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

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 ponderosa 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) differed between the two sizes of trees. For both size classes, 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 radiation (I _P) exceeded the light saturation for photosynthesis in ponderosa pine (900 μmol m^–2 (ground) s^–1), differences 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 g _t 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 difference of 15% of average annual precipitation. Lower g _t in the taller trees also likely lowers photosynthesis during the growing season. Received: 19 April 1999 / Accepted: 23 March 2000     

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

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 (g _s) and boundary layer (g _b) 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 g _b (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 g _b. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g _s when g _s 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 g _s and the leaf-bulk air vapor pressure difference (VPD) without taking g _b 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 g _s initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (G _t), becoming asymptotic at higher values of G _t. For both E and g _s a unique relationship appeared to describe the response of all species to variations in G _t. The relatively weak correlation observed between g _s 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.     

The effect of short-term flooding on the sap flow, gas exchange and hydraulic conductivity of young apricot trees

The effect of short-term flooding was examined in 2-year-old apricot trees (Prunus armeniaca cv. Búlida). Six apricot trees of similar appearance were submitted to two treatments: three were irrigated daily, while the others were flooded for a period of 50 h by submerging the pots in plastic water tanks. The trees were removed from the water, drained and then placed in the same conditions as the control plants. A decrease in transpiration in the flooded trees with respect to the control plants was evident. The daily pattern of soil O₂ concentration and plant hydraulic resistance followed a similar trend during the flooding. However, this relationship was not maintained throughout the experiment, since the O₂ values increased rapidly when the waterlogging ceased, while plant hydraulic resistance only recovered at the end of the experiment when the original root system, damaged by flooded conditions, was replaced with new roots. In flooded trees, the midday leaf water potential decreased progressively from the beginning of flooding, but gradually recovered when the waterlogging ceased. Leaf conductance values of treated plants were slow to recover, reaching values of the control plants 8 days after the leaf water potential had recovered. The close relationship observed during most of the experiment between the leaf water parameters, leaf conductance and plant hydraulic conductance indicate that hydraulic messages are likely to play a dominant role in co-ordinating the observed responses of the shoot.     

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day⁻¹ in a 35 m-tall Anacardium excelsum tree to 46 kg day⁻¹ in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (g _s) and crown conductance (g _c), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (G _t). Vapor phase conductance in all five trees responded similarly to variation in G _t. Large diurnal variations in G _t were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration. Received: 1 October 1997 / Accepted: 6 March 1998     

Xylem hydraulic physiology: The functional backbone of terrestrial plant productivity

Land plants are completely dependent on a passive system of water transport for their survival. The great bulk of the xylem tissue is non-living and consequently has no short term capacity to acclimate or adjust to changes in hydraulic demand. Yet there exists an extraordinary degree of coordination between the hydraulic and photosynthetic systems of plants that defies developmental explanation. The connection between hydraulic capacity and photosynthetic assimilation arises as a product of the shared stomatal pathway for water and CO₂ exchange in the leaf. A combination of optimization in both water use and structural xylem investment has led to a situation in vascular plants where the form and function of all individuals is moulded by the link between hydraulic and photosynthetic systems. Unlike competing models of hormonal control of gas exchange, hydraulic limitation of productivity under optimal and drought conditions accounts for much of the observed variation in plant gas exchange in natural systems. The plant water transport system places a hard physical limit to plant productivity and survival. Identifying the developmental control of key xylem traits will yield the potential for achieving new performance capabilities in plants.     

Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Stomatal sensitivity to vapour pressure deficit of temperate and tropical evergreen rainforest trees of Australia

Although rainforests of eastern Australia grow in regions of high precipitation, there is a shift from a summer dry season in the temperate south to a winter dry season in the tropical north. Therefore, rainforest trees of eastern Australia provide an opportunity to investigate stomatal sensitivity of mesic trees to vapour pressure deficit (VPD) along a gradient in seasonality of precipitation. Eight rainforest canopy tree species were selected to cover the latitudinal range of rainforests in eastern Australia. Seedlings of these species were grown for a year in glasshouses under ambient conditions or at low VPD and water vapour exchange of leaves was measured during summer. Tropical species, which experience summer-dominant precipitation, showed higher stomatal sensitivities to VPD than temperate species, which experience winter-dominant precipitation. Growing plants under a low VPD increased stomatal sensitivity to increasing VPD in most species. The high stomatal sensitivity to VPD of the tropical species is consistent with the infrequent water stress experienced during their growing season and suggests a high susceptibility to water deficits. In contrast, temperate species may use other mechanisms to maintain photosynthesis under the relatively drier conditions of the temperate growing season.     

