Showing papers on "Transpiration published in 2000"

Hormonal changes induced by partial rootzone drying of irrigated grapevine

Abstract: Partial rootzone drying (PRD) is a new irrigation technique which improves the water use efficiency (by up to 50%) of wine grape production without significant crop reduction. The technique was developed on the basis of knowledge of the mechanisms controlling transpiration and requires that approximately half of the root system is always maintained in a dry or drying state while the remainder of the root system is irrigated. The wetted and dried sides of the root system are alternated on a 10-14 d cycle. Abscisic acid (ABA) concentration in the drying roots increases 10-fold, but ABA concentration in leaves of grapevines under PRD only increased by 60% compared with a fully irrigated control. Stomatal conductance of vines under PRD irrigation was significantly reduced when compared with vines receiving water to the entire root system. Grapevines from which water was withheld from the entire root system, on the other hand, show a similar reduction in stomatal conductance, but leaf ABA increased 5-fold compared with the fully irrigated control. PRD results in increased xylem sap ABA concentration and increased xylem sap pH, both of which are likely to result in a reduction in stomatal conductance. In addition, there was a reduction in zeatin and zeatin-riboside concentrations in roots, shoot tips and buds of 60, 50 and 70%, respectively, and this may contribute to the reduction in shoot growth and intensified apical dominance of vines under PRD irrigation. There is a nocturnal net flux of water from wetter roots to the roots in dry soil and this may assist in the distribution of chemical signals necessary to sustain the PRD effect. It was concluded that a major effect of PRD is the production of chemical signals in drying roots that are transported to the leaves where they bring about a reduction in stomatal conductance.

Water uptake by roots: effects of water deficit

Abstract: The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anatomical structure of roots results in a composite transport of both water and solutes. In the model, the parallel apoplastic and cell-to-cell (symplastic and transcellular) pathways play an important role as well as the different tissues and structures arranged in series within the root cylinder (epidermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Casparian bands and suberin lamellae in the root's endo- and exodermis are discussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in certain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exodermis. In the absence of transpiration, water flow is osmotic in nature which causes a high resistance as water passes across many membranes on its passage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non-linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients are in line with the idea of some apoplastic bypasses for water within the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by water channels dominates. Since water channels are under metabolic control, this component represents an 'active' element of regulation. Composite transport allows for an optimization of the water balance of the shoot in addition to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses of plants to water deficit and other stresses. During water deficit, the cohesion-tension mechanism of the ascent of sap in the xylem plays an important role. Results are summarized which prove the validity of the coehesion/tension theory. Effects of the stress hormone abscisic acid (ABA) are presented. They show that there is an apoplastic component of the flow of ABA in the root which contributes to the ABA signal in the xylem. On the other hand, (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the root level which is in agreement with the composite transport model. Hydraulic water flow in the presence of gradients in hydrostatic pressure remains unchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp(r)) level. They are in line with known effects of nutrient deprivation on root Lp(r )and the diurnal rhythm of root Lp(r )recently found in roots of LOTUS.

Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport

Abstract: Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Ψ) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of Ψ gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low Ψ. In contrast, in leaves under reductions in Ψ of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low Ψ in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Ψ is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Ψ of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of Ψ in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment.

Belowground consequences of vegetation change and their treatment in models

Abstract: The extent and consequences of global land-cover and land-use change are increasingly apparent. One consequence not so apparent is the altered structure of plants belowground. This paper examines such belowground changes, emphasizing the interaction of altered root distributions with other factors and their treatment in models. Shifts of woody and herbaceous vegetation with deforestation, afforestation, and woody plant en- croachment typically alter the depth and distribution of plant roots, influencing soil nutrients, the water balance, and net primary productivity (NPP). For example, our analysis of global soil data sets shows that the major plant nutrients C, N, P, and K are more shallowly distributed than are Ca, Mg, and Na, but patterns for each element vary with the dominant vegetation type. After controlling for climate, soil C and N are distributed more deeply in arid shrublands than in arid grasslands, and subhumid forests have shallower nutrient dis- tributions than do subhumid grasslands. Consequently, changes in vegetation may influence the distribution of soil carbon and nutrients over time (perhaps decades to centuries). Shifts in the water balance are typically much more rapid. Catchment studies indicate that the water yield decreases 25-40 mm for each 10% increase in tree cover, and increases in transpiration of water taken up by deep roots may account for as much as 50% of observed responses. Because models are increasingly important for predicting the consequences of vegetation change, we discuss the treatment of belowground processes and how different treatments affect model outputs. Whether models are parameterized by biome or plant life form (or neither), use single or multiple soil layers, or include N and water limitation will all affect predicted outcomes. Acknowledging and understanding such differences should help constrain predictions of vegetation change.

