Measuring xylem hydraulic vulnerability for long-vessel species: an improved methodology with the flow centrifugation technique

The hydraulic properties of Pinus pinea, Pinus halepensis and Tetraclinis articulata were studied in a coastal dune area from Eastern Spain. The measured variables include vulnerability to xylem embolism (vulnerability curves), hydraulic conductivity and carbon isotopic discrimination in leaves. Leaf water potentials were also monitored in the three studied populations during an extremely dry period. Our results showed that roots had always wider vessels and higher hydraulic conductivity than branches. Roots were also more vulnerable to xylem embolism and operated closer to their hydraulic limit (i.e., with narrower safety margins). Although it was not quantified, extensive root mortality was observed in the two pines during the study period, in agreement with the high values of xylem embolism (> 75%) predicted from vulnerability curves and the water potentials measured in the field. T. articulata was much more resistant to embolism than P. pinea and P. halepensis. Since T. articulata experienced also lower water potentials, safety margins from hydraulic failure were only slightly wider in this species than in the pines. Combining species and tissues, high resistance to xylem embolism was associated with low hydraulic conductivity and with high wood density. Both relationships imply a cost of having a resistant xylem. The study outlined very different water-use strategies for T. articulata and the pines. Whereas T. articulata had a conservative strategy that relied on the low vulnerability of its conducting system to drought-induced xylem embolism, the two pines showed regulatory mechanisms at different levels (i.e., embolism, root demography) that constrained the absorption of water when it became scarce.

The reliability of a double-ended pressure sleeve technique was evaluated on three woody angiosperm species with contrasting maximum vessel lengths. Vulnerability curves (VCs) were constructed by varying sample length and the size of the pressure sleeves. VCs were compared against curves obtained with reference techniques. For the two diffuse-porous species, Betula pendula and Prunus persica, VCs built with shoot segments shorter than maximum vessel length strongly overestimated species vulnerability. Furthermore, increasing the size of the pressure sleeve also tended to lead to overestimated VCs. For the ring-porous species Quercus robur, the technique strongly overestimated vulnerability to embolism, whatever the sample length or chamber tested. In conclusion, the double-ended pressure sleeve technique only gives reliable VCs on diffuse-porous angiosperms with short pressure sleeves, only when segments are longer than maximum vessel length.

Groundwater flow resulting from a proposed hydraulic fracturing (fracking) operation was numerically modeled using 91 scenarios. Scenarios were chosen to be a combination of hydrogeological factors that a priori would control the long‐term migration of fracking fluids to the shallow subsurface. These factors were induced fracture extent, cross‐basin groundwater flow, deep low hydraulic conductivity strata, deep high hydraulic conductivity strata, fault hydraulic conductivity, and overpressure. The study considered the Bowland Basin, northwest England, with fracking of the Bowland Shale at ∼2,000 m depth and the shallow aquifer being the Sherwood Sandstone at ∼300–500 m depth. Of the 91 scenarios, 73 scenarios resulted in tracked particles not reaching the shallow aquifer within 10,000 years and 18 resulted in travel times less than 10,000 years. Four factors proved to have a statistically significant impact on reducing travel time to the aquifer: increased induced fracture extent, absence of deep high hydraulic conductivity strata, relatively low fault hydraulic conductivity, and magnitude of overpressure. Modeling suggests that high hydraulic conductivity formations can be more effective barriers to vertical flow than low hydraulic conductivity formations. Furthermore, low hydraulic conductivity faults can result in subsurface pressure compartmentalization, reducing horizontal groundwater flow, and encouraging vertical fluid migration. The modeled worst‐case scenario, using unlikely geology and induced fracture lengths, maximum values for strata hydraulic conductivity and with conservative tracer behavior had a particle travel time of 130 years to the base of the shallow aquifer. This study has identified hydrogeological factors which lead to aquifer vulnerability from shale exploitation.

