Embolism resistance of different aged stems of a California oak species (Quercus douglasii): optical and microCT methods differ from the benchtop-dehydration standard.

Many studies have demonstrated linkages between the occurrence of fog and ecophysiological functioning in cloud forests, but few have investigated hydraulic functioning as a determining factor that explains sharp changes in vegetation. The objective of this study was to compare the plant water status during cloud-immersed and non-immersed conditions and hydraulic vulnerability in branches and roots of species across a temperate, mountain fog ecotone. Because cloud forests are often dark, cool and very moist, we expected cloud forest species to have less drought-tolerant characteristics (i.e., lower Pe and P50-the pressures required to induce a 12 and 50% loss in hydraulic conductivity, respectively) relative to non-cloud forest species in adjacent (lower elevation) forests. Additionally, due to the ability of cloud forest species to absorb cloud-fog water, we predicted greater improvements in hydraulic functioning during fog in cloud forest species relative to non-cloud forest species. Across the cloud forest ecotone, most species measured were very resistant to losses in conductivity with branch P50 values from -4.5 to -6.0 MPa, hydraulic safety margins (Ψmin - P50) >1.5 MPa and low calculated hydraulic conductivity losses. Roots had greater vulnerabilities, with P50 values ranging from -1.4 to -2.5 MPa, leading to greater predicted losses in conductivity (∼20%). Calculated values suggested strong losses of midday leaf hydraulic conductance in three of the four species, supporting the hydraulic segmentation hypothesis. In both cloud forest and hardwood species, Ψs were greater on foggy days than sunny days, demonstrating the importance of fog periods to plant water balance across fog regimes. Thus, frequent fog did not result in systemic changes in hydraulic functioning or vulnerability to embolism across our temperate cloud forest ecotone. Finally, roots functioned with lower hydraulic conductivity than branches, suggesting that they may serve as more sensitive indicators of hydraulic functioning in these mesic, foggy ecosystems.

The vascular system of grapevine (Vitis spp.) has been reported as being highly vulnerable, even though grapevine regularly experiences seasonal drought. Consequently, stomata would remain open below water potentials that would generate a high loss of stem hydraulic conductivity via xylem embolism. This situation would necessitate daily cycles of embolism repair to restore hydraulic function. However, a more parsimonious explanation is that some hydraulic techniques are prone to artifacts in species with long vessels, leading to the overestimation of vulnerability. The aim of this study was to provide an unbiased assessment of (1) the vulnerability to drought-induced embolism in perennial and annual organs and (2) the ability to refill embolized vessels in two Vitis species X-ray micro-computed tomography observations of intact plants indicated that both Vitis vinifera and Vitis riparia were relatively vulnerable, with the pressure inducing 50% loss of stem hydraulic conductivity = -1.7 and -1.3 MPa, respectively. In V. vinifera, both the stem and petiole had similar sigmoidal vulnerability curves but differed in pressure inducing 50% loss of hydraulic conductivity (-1.7 and -1 MPa for stem and petiole, respectively). Refilling was not observed as long as bulk xylem pressure remained negative (e.g. at the apical part of the plants; -0.11 ± 0.02 MPa) and change in percentage loss of conductivity was 0.02% ± 0.01%. However, positive xylem pressure was observed at the basal part of the plant (0.04 ± 0.01 MPa), leading to a recovery of conductance (change in percentage loss of conductivity = -0.24% ± 0.12%). Our findings provide evidence that grapevine is unable to repair embolized xylem vessels under negative pressure, but its hydraulic vulnerability segmentation provides significant protection of the perennial stem.