The HIC signalling pathway links CO2 perception to stomatal development

Abstract: Stomatal pores on the leaf surface control both the uptake of CO2 for photosynthesis and the loss of water during transpiration. Since the industrial revolution, decreases in stomatal numbers in parallel with increases in atmospheric CO2 concentration have provided evidence of plant responses to changes in CO2 levels caused by human activity1,2. This inverse correlation between stomatal density and CO2 concentration also holds for fossil material from the past 400 million years3 and has provided clues to the causes of global extinction events4. Here we report the identification of the Arabidopsis gene HIC (for high carbon dioxide), which encodes a negative regulator of stomatal development that responds to CO2 concentration. This gene encodes a putative 3-keto acyl coenzyme A synthase—an enzyme involved in the synthesis of very-long-chain fatty acids5. Mutant hic plants exhibit up to a 42% increase in stomatal density in response to a doubling of CO2. Our results identify a gene involved in the signal transduction pathway responsible for controlling stomatal numbers at elevated CO2.

Relative humidity‐ and ABA‐induced variation in carbon and oxygen isotope ratios of cotton leaves

Abstract: Cotton (Gossypium hirsutum L. cv. CS50) plants were grown at two levels of relative humidity (RH) and sprayed daily with abscisic acid (ABA) at four concentrations. Plants grown at lower humidity had higher transpiration rates, lower leaf temperatures and lower stomatal conductance. Plant biomass was also reduced at low humidity. Within each humidity environment, increasing ABA concentration generally reduced stomatal conductance, evaporation rates, superficial leaf density and plant biomass, and increased leaf temperature and specific leaf area. As expected, decreased stomatal conductance resulted in decreased carbon isotope discrimination in leaf material (Δ13Cl). Plants grown at low humidity were more enriched in 18O than those grown at high RH, as theory predicts. Within each humidity environment, increasing ABA concentration increased oxygen isotope enrichment of leaf cellulose (Δ18Oc) and whole-leaf tissue (Δ18Ol). Values of Δ13Cl and Δ18Ol predicted by theoretical models were close to those observed, accounting for 79% of the measured variation in Δ13Cl and 95% of the measured variation in Δ18Ol. Supporting theory, Δ13Cl and Δ18Ol in whole-leaf tissue were negatively related.

Influence of soil porosity on water use in Pinus taeda

Abstract: We analyzed the hydraulic constraints im- posed on water uptake from soils of different porosities in loblolly pine (Pinus taeda L.) by comparing genetical- ly related and even-aged plantations growing in loam versus sand soil. Water use was evaluated relative to the maximum transpiration rate (E crit ) allowed by the soil- leaf continuum. We expected that trees on both soils would approach Ecrit during drought. Trees in sand, how- ever, should face greater drought limitation because of steeply declining hydraulic conductivity in sand at high soil water potential ( Ψ S ). Transport considerations sug- gest that trees in sand should have higher root to leaf ar- ea ratios (AR:AL), less negative leaf xylem pressure ( Ψ L), and be more vulnerable to xylem cavitation than trees in loam. The A R :A L was greater in sand versus loam (9.8 vs 1.7, respectively). This adjustment maintained about 86% of the water extraction potential for both soils. Trees in sand were more deeply rooted (>1.9 m) than in loam (95% of roots <0.2 m), allowing them to shift water uptake to deeper layers during drought and avoid hydraulic failure. Midday Ψ L was constant for days of high evaporative demand, but was less negative in sand (-1.6 MPa) versus loam (-2.1 MPa). Xylem was more vulnerable to cavitation in sand versus loam trees. Roots in both soils were more vulnerable than stems, and expe- rienced the greatest predicted loss of conductivity during drought. Trees on both soils approached Ecrit during drought, but at much higher Ψ S in sand (<-0.4 MPa) than in loam (<-1.0 MPa). Results suggest considerable phenotypic plasticity in water use traits for P. taeda which are adaptive to differences in soil porosity.

