Showing papers on "Transpiration published in 1995"

Leaf nitrogen, photosynthesis, conductance and transpiration : scaling from leaves to canopies

Abstract: A model is presented which solves simultaneously for leaf‐scale stomatal conductance, CO2 assimilation and the energy balance as a function of leaf position within canopies of well‐watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen‐ and light‐use efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are ∼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gc should be calculated as Gc=AIƒ(fslgsl+fshgsh), where I” is leaf area index, fsi and fsh are the fractions of sunlit and shaded leaves at each level, and gsi and gsh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big‐leaf’ version of the Penman‐Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen‐ and radiation‐use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2‐ to 3‐fold variation in these efficiencies.

A reinterpretation of stomatal responses to humidity

Abstract: The stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ∂g/∂E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance gm obtained by extrapolation to zero transpiration, and a maximum rate of transpiration Em obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO2 concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.

A Model for Predicting Continental‐Scale Vegetation Distribution and Water Balance

Abstract: A Mapped atmosphere-Plant-Soil System (MAPSS) has been constructed for simulating the potential biosphere impacts and biosphere-atmosphere feedbacks from climatic change The system calculates the potential vegetation type and leaf area that could be supported at a site, within the constraints of the abiotic climate Both woody vegetation and grass are supported and compete for light and water The woody vegetation can be either trees or shrubs, evergreen or deciduous, and needleleaved or broadleaved A complete site water balance is calculated and integrates the vegetation leaf area and stomatal conductance in canopy transpiration and soil hydrology The MAPSS model accurately simulates the distributions of forests, grasslands, and deserts and reproduces observed monthly runoff The model can be used for predictions of new vegetation distribution patterns, soil moisture, and runoff patterns in alternative climates 112 refs, 11 figs, 4 tabs

Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.).

Abstract: Summary To quantify the effects of crown thinning on the water balance and growth of the stand and to analyze the ecophysiological modifications induced by canopy opening on individual tree water relations, we conducted a thinning experi ment in a 43-year-old Quercus petraea stand by removing trees from the upper canopy level. Soil water content, rainfall inter ception, sap flow, leaf water potential and stomatal conduc tance were monitored for two seasons following thinning. Seasonal time courses of leaf area index (LAI) and girth increment were also measured. Predawn leaf water potential was significantly higher in trees in the thinned stand than in the closed stand, as a conse quence of higher relative extractable water in the soil. The improvement in water availability in the thinned stand resulted from decreases in both interception and transpiration. From Year 1 to Year 2, an increase in transpiration was observed in the thinned stand without any modification in LAI, whereas changes in transpiration in the closed stand were accompanied by variations in LAI. The different behaviors of the closed and open canopies were interpreted in terms of coupling to the atmosphere. Thinning increased inter-tree variability in sap flow density, which was closely related to a leaf area competi tion index. Stomatal conductance varied little inside the crown and differences in stomatal conductance between the treat ments appeared only during a water shortage and affected mainly the closed stand. Thinning enhanced tree growth as a result of a longer growing period due to the absence of summer drought and higher rates of growth. Suppressed and dominant trees benefited more from thinning than trees in the codomi nant classes.

Effects of arbuscular-mycorrhizal glomus species on drought tolerance: physiological and nutritional plant responses.

Abstract: The tolerance of lettuce plants (Lactuca sativa L. cv. Romana) to drought stress differed with the arbuscular-mycorrhizal fungal isolate with which the plants were associated. Seven fungal species belonging to the genus Glomus were studied for their ability to enhance the drought tolerance of lettuce plants. These fungi had different traits that affected the drought resistance of host plants. The ranking of arbuscular-mycorrhizal fungal effects on drought tolerance, based on the relative decreases in shoot dry weight, was as follows: Glomus deserticola > Glomus fasciculatum > Glomus mosseae > Glomus etunicatum > Glomus intraradices > Glomus caledonium > Glomus occultum. In this comparative study specific mycorrhizal fungi had consistent effects on plant growth, mineral uptake, the CO(inf2) exchange rate, water use efficiency, transpiration, stomatal conductance, photosynthetic phosphorus use efficiency, and proline accumulation under either well-watered or drought-stressed conditions. The ability of the isolates to maintain plant growth effectively under water stress conditions was related to higher transpiration rates, levels of leaf conductance, and proline, N, and P contents. Differences in proline accumulation in leaves among the fungal symbioses suggested that the fungi were able to induce different degrees of osmotic adjustment. The detrimental effects of drought were not related to decreases in photosynthesis or water use efficiency. Neither of these parameters was related to P nutrition. The differences in P and K acquisition, transpiration, and stomatal conductance were related to the mycorrhizal efficiencies of the different fungi. Our observations revealed the propensities of different Glomus species to assert their protective effects during plant water stress. The greater effectiveness of G. deserticola in improving water deficit tolerance was associated with the lowest level of growth reduction (9%) under stress conditions. The growth of plants colonized by G. occultum was reduced by 70% after a progressive drought stress period. In general, the different protective effects of the mycorrhizal isolates were not associated with colonizing ability. Nevertheless, G. deserticola was the most efficient fungus and exhibited the highest levels of mycorrhizal colonization, as well as the greatest stimulation of physiological parameters.