Angiosperm tree species in temperate regions are broadly divided into diffuse-porous and ring-porous species based on their xylem anatomy.Diffuse-porous species show very little distinction between the diameter of vessel elements in early versus late wood,while ring-porous species have a bimodal distribution of vessel diameters associated with large,early season vessels and small late season vessels.These anatomical differences result in differences between these two kinds of tree species in stem water transport capacity and in the vulnerability to drought-induced cavitation.However,it is not clear if diffuse-porous and ring-porous species show differences in leaf hydraulic traits.Water transport resistance in leaves accounts for 30%—80% of the total hydraulic resistance of the whole-plant water transport pathway,and relatively few studies have focused on leaf hydraulics owing to methodological barriers;hence,elucidating the differences between diffuse-porous versus ring-porous species in leaf hydraulics and in leaf hydraulic trait coordination with stem hydraulic traits can be helpful in demonstrating the differences between diffuse-porous and ring-porous tree species in plant water use,geological distribution,leaf phenology and ecological adaptation. We compared hydraulic traits of branches and leaves in three diffuse-porous deciduous tree species(Populus tomentosa,Platanus hispanica,Prunus serrulata) and three ring-porous deciduous tree species(Robinia pseudoacacia,Albizia julibrissin,Fraxinus chinensis) growing in northwestern China.Branch and leaf water transport capacity was evaluated by their maximum hydraulic conductivities(Kmax),and the hydraulic vulnerability was evaluated with P50,which corresponds to the branch or leaf water potential at 50% loss of maximum hydraulic conductivities.For stems,P50 was inferred from the vulnerability curves generated by air injection or bench dehydration method.For leaves,the curves were constructed by measuring hydraulic conductance(Kleaf) in leaves rehydrated from a range of water potentials(ψleaf).Kleaf was measured by assessing kinetics of ψleaf relaxation upon leaf rehydration. The results showed that branch cross-sectional area-based maximum specific conductivities(Ks-max) of the ring-porous species were greater than those of the diffuse-porous species.Ring-porous species were more vulnerable to cavitation(P50 branch) than diffuse-porous species and a tradeoff relationship was evident between Ks-max and P50branch in branches.No differences were found between leaf water transport capacities(Kl-max) or the vulnerability to hydraulic dysfunction(P50leaf) in the two species types,and there was no tradeoff relationship between Kl-max and P50leaf.In the three diffuse-porous species,leaves were more vulnerable than branches to water stress-induced dysfunction,but in the ring-porous species,branches were more vulnerable than leaves.Pearson correlation analysis indicated that there was no correlation between branch and leaf hydraulic traits(Kmax and P50) in the six investigated woody species.These results suggest that in our study:(1) diffuse-porous and ring-porous species diverged mainly in branch but not in leaf hydraulics,so that leaf hydraulics alone cannot be used to explain the differences between these two tree types in ecological function and adaptation.(2) Branch and leaf hydraulic traits were relatively independent and may be correlated with branch and leaf structure,respectively.

Relationships between xylem anatomical traits and cavitation resistance have always been a major content of plant hydraulics. To know how plants cope with drought, it is extremely important to acquire detailed knowledge about xylem anatomical traits and assess the cavitation resistance accurately. This study aims to increase our knowledge in the methods determining cavitation resistance and xylem anatomical traits. We selected a semi-ring-porous species, Hippophae rhamnoides L., and a diffuse-porous species, Corylus heterophylla F., to clarify the reasons for the difference in cavitation resistance based on detailed xylem anatomical traits and reliable vulnerability curves (VCs). Both Cavitron and bench dehydration (BD) were used to construct VCs. Xylem anatomical traits, including pit membrane ultrastructure of these two species, were determined. The VCs obtained by the two different techniques were of different types for H. rhamnoides, its Cavitron VCs might be unreliable because of open-vessel artifacts. On the basis of BD VCs, H. rhamnoides showed higher cavitation resistance than C. heterophylla, and this is attributed to its low vessel connectivity as well as non-porous and thicker pit membranes.

Centrifuges provide a fast approach to quantify the embolism resistance of xylem in vulnerability curves (VCs). Since embolism formation is assumingly driven by pressure only, spintime is not standardized for flowcentrifuge experiments. Here, we explore to what extent embolism resistance could be spin-time dependent and hypothesize that changes in hydraulic conductivity (Kh) would shift VCs towards higher water potential (Ψ) values over time. We quantified time-based shifts in flow-centrifuge VCs and their parameter estimations for six angiosperm species by measuring Kh over 15min of spinning at a particular speed before a higher speed was applied to the same sample. We compared various VCs per sample based on cumulative spintime and modelled the relationship between Kh, Ψ and spin-time. Time-based changes of Kh showed considerable increases and decreases at low and high centrifuge speeds, respectively, which generally shifted VCs towards more positive Ψ values. Values corresponding to 50% loss of hydraulic conductivity (P50) became less negative by up to 0.72MPa in Acer pseudoplatanus L., and on average by 8.5% for all six species compared with VCs that did not consider spin-time. By employing an asymptotic exponential model, we estimated time-stable Kh, which improved the statistical significance of VCs in five of the six species studied. This model also revealed the instability of VCs at short spin times with embolism formation in flowcentrifuges following a saturating exponential growth curve. Although pressure remains the major determinant of embolism formation, spin-time should be considered in flow-centrifuge VCs because not considering the time-dependent stability of Kh overestimates embolism resistance. This spin-time artefact is species-specific and likely based on relatively slow gas diffusion that is associated with embolism propagation. The accuracy of VCs is improved by determining time-stable Kh values for each centrifuge speed without considerably extending the experimental time to construct VCs.