The water retention curve (WRC) and the hydraulic conductivity function (HCF) are key ingredients in most analytical and numerical models for flow and transport in unsaturated porous media. Despite their formal derivation for a representative elementary volume (REV) of soil complex pore spaces, these two hydraulic functions are rooted in pore‐scale capillarity and viscous flows that, in turn, are invoked to provide interpretation of measurements and processes, such as linking WRC with the more difficult to measure HCF. Numerous conceptual and parametric models were proposed for the representation of processes within soil pore spaces and inferences concerning the two hydraulic functions (WRC and HCF) from surrogate variables. We review some of the primary models and highlight their physical basis, assumptions, advantages, and limitations. The first part focuses on the representation and modeling of WRC, including recent advances such as capillarity in angular pores and film adsorption and present empirical models based on easy to measure surrogate properties (pedotransfer functions). In the second part, we review the HCF and focus on widely used models that use WRC information to predict the saturated and unsaturated hydraulic conductivity. In the third part, we briefly review issues related to parameter equivalence between models, hysteresis in WRC, and effects of structural changes on hydraulic functions. Recent technological advances and monitoring networks offer opportunities for extensive hydrological information of high quality. The increase in measurement capabilities highlights the urgent need for building a hierarchy of parameters and model structures suitable for different modeling objectives and predictions across spatial scales. Additionally, the commonly assumed links between WRC and HCF must be reevaluated and involve more direct measurements of HCF. The modeling of flow and transport through structured and special porous media may require special functions and reflecting modifications in the governing equations. Finally, the impact of dynamics and transient processes at fluid interfaces on flow regimes and hydraulic properties necessitate different modeling and representation strategies beyond the present REV‐based framework.

Despite the vast extent of desert soils on the earth's surface, our understanding of the moisture dynamics of near‐surface desert soils (i.e., the top centimeters to few meters of the soil profile) remain limited. The goal of this study was to explore the use of the Peters–Durner–Iden (or PDI) instead of bimodal van Genuchten (or BVG) hydraulic functions to improve water redistribution simulations using HYDRUS‐1D for drier soils in desert environments. The PDI hydraulic functions take capillary and film flow into account, whereas BVG hydraulic functions are limited to capillary flow. By comparing measured with simulated water content data, we found that moisture redistribution simulations were improved by using PDI instead of BVG soil water retention and hydraulic conductivity functions. Compared with the BVG simulations, the PDI simulations particularly improved for drier soil conditions (i.e., volumetric water contents ranging from 6 to 10%; suction heads between pF 2 and pF 3.8, and saturation degrees between 19 and 32%, respectively) for the studied sandy soil of Scaling Environmental Processes in Heterogeneous Arid Soils (SEPHAS) Lysimeter 1. For pF >3, the PDI functions predicted higher hydraulic conductivity than the BVG functions, which confirmed the hypothesis that a hydraulic conductivity function, which can capture film flow, may improve moisture distribution simulations for dry soils. For pF between 2 and 3, however, simulation results improved due to the difference in the water retention rather than the hydraulic conductivity function.

The relationship between leaf water potential, soil water potential, and transpiration depends on soil and plant hydraulics and stomata regulation. Recent concepts of stomatal response to soil drying relate stomatal regulation to plant hydraulics, neglecting the loss of soil hydraulic conductance around the roots. Our objective was to measure the effect of soil drying on the soil-plant hydraulic conductance of maize and to test whether stomatal regulation avoids a loss of soil-plant hydraulic conductance in drying soils. We combined a root pressure chamber, in which the soil-root system is pressurized to maintain the leaf xylem at atmospheric pressure, with sap flow sensors to measure transpiration rate. The method provides accurate and high temporal resolution measurements of the relationship between transpiration rate and xylem leaf water potential. A simple soil-plant hydraulic model describing the flow of water across the soil, root, and xylem was used to simulate the relationship between leaf water potential and transpiration rate. The experiments were carried out with 5-week-old maize grown in cylinders of 9 cm diameter and 30 cm height filled with silty soil. The measurements were performed at four different soil water contents (WC). The results showed that the relationship between transpiration and leaf water potential was linear in wet soils, but as the soil dried, the xylem tension increased, and nonlinearities were observed at high transpiration rates. Nonlinearity in the relationship between transpiration and leaf water potential indicated a decrease in the soil-plant hydraulic conductance, which was explained by the loss of hydraulic conductivity around the roots. The hydraulic model well reproduced the observed leaf water potential. Parallel experiments performed with plants not being pressurized showed that plants closed stomata when the soil-plant hydraulic conductance decreased, maintaining the linearity between leaf water potential and transpiration rate. We conclude that stomata closure during soil drying is caused by the loss of soil hydraulic conductivity in a predictable way.