Influence of leaf water status on stomatal response to humidity, hydraulic conductance, and soil drought in Betula occidentalis

Abstract: Whole-canopy measurements of water flux were used to calculate stomatal conductance (g s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψs). As expected, g s dropped in response to decreased k or ΨS, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g s were associated with approximately constant bulk leaf water potential (ψl), this does not logically exclude a feedback response between ΨL and g s . To test the influence of leaf versus root water status on g s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased ΨS was fully or partially reversed within 5 min of pressurizing the soil. Bulk ΨL remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g s , or caused it to decline; and bulk ΨL increased. Increased Ψl may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low ΨS. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.

Relationships between stem diameter, sapwood area, leaf area and transpiration in a young mountain ash forest.

Abstract: We examined relationships between stem diameter, sapwood area, leaf area and transpiration in a 15-year-old mountain ash (Eucalyptus regnans F. Muell.) forest containing silver wattle (Acacia dealbata Link.) as a suppressed overstory species and mountain hickory (Acacia frigescens J.H. Willis) as an understory species. Stem diameter explained 93% of the variation in leaf area, 96% of the variation in sapwood area and 88% of the variation in mean daily spring transpiration in 19 mountain ash trees. In seven silver wattle trees, stem diameter explained 87% of the variation in sapwood area but was a poor predictor of the other variables. When transpiration measurements from individual trees were scaled up to a plot basis, using stem diameter values for 164 mountain ash trees and 124 silver wattle trees, mean daily spring transpiration rates of the two species were 2.3 and 0.6 mm day(-1), respectively. The leaf area index of the plot was estimated directly by destructive sampling, and indirectly with an LAI-2000 plant canopy analyzer and by hemispherical canopy photography. All three methods gave similar results.

Hyphal contribution to water uptake in mycorrhizal plants as affected by the fungal species and water status

Abstract: Vesicular-arbuscular mycorrhizae may increase resistance of plants to drought by a number of mechanisms, such as increased root hydraulic conductivity, stomatal regulation, hyphal water uptake and osmotic adjustment. However, a substantial contribution of vesicular-arbuscular mycorrhizal (VAM) hyphae to water uptake has not been demonstrated unequivocally. The objective of this investigation was to examine the contribution of hyphae from two VAM fungi to water uptake and transport by the host plant. Lettuce (Lactuca saliva L.) plants were grown in a container divided by a screen into two compartments. One was occupied by roots, the other only by VAM hyphae, which the screen permitted to pass. Roots were colonized by the VAM fungi Glomus deserticola or Glomus fasciculatum, or were left uninoculated but P-supplemented. Water was supplied to the hyphal compartment at a distance of 10 cm from the screen (root). CO 2 exchange rate, water-use efficiency, transpiration, stomatal conductance and photosynthetic phosphorus-use efficiency of VAM or P-amended control plants were evaluated at three levels of water application in the hyphal compartment. Results indicate that much of the water was taken up by the hyphae in VAM plants. VAM plants, which had access to the hyphal compartment, had higher water and nutrient contents. G. deserticola functioned efficiently under water limitation and mycelium from G. fasciculatum-colonized plants was very sensitive to water in the medium. This discrepancy in VAM behaviour reflects the various abilities of each fungus according to soil water levels. Different abilities of specific mycelia were also expressed in terms of nutritional and leaf gas-exchange parameters. G. fasciculatum caused a significant increase in net photosynthesis and rate of water use efficiency compared to G. deserticola and P-fertilized plants. In contrast, the G. deserticola treatment was the most efficient affecting N, P and K nutrition, leaf conductance and transpiration. Since no differences in the intra- and extra-radical hyphal extension of the two endophytes were found, the results demonstrate that mycorrhizal hyphae can take up water and that there are considerable variations in both the behaviour of these two VAM fungi and in the mechanisms involved in their effects on plant water relations.