Hydraulic adjustment of maple saplings to canopy gap formation

The leaf-specific hydraulic conductivity (K _L) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A _L/A _S) and the specific hydraulic conductivity of xylem tissue (K _S). In environments with high atmospheric vapor pressure deficit (VPD), K _L may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K _L and lower A _L/A _S than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K _S increased with sapling size for both species. In A. rubrum, K _S did not differ between the two environments but lower A _L/A _S (P=0.05, ANCOVA) led to higher K _L for gap-grown saplings (P < 0.05, ANCOVA). In A. pensylvanicum, neither K _S, A _L/A _S, nor K_L differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with K _L (r ²=0.75, P < 0.05). A hypothetical large A. pensylvanicum sapling (2 m tall) had 2.4 times higher K _L and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r ²=0.69, P < 0.05). Calculations indicated that small A.pensylvanicum saplings (low K _L) could not transpire at the rate of large saplings (high K _L) without reaching theoretical thresholds for xylem embolism induction. The coordination between K _L and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The K _S of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing K _L. However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in K _L and K _S with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment. Received: 18 February 1997 / Accepted: 23 June 1997     

The bulk hydraulic conductivity of mangrove soil perforated with animal burrows

Flow of groundwater from mangrove swamp sediment to mangrove creeks is likely to be an important pathway in mangrove swamps, particularly for the removal of salt excluded at the mangrove root. The swamps are generally saturated with water, and are perforated with animal burrows, allowing significant groundwater flow to mangrove creeks to occur. The hydraulic conductivity of the sediment is thus an important physical parameter but is very difficult to measure in-situ. In this work, we describe a simple method for determining the hydraulic conductivity of mangrove sediment, including the effect of macropores such as crab burrows, which uses the existing animal burrows as piezometers. Experiments to measure the hydraulic conductivity of the sediment were carried out in a variety of mangrove forests. It was found that hydraulic conductivity varied from around 1 to 10 m per day, which is at least 10 times greater than would be expected if there were no burrows. In order to check the validity of the method, conventional piezometers were used to determine the free water table level in an area of mangroves fringing a creek. From these measurements, hydraulic conductivity was determined independently and found to be consistent with the new methodology.     

玉米生育期土壤—植物—大气连续体水流阻力与水势的分布

根据玉米生育期的田间试验资料分析了土壤-植物-大气连续体中水势和水流阻力的分布,结果表明土壤与植物叶片之间的水势差在玉米抽雄期前达0.8—1.0MPa,到抽雄期以后达1.0—1.5MPa,叶片与大气之间的水势差则在抽雄期前后分别达80—120MPa和60—80MPa;连续体内的水流阻力主要在叶片与大气之间。建立了连续体中玉米叶片水势的动态模拟公式,模拟叶水势具有较高的精度。最后,揭示了叶片蒸腾速率与叶-气系统水势差和水流阻力的关系,当叶片与大气之间的水势差达90—100MPa之后,蒸腾速率随叶-气间水势差增加而减小。     

Native root xylem embolism and stomatal closure in stands of Douglas-fir and ponderosa pine: mitigation by hydraulic redistribution

Hydraulic redistribution (HR), the passive movement of water via roots from moist to drier portions of the soil, occurs in many ecosystems, influencing both plant and ecosystem-water use. We examined the effects of HR on root hydraulic functioning during drought in young and old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine (Pinus ponderosa Dougl. Ex Laws) trees growing in four sites. During the 2002 growing season, in situ xylem embolism, water deficit and xylem vulnerability to embolism were measured on medium roots (2–4-mm diameter) collected at 20–30 cm depth. Soil water content and water potentials were monitored concurrently to determine the extent of HR. Additionally, the water potential and stomatal conductance (g_s) of upper canopy leaves were measured throughout the growing season. In the site with young Douglas-fir trees, root embolism increased from 20 to 55 percent loss of conductivity (PLC) as the dry season progressed. In young ponderosa pine, root embolism increased from 45 to 75 PLC. In contrast, roots of old-growth Douglas-fir and ponderosa pine trees never experienced more than 30 and 40 PLC, respectively. HR kept soil water potential at 20–30 cm depth above –0.5 MPa in the old-growth Douglas-fir site and –1.8 MPa in the old-growth ponderosa pine site, which significantly reduced loss of shallow root function. In the young ponderosa pine stand, where little HR occurred, the water potential in the upper soil layers fell to about –2.8 MPa, which severely impaired root functioning and limited recovery when the fall rains returned. In both species, daily maximum g_s decreased linearly with increasing root PLC, suggesting that root xylem embolism acted in concert with stomata to limit water loss, thereby maintaining minimum leaf water potential above critical values. HR appears to be an important mechanism for maintaining shallow root function during drought and preventing total stomatal closure.     