Recent work has suggested that plants differ in their relative reliance on structural avoidance of embolism versus maintenance of the xylem water column through dynamic traits such as capacitance, but we still know little about how and why species differ along this continuum. It is even less clear how or if different parts of a plant vary along this spectrum. Here we examined how traits such as hydraulic conductivity or conductance, xylem vulnerability curves, and capacitance differ in trunks, large- and small-diameter branches, and foliated shoots of four species of co-occurring conifers. We found striking similarities among species in most traits, but large differences among plant parts. Vulnerability to embolism was high in shoots, low in small- and large-diameter branches, and high again in the trunks. Safety margins, defined as the pressure causing 50% loss of hydraulic conductivity or conductance minus the midday water potential, were large in small-diameter branches, small in trunks and negative in shoots. Sapwood capacitance increased with stem diameter, and was correlated with stem vulnerability, wood density and latewood proportion. Capacitive release of water is a dynamic aspect of plant hydraulics that is integral to maintenance of long-distance water transport.

Unsaturated hydraulic conductivity function is one of key parameters to solve the flow phenomena in problems of landslide. Prediction models for hydraulic conductivity function related to soil-water retention curve equations in many geotechnical applications have been still used instead of direct measurement of the hydraulic conductivity function since prediction models from soil-water retention curve equations are attractive for their fast and easy use and low cost. However, many researchers found that prediction models for the hydraulic conductivity function can not predict the hydraulic conductivity exactly in comparison with experimental outputs. This research introduced an inverse analysis to evaluate the hydraulic conductivity function corresponding to experimental output from the flow pump system. Optimisation process was carried out to obtain the hydraulic conductivity function. This research showed that the inverse analysis with flow pump system was suitable to assess the hydraulic conductivity in unsaturated soil, and the prediction models for the hydraulic conductivity were led to the significant discrepancy from actual experimental outputs.

A knowledge of soil hydraulic properties—the water retention curve and unsaturated hydraulic conductivity—is required for soil water modelling and for various studies of soil hydrology. Taking measurements using traditional techniques is time consuming, the equipment is costly and the results can be uncertain. The evaporation method is frequently used for the simultaneous determination of hydraulic functions of unsaturated soil samples, i.e. the water retention curve and hydraulic conductivity function. Due to the limited range of common tensiometers, all the methodological variations of the evaporation method suffer from the limitation that the hydraulic functions can only be determined to a maximum of 70 kPa. The extended evaporation method (EEM) overcomes this restriction. Using new cavitation tensiometers and setting the air-entry pressure of the tensiometer’s porous ceramic cup as a final tension value allow both hydraulic functions to be quantified close to the wilting point. Additionally, soil shrinkage dynamics as well as soil water hysteresis can be quantified. Here, the HYPROP system was selected, a commercial device with vertically aligned tensiometers optimised to perform evaporation measurements. The HYPROP software was developed for recording, calculating, evaluating, fitting and exporting hydrological data. A good match between the results of soil hydraulic functions was shown when those obtained from traditional methods and the extended evaporation method were compared. Systematic deviations were not found.

Atmospheric nitrogen (N) deposition has been found to significantly affect plant growth and physiological performance in terrestrial ecosystems. Many individual studies have investigated how N addition influences plant functional traits, however these investigations have usually been limited to a single species, and thereby do not allow derivation of general patterns or underlying mechanisms. We synthesized data from 56 papers and conducted a meta-analysis to assess the general responses of 15 variables related to leaf economics, gas exchange, and hydraulic traits to N addition among 61 woody plant species, primarily from temperate and subtropical regions. Results showed that under N addition, leaf area index (+10.3%), foliar N content (+7.3%), intrinsic water-use efficiency (+3.1%) and net photosynthetic rate (+16.1%) significantly increased, while specific leaf area, stomatal conductance, and transpiration rate did not change. For plant hydraulics, N addition significantly increased vessel diameter (+7.0%), hydraulic conductance in stems/shoots (+6.7%), and water potential corresponding to 50% loss of hydraulic conductivity (P50, +21.5%; i.e., P50 became less negative), while water potential in leaves (−6.7%) decreased (became more negative). N addition had little effect on vessel density, hydraulic conductance in leaves and roots, or water potential in stems/shoots. N addition had greater effects on gymnosperms than angiosperms and ammonium nitrate fertilization had larger effects than fertilization with urea, and high levels of N addition affected more traits than low levels. Our results demonstrate that N addition has coupled effects on both carbon and water dynamics of woody plants. Increased leaf N, likely fixed in photosynthetic enzymes and pigments leads to higher photosynthesis and water use efficiency, which may increase leaf growth, as reflected in LAI results. These changes appear to have downstream effects on hydraulic function through increases in vessel diameter, which leads to higher hydraulic conductance, but lower water potential and increased vulnerability to embolism. Overall, our results suggest that N addition will shift plant function along a tradeoff between C and hydraulic economies by enhancing C uptake while simultaneously increasing the risk of hydraulic dysfunction.

