic precipitation (at pH 3.1, 4.1, or 5.1) for 3 or 4 consecutive growing seasons (total ozone exposures of about 165 ppm · hr to almost 570 ppm · hr). Forty-five trees were grown on weighing lysimeters and were used to develop whole-tree water-use budgets. After 3 yr of exposure, the 45 trees not grown on lysimeters were harvested, and the plant tissues were weighed. After the fourth growing season, the remaining 45 trees were harvested, and the weight of the plant tissues was determined. Significant effects of ozone or simulated acidic precipitation were not detected, with the exception of a trend for increased needle dry weight with increasing ozone and, in 1990 needles only, with decreasing pH. No effects were detected in total dry weight, aboveground dry weight, coarse or fine root weight, stem dry weight, or relative growth rate. No significant interactions of ozone and pH treatment were detected. Neither ozone nor pH treatment affected the rate of water use by red spruce saplings, even when they were subjected to a dry-down period resulting in drought stress. For. Sci. 43(3):355-361.

Changes in soil chemistry in efluent-irrigated Pinus radiata and Eucalyptus grandis plantations

Two tree species (Pinus radiata and Eucalyptus grandis) were planted in 1991 and irrigated weekly with either secondary-treated effluent or bore-water, in an effluent-irrigated plantation at Wagga Wagga, NSW. Changes in soil salinity, sodicity, alkalinity, and the accumulation of potentially toxic elements after 2 and 4 irrigation seasons were studied. The average sodium adsorption ratio (SAR) and electrical conductivity (EC) of effluent were 4·8 and 0·79 dS/m, while in bore water they were 3·3 and 0·70 dS/m, respectively. Effluent was applied at 3 rates: the water-use rate of the plantation less rainfall (medium treatment, M), a nominal rate 0·5 times M (low treatment, L), and a nominal rate 1·7 times M (high treatment, H). The bore-water treatment (W) was irrigated at its water-use rate less rainfall. Salinity of saturation paste extracts in the surface 0·05 m increased from a pre-treatment value of 1 dS/m to 2–3 dS/m after 4 years of effluent irrigation. The increase in soil salinity was consistent with the salt loading for the different irrigation treatments. Similarly, exchangeable sodium percentage (ESP) increased in all treatments. The ESP was generally <2% at the start of the experiment and increased to a maximum of 25% in the 0·3–0·4 m soil layer of treatment L. Values had changed throughout the 4 irrigation seasons and the soil profile does not appear to have reached a steady-state condition. Soil pH increased in all treatments. The largest increase (≈0·7) occurred in treatment H, while treatment W had the smallest increase (≈0·4). The increase in soil pH caused a decrease in exchangeable Al and Mn in the upper 0·4 m of the soil profile in all treatments. Effective cation exchange capacity (ECEC), approximated by the sum of the extractable cations, increased significantly in the upper 0·3 m of the soil profile after 2 and 4 irrigation seasons, but there was no significant change in ECEC below 0·3 m.

The Association of Biomass Allocation With Growth and Water Use Efficiency of Two Triticum aestivum Cultivars

We investigated traits that might improve performance of wheat (Triticum aestivum L.) in a dry environment, and examined if the response to drought is different for cultivars with a contrasting biomass allocation pattern, using two cultivars with either a high (Katya) or small (Mexipak) proportion of biomass allocated to the leaves. Plants were grown in pots placed in the field, under well-watered conditions and under intermittent drought. Katya allocated a greater proportion of its biomass to both leaves and roots, and a smaller proportion to stems than Mexipak did. The growth rate of Katya was not higher despite a higher investment in leaf area and a slightly higher rate of photosynthesis, possibly due to concomitantly higher respiratory losses. Under well-watered conditions, the efficiency of water use for growth was higher for Katya than for Mexipak, due to a lower water use at a similar growth rate. Differences in water use per plant were determined by the rate of water loss or uptake, rather than by the amount of roots or leaves. A high water use efficiency was independent of growth rate. We conclude that a lower rate of water use due to a lower transpiration per unit leaf area linked with a high leaf area can contribute to improve the performance of wheat cultivars in a water-limited environment.

Transpiration and root water uptake by olive trees

While the cultivated olive tree (Olea europaea L.) is known to be sclerophyllous and effective at tolerating drought, little is known of its short-term water-use dynamics for most studies have been based on longer-term, water-balance information. We present here, for the first time, heat-pulse measurements of the sap flux measured not only within the semi-trunk of an olive tree, but also within a root excavated close to the stump. One tree in the olive grove near Seville in Spain had regularly received basin irrigation during the summer, whereas the other, growing on this deep silt loam, had been without water for over 3 months. Following a flood irrigation of 730 L to a dyked area around the tree, the regularly-irrigated olive maintained a transpiration rate of 1.65 mm3 mm−2 d−1, on a leaf area basis, for only 3 days following the irrigation. This rate was maintained for a total consumption of 110 L. It then began again to limit its rate of water use with transpiration falling below that predicted for well-watered conditions by the Penman-Monteith equation. The flow of sap in the near-surface root dropped concomitantly. Meanwhile the unirrigated tree was using water at just 0.78 mm d−1. Yet following an irrigation of 870 L it only lifted its consumption to 1.12 mm d−1, on a leaf area basis. Neither did it recover its leaf water potential following this wetting because of an inability to refill cavitated vessels. These data again show olive to be a parsimonious and cautious consumer of soil water.

Water Use and Growth of Cotton in Response to Elevated CO2 in Wet and Drying Soil

Cotton (Gossypium hirsutum cv. Sicala 34) was grown at 352 ('low CO2') or 710 ('high CO2') μL L-1 atmospheric CO2 in continuously wet soil, or in drying soil, or in drying soil re-wetted after plant wilting. In wet soil, the approximately 15% reduction in transpiration per unit leaf area owing to high CO2 was only half that for other species, whereas effects on growth and leaf area were relatively larger. Consequently, water use per plant was 45-50% higher for high CO2 plants in contrast to other species for which the rate of water use is either the same or lower in high CO2. Greater plant water use early in a drying cycle caused the soil to dry faster under high CO2 than under low CO2. The addition of the consequential greater water stress at high CO2 in drying soil to the direct CO2 effect on stomata caused the transpiration rate of high CO2 plants to fall by up to 60% as the soil dried relative to plants drying at low CO2. After re-wetting the dry soil, the reduction in transpiration rate at high CO2 returned within hours to the value of 15% seen in wet soil. The results were inconsistent with the idea that water deficits increase the sensitivity of stomatal aperture to CO2. Other consequences of drier soil under high CO2 compared with low CO2 were: (a) unlike in many other species, in cotton, the relative growth enhancement by high CO2 is not higher under drying soil compared with wet soil owing to the opposite effect on soil water content; and (b) the increased water-use efficiency in drying soil relative to wet soil was greater in high CO2 plants than in low CO2. The confounding of indirect effects of soil water with the direct CO2 effects may explain the wide variability of literature reports about CO2 effects on stomatal conductance and water use.

Effects of Plant Size on Photosynthesis and Water Relations in the Desert Shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.