Determination of the hydraulic conductivity of Lamprothamnium by use of the pressure clamp

By use of the pressure-clamp technique, the hydraulic conductivity of the brackish-water alga Lamprothamnium was found to be 5·10^-6 cm s^-1 bar^-1. The dimensions of the internodes are so small that it is possible, for the first time, to measure a complete volume relaxation upon clamping the turgor pressure to a preset value by a feedback control of the pressure probe. As theoretically predicted, the values of the hydraulic conductivity obtained from the initial slope of the volume relaxation induced by the pressure clamp are in agreement (within experimental error) with those obtained from the half-time of the relaxation process. The cell volume also obtained from the analysis of the volume relaxation is the osmotically effective cell volume and is therefore slightly smaller than the value obtained by taking the dimensions of the cell including the cell wall.Abbreviations and symbols Lp hydraulic conductivity - P turgor pressure - S_v initial slope of volume relaxion - T_1/2 half-time of volume relaxation Dedicated to Professor Dr. H. Ziegler on the occasion of his 60th birthday     

Mechanics of interstitial-lymphatic fluid transport: theoretical foundation and experimental validation

Interstitial fluid movement is intrinsically linked to lymphatic drainage. However, their relationship is poorly understood, and associated pathologies are mostly untreatable. In this work we test the hypothesis that bulk tissue fluid movement can be evaluated in situ and described by a linear biphasic theory which integrates the regulatory function of the lymphatics with the mechanical stresses of the tissue. To accomplish this, we develop a novel experimental and theoretical model using the skin of the mouse tail. We then use the model to demonstrate how interstitial–lymphatic fluid movement depends on a balance between the elasticity, hydraulic conductivity, and lymphatic conductance as well as to demonstrate how chronic swelling (edema) alters the equipoise between tissue fluid balance parameters. Specifically, tissue fluid equilibrium is perturbed with a continuous interstitial infusion of saline into the tip of the tail. The resulting gradients in tissue stress are measured in terms of interstitial fluid pressure using a servo-null system. These measurements are then fit to the theory to provide in vivo estimates of the tissue hydraulic conductivity, elastic modulus, and overall resistance to lymphatic drainage. Additional experiments are performed on edematous tails to show that although chronic swelling causes an increase in the hydraulic conductivity, its greatly increased distensibility (due to matrix remodeling) dampens the driving forces for fluid movement and leads to fluid stagnation. This model is useful for examining potential treatments for edema and lymphatic disorders as well as substances which may alter tissue fluid balance and/or lymphatic drainage.     

樟树叶油地理变异的研究

对分布在福建省不同地区的樟树〔C innam omum camphora(L.)Presl〕叶油含量和主要化学成分进行分析,结果表明,J3(118°16′~119°29′)和W 1(28°18′~27°30′)交叉区域芳樟叶油含量(1.90%±0.30%)和芳樟醇含量(95.34%±1.10%)较高,可作为优良芳樟选育的重点区域;J2(117°03′~118°16′)和W 3(26°42′~25°54′)交叉区域桉樟叶油含量(1.36%±0.48%)和1,8-桉叶油素含量(10.44%±17.28%)较高,可以作为优良桉樟选育的重点区域。J2和W 2(27°30′~26°42′)交叉区域樟脑叶油含量(1.13%±0.65%)和樟脑含量(4.80%±13.97%)较高,可以作为优良脑樟选育的重点区域;J4(119°29′~120°43′)和W 6(24°18′~23°31′)交叉区域黄樟叶油含量(1.05%±0.34%)和黄樟油素含量(9.01%±16.73%)较高,可以作为优良黄樟选育的重点区域。樟树叶油含量和主成分类型及含量呈明显的地理分布。     

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

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 (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, 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 (ψ_l), this does not logically exclude a feedback response between Ψ_L 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 Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L 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 Ψ_L increased. Increased Ψ_l 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 Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

Sulphate deprivation depresses the transport of nitrogen to the xylem and the hydraulic conductivity of barley (Hordeum vulgare L.) roots

During the first 4 d after the removal of SO ₄ ^2- from cultures of young barley plants, the net uptake of ¹⁵N-nitrate and the transport of labelled N to the shoot both decline. This occurred during a period in which there was no measurable change in plant growth rate and where the incorporation of [³H]leucine into membrane and soluble proteins was unaffected. Reduced N translocation was associated with six- to eightfold increases in the level of asparagine and two- to fourfold increases in glutamine in root tissue; during the first 4 d of SO ₄ ^2- deprivation there were no corresponding increases in amides in leaf tissue. The provision of 1 mol · m^–3 methionine halted, and to some extent reversed the decline in NO ₃ ^- uptake and N translocation which occurred during continued SO ₄ ^2- deprivation. This treatment had relatively little effect in lowering amide levels in roots. Experiments with excised root systems indicated that SO ₄ ^2- deprivation progressively lowered the hydraulic conductivity, Lp, of roots; after 4 d the Lp of SO ₄ ^2- -deprived excised roots was only 20% of that of +S controls. In the expanding leaves of intact plants, SO ₄ ^2- deprivation for 5 d was found to lower stomatal conductance, transpiration and photosynthesis, in the order given, to 33%, 37% and 18% of control values. The accumulation of amides in roots is probably explained by a failure to export either the products of root nitrate assimilation or phloem-delivered amino-N. This may be correlated with the lowered hydraulic conductivity. Enhanced glutamine and-or asparagine levels probably repressed net uptake of NO ₃ ^- and ¹³NO ₃ ^- influx reported earlier (Clarkson et al. 1989, J. Exp. Bot. 40, 953–963). Attention is drawn to the similar hydraulic signals occurring in the early stages of several different types of mineral-nutrient stresses.Abbreviations Asn asparagine - Gln glutamine - Lp hydraulic conductivity J.L.K. is extremely grateful to the British Council for supporting his working visit to Long Ashton. We thank John Radin for helpful discussion and encouragement.     