Limitation of stomatal conductance by hydraulic traits: sensing or preventing xylem cavitation?

Abstract: We tested the hypothesis that hydraulic conductance per unit leaf surface area of plant shoots (KSL) determines the maximum diurnal stomatal conductance (gL) that can be reached by plants growing in the field. A second hypothesis was tested that some xylem cavitation cannot be avoided by transpiring plants and might act as a signal for regulating gL. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal gL, transpiration rate (EL), pre-dawn and minimum diurnal leaf water potential (Ψpd and Ψmin, respectively) were measured in the field. The critical Ψ level at which stem cavitation was triggered (Ψcav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum KSL of 1-year-old shoots. Both gL and EL were positively related to KSL. The whole-plant hydraulic conductance per unit leaf area (KWL) of all the species studied, calculated as the ratio of EL to ΔΨ (=Ψpd-Ψmin) was closely related to KSL. In every case, Ψmin (ranging between –0.85 and –1.35 MPa in the different species) dropped to the Ψcav range or was <Ψcav (ranging between –0.71 and –1.23 MPa), thus suggesting that some cavitation-induced embolism could not be avoided. The possibility is discussed that some cavitation-induced reduction in KSL is the signal for stomatal closure preventing runaway embolism. The lack of correlation of gL to Ψcav is discussed in terms of the inconsistency of Ψcav as an indicator of the vulnerability of plants to cavitation. No differences in hydraulic traits were observed between evergreen and deciduous species.

The effect of tree height on crown level stomatal conductance

Abstract: Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G Si ) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q o ). Under conditions of D = 1 kPa and saturating Q o , G Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood-to-leaf area ratio (A S :A L ) doubled. A simple hydraulic model explained the variation in G Si based on an inverse relationship with height, and a linear relationship with A S :A L . Thus, in F. sylvatica, adjustments in A S :A L partially compensate for the negative effect of increased flow-path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Abstract: Despite mounting evidence showing that C4 plants can accumulate more biomass at elevated CO2 partial pressure (p(CO2)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C4 plants to elevated p(CO2) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO2) can stimulate the growth of both well-watered and waterstressed C4 plants. First, through enhanced leaf CO2 assimilation rates due to increased intercellular p(CO2). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO2 fixation in the bundle sheath or the presence of C3-like photosynthesis in young C4 leaves are unlikely explanations for the high CO2-responsiveness of C4 photosynthesis. The interactions between elevated p(CO2), leaf temperature and shoot water relations on the growth and photosynthesis of C4 plants are identified as key areas needing urgent research.

Analyses of assumptions and errors in the calculation of stomatal conductance from sap flux measurements.

Abstract: We analyzed assumptions and measurement errors in estimating canopy transpiration (E(L)) from sap flux (J(S)) measured with Granier-type sensors, and in calculating canopy stomatal conductance (G(S)) from E(L) and vapor pressure deficit (D). The study was performed in 12-year-old Pinus taeda L. stands with a wide range in leaf area index (L) and growth rate. No systematic differences in J(S) were found between the north and south sides of trees. However, J(S) in xylem between 20 and 40 mm from the cambium was 50 and 39% of J(S) in the outer 20-mm band of xylem in slow- and fast-growing trees, respectively. Sap flux measured in stems did not lag J(S) measured in branches, and time and frequency domain analyses of time series indicated that variability in J(S) in stems and branches is mostly explained by variation in D. Therefore, J(S) was used to estimate transpiration, after accounting for radial patterns. There was no difference between D and leaf-to-air vapor pressure gradient, and D did not have a vertical profile in stands of either low or high L suggesting a strong canopy-atmosphere coupling. Therefore, D estimated at one point in the canopy can be used to calculate G(S) in such stands. Given the uncertainties in J(S), relative humidity, and temperature measurements, to keep errors in G(S) estimates to less than 10%, estimates of G(S) should be limited to conditions in which D >/= 0.6 kPa.