High temperatures threaten tree survival and regeneration. A few pine species, such as Pinus oocarpa and Pinus canariensis, resprout after complete defoliation, a likely consequence of evolving in volcanic environments. Pinus pinea and Pinus pinaster rely on other mechanisms to survive wildfires. We hypothesized that the needle water potential (Ψ) and needle osmotic potential (Ψs) would decrease more under hot wind in resprouting species, a strategy of needle sacrifice in accordance with the hydraulic segmentation hypothesis. We submitted two-year-old seedlings to a two-phase hot wind treatment, consisting of one hour at 39 °C followed by five minutes at 70 °C. Phase 2 killed all needles. In non-resprouting species, Ψ decreased steeply at the beginning of Phase 1 and remained between −2 MPa and −4 MPa afterward, maintaining the loss of stem hydraulic conductance below the 50% threshold. On average, resprouting species had 15% lower wood densities and kept 51% higher stem water contents than non-resprouting species after Phase 2. The loss of hydraulic conductance did not affect resprouting. The increase in hydraulic conductance toward the base of the stem was lowest in P. canariensis, suggesting a lower degree of conduit tapering in the only species that had not undergone heteroblastic change. We measured the lowest Ψ and highest Ψs in the most xeric P. canariensis and the opposite in the most mesic P. oocarpa, highlighting the roles of xylary and extra-xylary hydraulic resistances in compartmentalizing the needle to preserve the stem. The measurement of both Ψ and Ψs allowed us to characterize the strategies of response to hot wind in resprouting and non-resprouting pine species.

Vulnerability segmentation (VS) has been widely suggested to protect stems and trunks from hydraulic failure during drought events. In many ecosystems, some species have been shown to be non-segmented (NS species). However, it is unclear whether drought-induced mortality is related to VS. To understand this, we surveyed the mortality and recruitment rate and measured the hydraulic traits of leaves and stems as well as the photosynthesis of six tree species over five years (2012–2017) in a savanna ecosystem in Southwest China. Our results showed that the NS species exhibited a higher mortality rate than the co-occurring VS species. Across species, the mortality rate was not correlated with xylem tension at 50% loss of stem hydraulic conductivity (P50stem), but was rather significantly correlated with leaf water potential at 50% loss of leaf hydraulic conductance (P50leaf) and the difference in water potential at 50% loss of hydraulic conductance between the leaves and terminal stems (P50leaf-stem). The NS species had higher Huber values and maximum net photosynthetic rates based on leaf area, which compensated for a higher mortality rate and promoted rapid regeneration under the conditions of dry–wet cycles. To our knowledge, this study is the first to identify the difference in drought-induced mortality between NS species and VS species. Our results emphasize the importance of VS in maintaining hydraulic safety in VS species. Furthermore, the high mortality rate and fast regeneration in NS species may be another hydraulic strategy in regions where severe seasonal droughts are frequent.

We examined drought-induced changes in susceptibility of potted Scots pine (Pinus sylvestris L.) trees to a bark-beetle associated fungus (Leptographium wingfieldii Morelet, from the bark beetle Tomicus piniperda L.). Five-year-old field-grown trees were transplanted to 50-l pots and grown for 1 year before the treatments were applied. Trees in the drought treatment were subjected to several successive, 3-week-long drought cycles, with predawn water potential dropping below -2 MPa at peak drought intensity. The experimental drought cycles were more severe than the natural drought episodes usually recorded in Scots pine stands. Trees were then mass-inoculated with L. wingfieldii at a density close to the critical threshold density of inoculations (400 m(-2)) above which tree resistance is overcome. Inoculation of well-watered trees resulted in induced reaction zones around the inoculation points and very limited damage (resinosis) in the sapwood. Drought alone had no long-lasting consequences on tree water relations, except for a decrease in hydraulic conductance in the youngest segments of the main stem. However, the combination of mass-inoculation and drought stress after inoculation resulted in a dramatic loss of stem hydraulic conductivity that was paralleled by conspicuous damage to the sapwood (resinosis, drying and blue staining). There was a close correlation between amount of visible sapwood damage and loss of hydraulic conductivity. The intensity of induced reactions in the phloem was unaffected by drought stress. We conclude that tree defence against L. wingfieldii is decreased during severe drought stress, resulting in changes in the spread and action of the fungus in the sapwood but not in the phloem.