Inter-species variation of photosynthetic and xylem hydraulic traits in the deciduous and evergreen Euphorbiaceae tree species from a seasonally tropical forest in south-western China

The objective of the present study was to examine the functional coordination among hydraulic traits, xylem characteristics and gas exchange rates across three deciduous Euphorbiaceae tree species (Hevea brasiliensis, Macaranga denticulata and Bischofia javanica) and three evergreen Euphorbiaceae tree species (Drypetes indica, Aleurites moluccana and Codiaeum variegatum) from a seasonally tropical forest in south-western China. The deciduous tree species were more vulnerable to water stress-induced embolism than the evergreen tree species. However, the deciduous tree species generally had higher maximal rates of sapwood and leaf-specific hydraulic conductivity (K _S and K _L), respectively. Compared with the evergreen tree species, the deciduous tree species, however, possessed a lower density of sapwood and a wider diameter of xylem vessels. Regardless of leaf phenology, the hydraulic vulnerability and conductivity were significantly correlated with sapwood density and mean vessel diameter. Furthermore, the hydraulic vulnerability was positively correlated with water transport efficiency. In addition, the deciduous tree species exhibited higher maximal photosynthetic rates (A _max) and stomatal conductance (g _max), but lower water use efficiency (WUE). Interestingly, the A _max, g _max and WUE were strongly correlated with K _S and K _L across the deciduous and evergreen tree species. These results suggest that xylem structure, rather than leaf phenology, accounts for the difference in hydraulic traits between the deciduous tree species and the evergreen tree species. Meanwhile, our results show that there is a significant trade-off between hydraulic efficiency and safety, and a strong functional correlation between the hydraulic capacity and gas exchange rates across the deciduous and evergreen tree species.     

Reliability, repeatability and accuracy of the falling head method for hydraulic conductivity measurements under laboratory conditions

The aim of this study was to verify under lab conditions the reliability, repeatability and accuracy of the falling head method (FHM) for hydraulic conductivity measurements. The FHM is a reliable procedure that has slight variations (less than 10%) in repeated measurements and turns out to be a reliable technique to record the hydraulic conductivities typically described for clogged and unclogged subsurface-flow constructed wetlands (from 4 to ca. 360 m/day). The accuracy of the method is acceptable considering difficulties in the measurement of hydraulic conductivity in highly conductive media. Accordingly, results show measurement deviations of 20% when compared with a laboratory constant head method for highly conductive media (higher than 250 m/day), and 80% for media with low hydraulic conductivity (lower than 50 m/day). The main conclusion of the present paper is that of the FHM is a reliable and repeatable technique for hydraulic conductivity measurements and it is accurate enough for on-site clogging assessment in full-scale constructed wetlands.     

Root hydraulic conductance of six forest trees: possible adaptive significance of seasonal changes

ABSTRACT

Root hydraulic conductance (K_R was measured in terms of the ratio of volume flow through roots (F) and to the pressure (P) driving the flow in six forest trees growing in habitats characterized by different water availabilities Le. Acer campestre L., Castanea sativa Miller, Fraxinus ornus L., Fraxinus oxycarpa Bieb., Ceratonia siliqua L. and Olea oleaster Hoffmg. et Link. Measurements were made in May, August and November 1996. K_R as normalized for unit leaf surface area (K_RL), was higher in species growing in humid environments (A. campestre and C. sativa) than in others (C. siliqua and O. oleaster) typical of aria zones. A. campestre and C. sativa showed declining K_RL values from spring to autumn while the latter had highest K_RL values in summer. This is in agreement with the typical drought avoidance strategy of C. siliqua which is based on large water losses balanced by equal water uptake from the soil. Plots of F and of A (leaf surface area) to K_RL as well as annual percent changes in F, A_L and K_RL suggest that changes in K_RL are mainly due to analogous changes in F, except for O. oleaster where opposite balanced changes in F and A_L contributed in maintaining the K_RL constant from August to November.