Age-related decline in stand productivity: the role of structural acclimation under hydraulic constraints

Abstract: The decline in above-ground net primary productivity (Pa) that is usually observed in forest stands has been variously attributed to respiration, nutrient or hydraulic limitations. A novel model is proposed to explain the phenomenon and the co-occurring changes in the balance between foliage, conducting sapwood and fine roots. The model is based on the hypothesis that a functional homeostasis in water transport is maintained irrespective of age: hydraulic resistances through the plant must be finely tuned to transpiration rates so as to avoid extremely negative water potentials that could result in diffuse xylem embolism and foliage dieback, in agreement with experimental evidence. As the plant grows taller, allocation is predicted to shift from foliage to transport tissues, most notably to fine roots. Higher respiration and fine root turnover would result in the observed decline in Pa. The predictions of the model have been compared with experimental data from a chronosequence of Pinus sylvestris stands. The observed reduction in Pa is conveniently explained by concurrent modifications in leaf area index and plant structure. Changes in allometry and shoot hydraulic conductance with age are successfully predicted by the principle of functional homeostasis.

Influence of nutrient versus water supply on hydraulic architecture and water balance in Pinus taeda

Abstract: We investigated the hydraulic consequences of a major decrease in root-to-leaf area ratio (AR:AL) caused by nutrient amendments to 15-year-old Pinus taeda L. stands on sandy soil. In theory, such a reduction in AR:AL should compromise the trees’ ability to extract water from drying sand. Under equally high soil moisture, canopy stomatal conductance (GS) of fertilized trees (F) was 50% that of irrigated/fertilized trees (IF), irrigated trees (I), and untreated control trees (C). As predicted from theory, F trees also decreased their stomatal sensitivity to vapour pressure deficit by 50%. The lower GS in F was associated with 50% reduction in leaf-specific hydraulic conductance (KL) compared with other treatments. The lower KL in F was in turn a result of a higher leaf area per sapwood area and a lower specific conductivity (conducting efficiency) of the plant and its root xylem. The root xylem of F trees was also 50% more resistant to cavitation than the other treatments. A transport model predicted that the lower AR:AL in IF trees resulted in a considerably restricted ability to extract water during drought. However, this deficiency was not exposed because irrigation minimized drought. In contrast, the lower AR:AL in F trees caused only a limited restriction in water extraction during drought owing to the more cavitation resistant root xylem in this treatment. In both fertilized treatments, approximate safety margins from predicted hydraulic failure were minimal suggesting increased vulnerability to drought-induced dieback compared with non-fertilized trees. However, IF trees are likely to be so affected even under a mild drought if irrigation is withheld.

Stem radius changes and their relation to stored water in stems of young Norway spruce trees

Abstract: Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (Ψ s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as Ψ s was >–2.3±0.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When Ψ s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which – in the experiment with stem segments – was indicated by an increasing ratio between Δ V and Δ W. We assume that for Ψ s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies Ψ s rarely falls below –2.3±0.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.

Changes in chlorophyll, ribulose bisphosphate carboxylase-oxygenase, glycine betaine content, photosynthesis and transpiration in Amaranthus tricolor leaves during salt stress

Abstract: SummaryWe examined changes in leaf growth and chemical composition, including chlorophyll content, ribulose bisphosphate carboxylase-oxygenase (RuBisCO), and glycine betaine (GB) in relation to photosynthesis and transpiration responses to salt stress in Amaranthus tricolor leaves. To induce salt stress, plants were transferred to a growth medium containing 300 mM NaCl for 7 d followed by 7 d of relief from salinity. A decrease in leaf enlargement began 3 d after salt stress, and leaves subsequently showed the same degree of regrowth as controls after relief in non-salt medium. Chlorophyll content expressed on a leaf-area basis increased under conditions of salinity due to a reduction in leaf tissue water content. The decrease in chlorophyll content continued throughout the 7 d of relief from salinity. The RuBisCO and soluble protein contents when expressed on a leaf dry-weight basis decreased in response to salinity, and then gradually increased during the relief period. GB content increased slightly up ...