In this paper, we presented results of interpretation of Self-Potential (SP) signals produced by pumping test experiment of heterogeneous aquifer. Heterogeneity is represented by zone with low and high hydraulic conductivity in two types of configuration: planned boundary and strip-layer. We studied five models for each type of heterogeneity with two variants of hydraulic conductivity. We carried out three-dimensional modelling with the use of Modflow (for hydraulic heads and electrical potentials) and ElSources (for current source term). We obtained SP distributions Self-Potential produced by pumping test which indicate heterogeneity. We calculated the values of the anomalous field for a detailed study of the heterogeneties. We found a correlation between SP and drawdown for the case of heterogeneity with low hydraulic conductivity. We defined the both boundaries of the strip clearly for zone with low hydraulic conductivity. The outer boundary of the strip-layer was not fixed due to the high hydraulic conductivity of the heterogeneity and low hydraulic gradient at the boundary. We estimated dependence of horizontal thickness of heterogeneity on SP distribution. The results can serve as a base for defining the heterogeneties in the aquifer.

Vessel-length distribution in stems of some American woody plants

Drought vulnerability of trees and other woody plants is much debated in the context of climate change, which creates a high interest in understanding plant water relations.The role and functioning of internal water storage is crucial, but still insufficiently understood.Drought vulnerability is typically assessed by considering loss in conductivity in function of decreasing xylem water potential, in a so-called 'vulnerability curve'.The xylem water potential at which a certain percentage of conductivity is lost (usually 50%) gives an indication of the vulnerability to cavitation.In a 'desorption curve', we can examine the release of water from internal storage tissues with decreasing water potential.Both curves are very valuable, but rely on a sequence of manual measurements (xylem water potential, hydraulic conductivity and water content) and are time-consuming.Therefore, we propose a new type of vulnerability curve that is based on continuous measurements of diameter shrinkage and ultrasonic acoustic emissions (UAE).We monitored weight loss, xylem diameter shrinkage and UAE and measured xylem water potential during the dehydration of excised branches of Vitis vinifera L. 'Johanniter'.The vulnerability curves could be interpreted in terms of water loss in elastic and inelastic tissues.The proposed method can be a tool to assess hydraulic capacitance and conductivity of the xylem.

We studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Ψ(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina, Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the "vulnerability curve" of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Ψ(x) and the Ψ(crit) representing the pressure at complete loss of transport. We also measured minimum in situ Ψ(x)(Ψ(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Ψ(xmin) than riparian species. Values of Ψ(crit) were correlated with minimum Ψ(x). Safety margins (Ψ(xmin)-Ψ(crit)) tended to increase with decreasing Ψ(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Ψ(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in part from a weak but significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats.

Here, we summarize studies on the effects of elevated [CO2 ] (CO2e ) on the structure and function of plant hydraulic architecture and explore the implications of those changes using a model. Changes in conduit diameter and hydraulic conductance due to CO2e vary among species. Ring-porous species tend towards an increase in conduit size and consequently conductivity. The effect in diffuse-porous species is much more limited. In conifers, the results are mixed, some species show minor changes in xylem structure, while other studies found increases in tracheid density and diameter. Non-woody plants generally exhibited the reverse pattern with narrower conduits and lower hydraulic conductivity under CO2e . Further, changes in drought-resistance traits suggest that non-woody plants were the most affected by CO2e , which may permit them to better resist drought-induced embolism under future conditions. Due to their complexity, acclimation in hydraulic traits in response to CO2e is difficult to interpret when relying solely on measurements. When we examined how the observed tissues-specific trends might alter plant function, our modelling results suggest that these hydraulic changes would lead to reduced conductance and more frequent drought stress in trees that develop under CO2e with a more pronounced effect in isohydric than in anisohydric species.