Predicting the effects of climate change on water yield and forest production in the northeastern United States

Abstract: Rapid and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO2 are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes. This paper describes an improved version of a forest carbon and water balance model (PnET-II) and the application of the model to predict stand- and regional-level effects of changes in temperature, precipitation and atmospheric CO2 concentration. PnET-II is a simple, generalized, monthly time-step model of water and carbon balances (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements from the original model include a complete carbon balance and improvements in the prediction of canopy phenology, as well as in the computation of canopy structure and photosynthesis. The model was parameterized and run for 4 forest/site combinations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soil-based water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire NewEngland/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr-1 for the 4 forest/site combinations, and from -150 to 350 g C m-2 yr-1 for the region, with a regional average of 76 g C m-2 yr-1. A combination of increased temperature (+6*C), decreased precipitation (-15%) and increased water use efficiency (2x, due to doubling of CO2) resulted generally in increases in NPP and decreases in water yield over the region.

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. II. Model testing and application

Abstract: The scaling of CO 2 and water vapour transfer from leaf to canopy dimensions was achieved by integrating mechanistic models for physiological (photosynthesis, stomatal conductance and soil/root and bole respiration) and micrometeorological (radiative transfer, turbulent transfer and surface energy exchanges) processes. The main objectives of this paper are to describe a canopy photosynthesis and evaporation model for a temperate broadleaf forest and to test it against field measurements. The other goal of this paper is to use the validated model to address some contemporary ecological and physiological questions concerning the transfer of carbon and water between forest canopies and the atmosphere. In particular, we examine the role of simple versus complex radiative transfer models and the effect of environmental (solar radiation and CO 2 ) and ecophysiological (photosynthetic capacity) variables on canopy-scale carbon and water vapour fluxes.

Estimating stand transpiration in a Eucalyptus populnea woodland with the heat pulse method: measurement errors and sampling strategies

Abstract: Summary Sap flow measurement techniques, such as the heat pulse (compensation) method, are practical means for estimating the water use of individual trees and are often the only reasonable alternative for measuring forest and woodland transpiration in complex heterogeneous terrain. The need to scale estimates of water use from a sample of individual stems to a stand (population) of known area may be satisfied by applying scalars of flux based on tree size or domain. We estimated the aggregate errors in applying the heat pulse technique to the estimation of stand transpiration in a poplar box (Eucalyptus populnea F.J. Muell.) woodland in southeastern Queensland, Australia, by a combination of precision analyses, experimental validation and Monte Carlo simulations of sampling errors. Errors in sap flux density measurements were approximately 13%. The potential error in the flux estimates for individual stems with stratified sampling of sap flux density with depth and bole quadrant based on four sensors was an additional 25%. Conducting wood area, diameter at 1.3 m, leaf area and domain based on Ecological Field Theory all proved excellent scalars of flux at the stand level. With a sample size of six trees stratified by diameter, coefficients of variation in scaling to the stand level were approximately 5% for any of these scalars. The greatest potential source of error in estimating stand transpiration by the heat pulse method was in the measurement of the fluxes of individual stems; scaling these measurements to a homogeneous stand of trees involved less uncertainty.

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties.

Abstract: Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (gs) and boundary layer (gb) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s−1 stalling speed of the anemometer used and was rarely above 0.5 m s−1, resulting in uniformly low gb (c. 200–300 mmol m−2 s−1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than gb. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in gs when gs was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of gs and the leaf-bulk air vapor pressure difference (VPD) without taking gb into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and gs initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (Gt), becoming asymptotic at higher values of Gt. For both E and gs a unique relationship appeared to describe the response of all species to variations in Gt. The relatively weak correlation observed between gs and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.