Crop Responses to Environment

Abstract: INTRODUCTION GENERAL PRINCIPLES Complete Understanding Requires Information from Several Levels of Biological Organization Separating Causes and Effects Can Be Difficult Limiting Factors, Synergisms, and Source/Sink Effects Optimization and Efficiency Genetic and Environmental Influences on Plants EXPERIMENTAL APPROACHES AND QUANTITATIVE METHODS Value of Experimental Studies in Different Fields or Seasons Having Contrasting Environments Value of Experimental Studies in Controlled Environments Value of Experimental Studies with Different Environments Imposed in the Same Field Quantitative Methods CROP PHYSIOLOGICAL RESPONSES TO LIGHT, PHOTOSYNTHESIS, AND RESPIRATION Photosynthesis and Productivity Photosynthesis and Adaptation Mitochondrial Respiration Photorespiration Growth Analysis CROP PHYSIOLOGICAL RESPONSES TO TEMPERATURE AND CLIMATIC ZONES Seed Germination, Storage, and Dormancy Resumption of Active Growth by Perennials Vegetative Growth Reproductive Development Climactic Zones Comparison Method for Determining Where Crops Can Be Grown CROP DEVELOPMENTAL RESPONSES TO TEMPERATURE, PHOTOPERIOD, AND LIGHT QUALITY Heat-Unit Systems for Predicting Plant Development Chilling Requirements of Plants Plant Developmental Responses to Photoperiod Light Quality Effects on Plant Development RADIATION AND ENERGY BALANCES, AND PREDICTING CROP WATER USE AND TEMPERATURE Solar Radiation at the Surface of the Earth Types of Radiation in the Earth's Environment and Optical Qualities of Plants Radiation and Energy Balances Predicting Crop Water Use Predicting Temperature Differences between Crop Canopy and Air CROP TRANSPIRATION AND WATER RELATIONS Transpiration Stomatal Responses to Environment Optimal Stomatal Function Adaptive Significance of Plant Differences in the Level of Daily Water Use Adaptive Significance of Plant Differences in Transpiration Efficiency Liquid Water Transport form Soil to Leaves Components of Total Water Potential Flow of Water from Root to Shoot Crop Water Relations CROP ADAPTATION TO WATER-LIMITED ENVIRONMENTS Crop Species Differences in Drought Resistance Mechanisms of Drought Resistance HYDROLOGICAL BUDGET OF CROPPING SYSTEMS, IRRIGATION, AND CLIMATIC ZONES Irrigation Management Climactic Zone definition based on Water CROP RESPONSES TO SALINITY AND OTHER LIMITING SOIL CONDITIONS Plant Responses to Extremes of Soil Texture and High Soil Bulk Density Salinity Tolerance Tolerance to High Boron Tolerance to High Aluminum INTERACTION OF CROP RESPONSES TO PESTS AND ABIOTIC FACTORS Crop Phonology and the Escape or Aggravation of Pest Problems Crop Resistance to Pests CONSIDERATION OF CROP RESPONSES TO ENVIRONMENT IN PLANT BREEDING Defining Crop Ideotype Traits Testing the Value of Crop Ideotype Traits Perspectives for Future Use of Crop Ideotypes in Plant Breeding REFERENCES

Effects of long-term rainfall variability on evapotranspiration and soil water distribution in the Chihuahuan Desert: A modeling analysis