Earthen barriers or clay liners are a major concern in geo-environmental engineering. They are designed to preclude or reduce leachate migration. Hence, a low hydraulic conductivity (k) is an important parameter in the design of clay liners. Materials such as bentonite and lateritic clays, which have a low hydraulic conductivity at high dry densities, are used in the construction of clay liners. Compacted expansive clays which are high in montmorillonite content also have a very low hydraulic conductivity. When expansive clays are blended with fly ash, an industrial waste, the hydraulic conductivity further reduces as the ash-clay blends result in increased dry densities at increased fly ash contents. Hence, fly ash-stabilised expansive clay can also be proposed as an innovative clay liner material. It is, therefore, required to study various physical and engineering properties of this new clay liner material. Liquid limit (LL) and free swell index (FSI) are important index properties to be studied in the case of this clay liner material. The hydraulic conductivity of this new clay liner material depends on the fly ash content in the blend. Further, parameters such as solute concentration and kinematic viscosity also influence hydraulic conductivity of clay liners. This paper presents experimental results obtained on hydraulic conductivity (k) of fly ash-stabilised expansive clay liner at varying fly ash content and solute concentration. The tests were performed with deionised water (DIW), CaCl2, NaCl and KCl as permeating fluids. Fly ash content in the blend was varied as 0, 10, 20 and 30 % by weight of the expansive clay, and the solute concentration was varied as 5 mM (milli molar), 10, 20, 50, 100 and 500. It was found that hydraulic conductivity (k) decreased with increasing fly ash content, solute concentration and kinematic viscosity. Further, hydraulic conductivity (k) was correlated with LL and FSI of the clay liner material for different fly ash contents and solute concentrations. Useful correlations were obtained.

Contrast in vulnerability to freezing-induced xylem embolism contributes to divergence in spring phenology between diffuse- and ring-porous temperate trees

Hydraulic conductivity, swelling, and liquid sorption capacity (i.e., maximum organic liquid mass bound per mass organoclay solid) were measured for an organoclay with dimethylammonium bound to the surface. Five fuels (No. 1 fuel oil, No. 2 fuel oil, diesel, jet fuel, and gasoline), four pure organic liquids (methanol, phenol, ethylbenzene, and dioctyl phthalate), ranging from hydrophilic to hydrophobic, and Type II deionized (DI) water were used as liquids for solvation and permeation. The more hydrophilic liquids (methanol and phenol) and DI water resulted in low swelling (≤6 mL/2 g) or liquid sorption capacity (≤202%) and high hydraulic conductivity (>10−6 m/s). The term hydraulic herein refers to liquid and applies to all permeant liquids used. The less-refined fuels composed of heavier distillates (fuel oil and diesel) and the phthalate resulted in low swelling (10–12 mL/2 g) and liquid sorption capacity (<235%) and intermediate to low hydraulic conductivity (10−10 to 10−11 m/s). The highly refined fuels composed of lighter distillates and ethylbenzene resulted in higher swelling (>20 mL/2 g), high liquid sorption capacity (<340%), and very low hydraulic conductivity (typically, <10−11 m/s). The swelling, liquid sorption capacity, and hydraulic conductivity of this organoclay are related systematically; however, none of these properties correlates systematically with the common parameters describing hydrophobicity, namely, solubility or the octanol-water partition coefficient. When the swell index is at least 10 mL/2 g, this organoclay has hydraulic conductivity of less than 10−10 m/s. Below 10 mL/2 g, the hydraulic conductivity increases rapidly as the swell index decreases. Sand-organoclay mixtures with uniform sand require more organoclay to achieve low hydraulic conductivity and are more sensitive to the swell index. For this organoclay, a mixture with at least 50% organoclay is recommended to ensure low hydraulic conductivity to gasoline and jet fuel. Diesel and fuel oil can require at least 75% of this organoclay to achieve low hydraulic conductivity.

Vulnerability curves (VCs) have been measured extensively to describe the differences in plant vulnerability to cavitation. Although the roles of hydraulic conductivity (Ks,max) and hydraulic safety (P50, embolism resistance), both of which are parameters of VCs ('sigmoidal' type), in tree demography have been evaluated across different forests, the direct linkages between VCs and tree demography are rarely explored. In this study, we combined measured VCs and plot data of 16 tree species in Panamanian seasonal tropical forests to investigate the connections between VCs and tree mortality, recruitment and growth. We found that the mortality and recruitment rates of evergreen species were most significantly positively correlated with P50. However, the mortality and recruitment rates of deciduous species only exhibited significant positive correlations with parameter a, which describes the steepness of VCs and indicates the sensitivity of conductivity loss with water potential decline, but is often neglected. These differences among evergreen and deciduous species may contribute to the poor performance of existing quantitative relationships (such as the fitting relationships for all 16 species) in capturing tree mortality and recruitment dynamics. Additionally, evergreen species presented a significant positive relationship between relative growth rate (RGR) and Ks,max, while deciduous species did not display such relationship. The RGR of both evergreen and deciduous species also displayed no significant correlations with P50 and a. Further analysis demonstrated that species with steeper VCs tended to have high mortality and recruitment rates, while species with flatter VCs were usually those with low mortality and recruitment rates. Our results highlight the important role of parameter a in tree demography, especially for deciduous species. Given that VC is a key component of plant hydraulic models, integrating measured VC rather than optimizing its parameters will help improve the ability to simulate and predict forest response to water availability.