Partitioning of water resources among plants of a lowland tropical forest

Abstract: Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, δD) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water δD values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water δD values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average δD values were-13‰, for 0–30 cm depth and-36.7‰ for 30–100 cm depth. Soil water δD values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water δD of deciduous trees (average=-25.3±1.4‰) was higher than that of evergreen trees (average=-36.3±3.5‰), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, δD of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.

Diurnal changes in the radius of a subalpine Norway spruce stem : their relation to the sap flow and their use to estimate transpiration

Abstract: Diurnal changes in the stem radius of a subalpine mature Norway spruce were measured simultaneously with the flow of sap in xylem. Matric potentials in the soil were > -35 kPa. The kinetics of the flow were closely related to the changes in the radius of the stem resulting from depletion of its extensible tissues. The radius of the stem oscillated daily and, fairly independently of this, fluctuated over several days. The daily shrinkage (Δd) was correlated with the daily flow through its base (Qd). When the crown transpired little and was nearly saturated during rainy days, ASd tended to increase relative to Qd. Using a linear relation, the estimates of flow by Δd deviated less than ± 10% from the values measured by heat balance, provided that the periods of calibration in their ratio of dry to rainy days were comparable to those estimated. If the two periods differed in this respect, the estimates of flow deviated up to 42%. A quadratic relation yielded estimates that depended less on weather. It reduced maximal deviations to ±22%. Since Δd additionally may represent the time pattern of the daily transpiration better than Qd, analysing changes in the radius of stems may supplement or partly replace measurements of sap flow in stems.

Export of Abscisic Acid, 1-Aminocyclopropane-1-Carboxylic Acid, Phosphate, and Nitrate from Roots to Shoots of Flooded Tomato Plants (Accounting for Effects of Xylem Sap Flow Rate on Concentration and Delivery).

Abstract: We determined whether root stress alters the output of physiologically active messages passing from roots to shoots in the transpiration stream. Concentrations were not good measures of output. This was because changes in volume flow of xylem sap caused either by sampling procedures or by effects of root stress on rates of whole-plant transpiration modified concentrations simply by dilution. Thus, delivery rate (concentration x sap flow rate) was preferred to concentration as a measure of solute output from roots. To demonstrate these points, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid, phosphate, nitrate, and pH were measured in xylem sap of flooded and well-drained tomato (Lycopersicon esculentum Mill., cv Ailsa Craig) plants expressed at various rates from pressurized detopped roots. Concentrations decreased as sap flow rates were increased. However, dilution of solutes was often less than proportional to flow, especially in flooded plants. Thus, sap flowing through detopped roots at whole-plant transpiration rates was used to estimate solute delivery rates in intact plants. On this basis, delivery of ACC from roots to shoots was 3.1-fold greater in plants flooded for 24 h than in well-drained plants, and delivery of phosphate was 2.3-fold greater. Delivery rates of abscisic acid and nitrate in flooded plants were only 11 and 7%, respectively, of those in well-drained plants.

Differential response of leaf conductance, carbon isotope discrimination and water‐use efficiency to nitrogen deficiency in maritime pine and pedunculate oak plants

Abstract: SUMMARY Growth, CO2 assimilation rate (A), leaf conductance (g), transpiration efficiency (W= ratio biomass production/plant water use) and carbon isotope discrimination (Δ) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand–peat mixture with three levels of fertilization: F100, optimal complete fertilization; F25, 25% of the optimal fertilizer supply; F0, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water-use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time-integrated (A/g derived from Δ measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between F100 and F25, has not been reported before. In oak, both A and g were decreased in F25 and F0 as compared with F100. W was not affected – and instantaneous as well as time-integrated A/g values were only slightly decreased – in relation to decreasing plant N concentration. For F100, no difference in W was noticed between pine and oak though the Δ values were 2·6‰ lower in oak. We speculate that this discrepancy was linked with higher plant-carbon losses through processes like respiration, fine-root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W, but has to be used with caution when comparing different species.