Abstract: We used the patch arid land simulator (PALS-FT) – a simple, mechanistic ecosystem model – to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: annuals, perennial forbs, C4 grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenological activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plant community types (grass- vs. shrub-dominated) were carried out for the Jornada Basin, New Mexico, using 100 years of daily precipitation data (1891–1990). In order to emphasize variability associated with rainfall and fundamental differences in FT composition between communities, the seasonal patterns cover of perennials were held constant from year to year. Because the relative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model predictions of ET by allowing their cover to be variable, fixed, or absent. Over the entire 100-yr period, total annual ET is highly correlated with total annual rainfall in both community types, although T and E alone are less strongly correlated with rainfall, and variation in transpiration is nearly 3 times greater than evaporation and 2 times greater than variation in rainfall (CV of rainfall = 35%). Water use shows a relatively high similarity between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year comparison between communities, T/ET was significantly greater in the grass-dominated community, reflecting the fact that over the long term more than half of the rain occurs in the summer and is used slightly more efficiently (T?E) by the C4-grass community than the shrub community, although we found some rainfall patterns that resulted in much greater T/ET in the shrub community in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulation, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub-dominated site). These predictions suggest that because of the relatively shallow distribution of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular `windows' of moisture availability via differences in phenologies. We found very little differences in average, long-term model predictions of T, E, and ET when annual plant cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studies about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variability that is possible.

Transpiration and whole-tree conductance in ponderosa pine trees of different heights

Abstract: Changes in leaf physiology with tree age and size could alter forest growth, water yield, and carbon fluxes. We measured tree water flux (Q) for 14 pondero- sa pine trees in two size classes (12 m tall and ~40 years old, and 36 m tall and ~ 290 years old) to determine if transpiration (E) and whole-tree conductance (g t ) dif- fered between the two sizes of trees. For both size class- es, E was approximately equal to Q measured 2 m above the ground: Q was most highly correlated with current, not lagged, water vapor pressure deficit, and night Q was <12% of total daily flux. E for days 165-195 and 240-260 averaged 0.97 mmol m -2 (leaf area, projected) s -1 for the 12-m trees and 0.57 mmol m -2 (leaf area) s -1 for the 36-m trees. When photosynthetically active radia- tion (I P ) exceeded the light saturation for photosynthesis in ponderosa pine (900 µmol m -2 (ground) s -1 ), differ- ences in E were more pronounced: 2.4 mmol m -2 (leaf area) s-1 for the 12-m trees and 1.2 mmol m-2 s-1 for the 36-m trees, yielding gt of 140 mmol m -2 (leaf area) s -1 for the 12-m trees and 72 mmol m -2 s -1 for the 36-m trees. Extrapolated to forests with leaf area index =1, the 36-m trees would transpire 117 mm between 1 June and 31 August compared to 170 mm for the 12-m trees, a dif- ference of 15% of average annual precipitation. Lower g t in the taller trees also likely lowers photosynthesis dur- ing the growing season.

Leaf water relations and stomatal behavior of four allopatric Eucalyptus species planted in Mediterranean southwestern Australia.

Abstract: In 1986, four allopatric Eucalyptus species (E. camaldulensis Dehnh, E. saligna Smith, E. leucoxylon F. Muell and E. platypus Hook.) were planted together in a 480-mm rainfall zone, in 8-m wide contour belts as part of a plan to minimize waterlogging and secondary salinization. Throughout 1997, 1998 and 1999, there was significant inter-specific variation in predawn leaf water potential (Psi(pd)); however, maximum stomatal conductance (g(sm)) only differed significantly between species in mid to late summer. Relationships between g(sm) and Psi(pd) were significant and showed that stomata of E. camaldulensis were significantly more sensitive to Psi(pd), and presumably soil water potential, than stomata of E. leucoxylon or E. platypus. When applied to the Psi(pd) data, these relationships predicted that g(sm), and by inference transpiration, varied much less between species than Psi(pd). Diurnal measurements throughout the season confirmed this prediction, and showed that E. camaldulensis and E. saligna avoided drought by gaining access to deeper water, whereas E. leucoxylon and E. platypus maintained greater g(sm) at a given water stress than E. camaldulensis or E. saligna. Osmotic potentials measured after rehydration and water release curves of the leaves indicated that different mechanisms accounted for the apparent drought tolerance of E. leucoxylon and E. platypus. In summer, E. leucoxylon reduced osmotic potential at full and zero turgor by similar amounts compared with winter. In summer, E. platypus had a significantly lower bulk elastic modulus and relative water content at turgor loss point than E. camaldulensis, E. saligna or E. leucoxylon. This elastic adjustment resulted in a larger difference between osmotic potential at full and zero turgor in summer than in winter. The inherently low osmotic potential in E. leucoxylon and elastic adjustment in E. platypus resulted in turgor loss at a similar and significantly lower water potential than in E. camaldulensis or E. saligna. These results have implications for species selection for planting to manage groundwater recharge in areas prone to waterlogging and secondary salinization.