Root water uptake by kiwifruit vines following partial wetting of the root zone

Abstract: Rates of sap flow and root-water uptake by two 7-year old kiwifruit vines (Acinidia deliciosa) were studied in an orchard with the aim of determining the ability of the vines to alter their spatial pattern of root-water uptake following differential wetting of the root zone. Time-domain reflectometry (TDR) was used to monitor changes in the soil's volumetric water content, π. The heat-pulse technique was used to monitor sap flow not only in the stem but also in several large roots to see how root flow responded with local changes in soil water availability. Prior to irrigation there was a broad correspondence between the pattern of water uptake and the distribution of root-length density. However, following irrigation, we observed a preferential uptake of water from the wetter parts of the soil and a corresponding decline in water uptake from the drier parts of the soil. Observations of root uptake by TDR following irrigation also revealed the inordinate activity of near-surface roots. The vine would preferentially draw upon near-surface water if it were available. Kiwifruit vines are able to shift rapidly their pattern of uptake, in a matter of days, away from drier parts of the root zone and begin to extract water preferentially from those regions where it is more freely available. Upon full wetting of the root zone, previously inactive roots in the dry soil of the root zone were quickly able to recover their activity. Indeed their activity following rewatering was found to be greater than it had been prior to the period of soil dryness. A rapid flush of new root growth is considered to be the mechanism that leads to this enhanced activity.

Watering up After Clear‐Cutting on Forested Wetlands of the St. Lawrence Lowland

Abstract: Clear-cutting on wetlands of the St. Lawrence lowlands raised the water table levels on seven of the eight studied sites encompassing five forest types and four soil subgroups. Water table levels dropped 3 cm after clear-cutting on the eighth site, which was the wettest site with precut water levels within the top 10 cm. This was explained by evaporation from exposed water surfaces. The magnitude of the water table rises increased with the depth of the precut water table. The seasonal mean and maximum rise were respectively 20 and 52 cm on a poorly drained mineral soil which had the lowest precut water table levels. The smallest rises, with means around 7 cm, were associated with high precut water table on bogs and on fens. The watering up was not reduced on fens where a lateral flow occurs. This study indicated that transition sites between the bogs or fens and the uplands were most susceptible to hydrologic changes after clear-cutting. Watering up was caused by reduced evapotranspiration, of which the major component was interception. The rise of the water table observed in the clear-cut and the bordering forest indicates that clear-cutting in narrow strips is not an effective solution to avoid water table rise. Silvicultural treatments to maintain interception and transpiration by leaving logging debris, small trees, and preestablished regeneration would be more effective.

Water losses in the Patagonian steppe: A modelling approach

Abstract: In this paper we sought to answer questions related to the long-term soil water dynamics of the Patagonian steppe: What are the magnitude and seasonal dynamics of transpiration, evaporation, and deep percolation? How do these fluxes respond to fluctuations in annual precipitation? What is the pattern of soil water availability? We developed a soil water model for the steppe with a daily time step. The model gives weekly cumulative values of transpiration, evaporation, and drainage, maximum and minimum water content for the different soil layers in each week, and the weekly fre- quency of days with soil water potential higher than -1 MPa for each layer. The model was tested against three sets of experimental data. Simulated data of total water losses were significantly correlated with observed data, and the slope did not differ significantly from 1 nor the y-intercept from 0. On a long term basis, evaporation accounted for 56% of total water loss, transpiration 34%, and deep percolation the remaining 10%. Transpiration and evaporation had asyn- chronic dynamics. Evaporation was high during the coldest and wettest months of the year (mainly winter months). Transpiration, on the contrary, reached maximum values when energy and water availability were simultaneously high in late spring-early summer. Drain- age took place during the coldest months, when most of precipitation occurred, and the soil remained near field capacity. Both evaporation and transpiration had a positive response to an increase in precipitation. However, the proportion of total water loss following these pathways decreased with in- creasing precipitation. Drainage had a positive exponential relationship with winter pre- cipitation. Probabilities of soil water potential higher than -1 MPa in the upper soil layer were very low during most of the warm season (P < 0.15). At the beginning of the growing season the wettest layer was located at an intermediate depth (10-20 and 20-40 cm), and moved downward so at the end, only deep roots had high soil water availability.