Forest growth and species distribution in a changing climate.

Abstract: Climate change has many potential effects on plants, some detrimental to growth, others beneficial. Increasing CO(2) concentration can increase photosynthetic rates, with the greatest increases likely to be in C(3) plants growing in warm dry conditions. Increasing temperature directly affects plant growth through effects on photosynthetic and respiration rates. However, plants have a considerable ability to adapt to changing conditions and can tolerate extremely high temperatures, provided that adequate water is available. Increasing temperature may increase vapor pressure deficits of the air, and thereby increase transpiration rates from most plant canopies. Effects are likely to vary among plant communities, with forests generally experiencing greater increases in transpiration rates than grasslands. These increases in transpiration are likely to be reduced by stomatal closure in response to increasing CO(2) concentration. In many areas, precipitation will probably increase with global warming; however, these increases may be insufficient to meet the increased transpirational demand by plant canopies. Increasing temperature is likely to increase soil organic matter decomposition rates so that nutrients may be more readily mineralized and made available to plants. In highly fertile systems, this could lead to nutrient losses through leaching. For different combinations of increases in temperature and CO(2) concentration, and for systems primarily affected by water or nutrient limitations, different overall effects on plant productivity can be expected. Responses will be negative in some circumstances and positive in others, but on the whole, catastrophic changes to forest growth seem unlikely under most conditions. In contrast,ecological consequences of climate change are potentially more serious. The distribution of many species tends to be limited to a narrow range of environmental conditions. Climate conditions over much of a species' current natural range may therefore become unsuitable, leading to significant decline of forests or of particular species within forests.

Sap flux of co-occurring species in a western subalpine forest during seasonal soil drought

Abstract: Co-occurring species may utilize vastly different strategies to cope with limited water resources, particularly in areas subjected to predictable and recurring drought. While these physiological responses have commonly been measured at the leaf level, in small seedlings, and integrated in fluxes of whole stands or watersheds, sap flux measure- ments in large trees have become a useful tool for monitoring transpiration of individual canopies over long time periods. In this study, sap flux ( JS) was measured with constant heat sap flow gauges for co-occurring species which have been previously evaluated at the leaf level. Measurements were taken in a subalpine stand containing most of the dominant species of the central Rocky Mountains (Pinus contorta, Abies lasiocarpa, Populus tre- muloides, and Pinus flexilis). Daily JS values were parabolically related to daytime average atmospheric vapor pressure deficit (D) in all species, with a broad range of maximum JS values occurring between 1.2 and 1.8 kPa. Populus tremuloides had the greatest increases in JS with increasing D, while Pinus contorta showed the lowest JS. A decrease in maximum JS was observed for all species later in the season when soil moisture declined from 0.35 to 0.24 m 3 /m 3 at 0-45 cm. Late-season JS in A. lasiocarpa decreased 50% due to stomatal closure in response to the soil moisture deficit, regardless of daily D. In contrast, the Pinus species were sensitive to D, showing larger late-season reductions in JS on high than on low D days. Populus tremuloides showed less sensitivity to soil moisture than the other species, with relatively high JS continuing late into the season and intermediate change in the response of JS to D with decreasing soil moisture. Stand-level estimates of transpiration by plots dominated by Pinus contorta and A. lasiocarpa (2.6 6 0.6 mm/d) were found to be similar to plots dominated by Populus tremuloides(2.7 6 0.6 mm/d) despite the nearly fourfold higher leaf area indices for the conifers.