Morphological and physiological consequences of a positive plant interaction

Abstract: It is known that plants can create acceptable environments for other plant species that may be unable to physiologically adjust to harsh physical conditions in isolation. In this study, we explore the morphological and physiological responses of a positive interaction on a common New England salt marsh shrub, Iva frutescens L. In previous experiments, we have shown that Iva benefits from the presence of the black rush, Juncus gerardi Loisel. Juncus removal in the lower marsh caused increases in soil salinity and anoxia, which resulted in lower Iva biomass, growth, and survival. We hypothesize that the positive effect of Juncus on Iva is a consequence of (1) the turf morphology of Juncus shading soil surfaces, thereby minimizing salt accumulation, and (2) the increased soil oxygen content surrounding Juncus caused by radial oxygen loss from its rhizosphere. However, the specific morphological and physiological benefits conferred on Iva are un- known. We conducted experiments in the salt marsh to isolate the effects of Juncus neighbors, waterlogging, and salt water stress on Iva. We found that Iva, when exposed to salty, oxygen- deprived soil conditions created in the absence of Juncus, responded more negatively to salinity than waterlogging, although the interaction between salinity and waterlogging was often significant. Iva without neighbors showed decreases in plant height, leaf density, leaf area, and flower density compared to controls or plants with fresh-water additions. Leaves of Iva without neighbors responded to salt stress and waterlogging by increasing leaf thickness and succulence. The increase in soil salinity of plants without neighbors caused plants to have corre- spondingly low water potentials, indicating that they were severely water stressed. Phys- iologically, Iva plants had a low mean photosynthetic rate, transpiration rate, water-use efficiency, and stomatal conductance compared to controls or Iva with fresh-water additions. Low water-use efficiency in Iva without neighbors was correlated with stomatal closure but also with high intercellular CO2 concentrations. The mechanisms behind this apparent anomaly may include both nonstomatal inhibition of photosynthesis as well as patchy stomatal closure, although it is uncertain which predominates from our data. Our results show that the presence of Juncus, with its superior ability to withstand waterlogging and salt stress, can create a hospitable environment for Iva, extending its distribution to lower intertidal habitats in which it does not have the morphological or physiological plasticity to normally cope.

Arbuscular mycorrhizas and water relations in maize under drought stress at tasselling

Abstract: SUMMARY A greenhouse experiment was conducted to investigate the influence of an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) on drought tolerance in tropical maize. Freshly regenerated seeds of selection cycles 0 (cv. CO, drought-sensitive) and 8 (cv. C8, drought-resistant) of the lowland tropical population 'TuxpefiQ sequia' were used for this study. Maize plants were subjected to drought stress for three weeks following tasselling (75-95 days after sowing). During the drought stress period, midday leaf water potential (LWP), stomatal resistance (SR) and transpiration rate (TR) were monitored daily, and green leaf area (GLA) determined at alternate days to assess the effects of mycorrhizal inoculation. Mycorrhizal plants of the cultivars CO and C8 had higher (less negative) LWP and TR, and lower SR throughout the experiment. The GLA was 27-5 % higher in mycorrhizal than non-mycorrhizal CO plants under drought conditions. Results indicate that mycorrhizas significantly improve tolerance to moderate drought stress imposed at tasselling, especially for the sensitive CO maize cultivar.

Temporal and spatial variations in the oxygen‐18 content of leaf water in different plant species

Abstract: Temporal variations in the δ18 oxygen (δ18O) content of water transpired by leaves during a simulated diurnal cycle fluctuated around the δ18O content of the source water. Reconstructed variations in the δ18O values of leaf water differed markedly from those predicted by conventional models. Even when transpiring leaves were maintained under constant conditions for at least 3 h, strict isotopic steady-state conditions of leaf water (equality of the 18O/16O ratios in the input and transpired water) were rarely attained in a variety of plant species (Citrus reticu-lata, Citrus paradisi, Gossypium hirsutum, Helianthus annuns, Musa musaceae and Nicotinia tabacum). Isotopic analysis of water transpired by leaves indicated that leaves approach the isotopic steady state in two stages. The first stage takes 10 to 35 min (with a rate of change of about 3–3%h−1), while in the second stage further approach to the isotopic steady state is asymptotic (with a rate of change of about 0–4% h−1), and under conditions of low transpiration leaves can last for many hours. Substantial spatial isotopic heterogeneity was maintained even when leaves were at or near isotopic steady state. An underlying pattern in this isotopic heterogeneity is often discerned with increasing 18O/16O ratios from base to tip, and from the centre to the edges of the leaves. It is also shown that tissue water along these spatial isotopic gradients, as well as the average leaf water, can have 18O/16O ratios both lower and higher than those predicted by the conventional Craig and Gordon model. We concluded, first, that at any given time during the diurnal cycle of relative humidity the attainment of an isotopic steady state in leaf water cannot be assumed a priori and, secondly, that the isotopic enrichment pattern of leaf water reflects gradual enrichment along the water-flow pathway (e.g. as in a string of pools), rather than a single-step enrichment from source water, as is normally assumed.

How Do Stomata Read Abscisic Acid Signals

Abstract: When abscisic acid (ABA) was fed to isolated epidermis of Commelina communis L., stomata showed marked sensitivity to concentrations of ABA lower than those commonly found in the xylem sap of well-watered plants. Stomata were also sensitive to the flux of hormone molecules across the epidermal strip. Stomata in intact leaves of Phaseolus acutifolius were much less sensitive to ABA delivered through the petiole than were stomata in isolated epidermis, suggesting that mesophyll tissue and/or xylem must substantially reduce the dose or activity of ABA received by guard cells. Delivery of the hormone to the leaf was varied by changing transpiration flux and/or concentration. Varying delivery by up to 7-fold by changing transpiration rate had little effect on conductance. At a given delivery rate, variation in concentration by 1 order of magnitude significantly affected conductance at all but the highest concentration fed. The results are discussed in terms of the control of stomatal behavior in the field, where the delivery of ABA to the leaf will vary greatly as a function of both the concentration of hormone in the xylem and the transpiration rate of the plant.

Variation in carbon isotope discrimination and photosynthetic gas exchange among populations of Pseudotsuga menziesii and Pinus ponderosa in different environments

Abstract: Seedlings representing 25 populations of Pseudotsuga menziesii and 26 populations of Pinus ponderosa were grown in a common garden in Moscow, ID, USA. The seeds were collected across the natural distribution of each species, at altitudes ranging from 170 to 2774 m above sea level, latitudes from 33 °N to 53 °N, and longitudes from 105 °W to 124 °W. Lipid-free seeds from mother trees and leaf tissue from the 2-year-old progeny were analysed. The design enabled us not only to measure genetically determined variation in carbon isotope discrimination (Δ) and gas exchange characteristics but also to compare performance in the common garden and in situ. In the common garden, significant population variation in Δ, gas exchange and specific leaf area was detected among seedlings of Pseudotsuga menziesii. Coastal, low-altitude genotypes had significantly lower Δ than interior, high-altitude genotypes. In Pinus ponderosa, populations varied only in specific leaf area. These broadly distributed sympatric species differ in genetic structure with respect to gas-exchange characteristics. In the common garden, high Δ of both species was associated with high stomatal conductance relative to photosynthetic rate. Specific leaf area, although strongly correlated with Δ, varied in the wrong direction to explain variation in Δ. In situ Δ was correlated with both altitude and vapour pressure deficit (VPD) in Pseudotsuga menziesii (r 2 =0.42, P=0002 and r 2 =025, P 0.77 and r 2 =0.06, P>0.21 for altitude and VPD, respectively). Foliage Δ from the progeny grown in the common garden was significantly correlated, in both species, with Δ of the in situ maternal photosynthate in the seeds; however, the correlation was negative in Pseudotsuga menziesii and positive in Pinus ponderosa. The negative correlation indicates a strong acclimatory response, perhaps to VPD, in Pseudotsuga menziesii. Altitudinal decreases in intercellular CO 2 partial pressures inferred from isotopic data were insufficient to compensate for increased VPD. Photosynthetic water-use efficiency (net photosynthesis/transpiration) was estimated to decrease by two- to fourfold from sea level to 2800m altitude within the distribution limits of these species.

Role of transpiration suppression by evaporation of intercepted water in improving irrigation efficiency

Abstract: Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (Igross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from Igross irrigation application losses for a reduced, or net, interception (Inet) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopyintercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 mm, estimated average Igross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced Igross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced Igross loss an additional 1.2%. Further 4–6% reductions in Igross losses were predicted when aerodynamic and canopy resistances were considered. Irrigation water applied only at the soil surface by LEPA irrigation had little effect on the microclimate within the canopy and consequently on Tr or ET, or irrigation application efficiency.