Soil water retention and hydraulic conductivity functions are crucial input data for models that simulate water movement in soils. When these functions have to be generated for areas of land, the intended application of the results of the modelling determines the level of generalization at which the problem should be addressed. In the research described in this thesis a soil map at a scale corresponding to the identified level of generalization, is used as the basic document from which soil hydraulic functions are derived for an area of land. The hydraulic functions of the major pedological horizons distinguished during the soil survey are measured. Pedological differences do not necessarily correspond with soil hydraulic differences. This results in a limited number of hydraulic "building blocks" with a significantly different soil physical behaviour. Transforming the major pedological horizons into "building blocks" provides the information to transform the soil map into a map of soil physical units.Direct measurement of the hydraulic functions is cumbersome and costly. As an alternative, the existing data base of measured hydraulic functions is analysed and pedotransfer functions are developed. The use of pedotransfer functions is found to be a cost-effective method of translating the basic soil data recorded during soil survey into hydraulic functions.A concept of functional criteria is introduced for evaluating differences in the hydraulic functions measured in the major pedological horizons distinguished and for evaluating different methods of generating these functions. Functional criteria are practical aspects of soil behaviour calculated with the hydraulic functions as input. Hydraulic functions are not an aim in themselves, but serve as input data for simulation models. Therefore, in this study the evaluation of differences in these functions is based on the evaluation of differences in calculated functional criteria and not on a statistical comparison of the functions themselves.Finally, aspects of spatial and temporal variability are investigated. The scaling technique is successfully applied to quantify the complex spatial variability in measured hydraulic functions in a distribution function of scale factors. However, using the results of scaling to calculate the variability in model output results in a conservative estimate of this variability. Model output is also affected by temporal variability in meteorological and water table data. Meteorological and water table data from a 30-year period are used to calculate moisture deficits and trafficability. The influence of temporal variability is reflected by presenting graphs that show the probability of the occurrence of moisture deficits and adequate trafficability.

Acoustic emission analysis is promising to investigate the physiological events leading to drought-induced injury and mortality. However, their nature and source are not fully understood, making this technique difficult to use as a direct measure of the loss of xylem hydraulic conductance. Acoustic emissions were recorded during severe dehydration in lavender plants (Lavandula angustifolia) and compared with the dynamics of embolism development and cell damage. The timing and characteristics of acoustic signals from two independent recording systems were compared by principal component analysis (PCA). Changes in water potential, branch diameter, loss of hydraulic conductance, and cellular damage were also measured to quantify drought-induced damages. Two distinct phases of acoustic emissions were observed during dehydration: the first one associated with a rapid loss of diameter and a significant increase in loss of xylem conductance (90%), and the second with slower changes in diameter and a significant increase in cellular damage. Based on PCA, a developed algorithm discriminated hydraulic-related acoustic signals from other sources, proposing a reconstruction of hydraulic vulnerability curves. Cellular damage preceded by hydraulic failure seems to lead to a lack of recovery. The second acoustic phase would allow detection of plant mortality.

Plant hydraulic traits related to leaf drought tolerance, like the water potential at turgor loss point (TLP) and the water potential inducing 50% loss of hydraulic conductance (P50), are extremely useful to predict the potential impacts of drought on plants. While novel techniques have allowed the inclusion of TLP in studies targeting a large group of species, fast and reliable protocols to measure leaf P50 are still lacking. Recently, the optical method coupled with the gas injection (GI) technique has been proposed as a possibility to speed up the P50 estimation. Here, we present a comparison of leaf optical vulnerability curves (OVcs) measured in three woody species, namely Acer campestre (Ac), Ostrya carpinifolia (Oc) and Populus nigra (Pn), based on bench dehydration (BD) or GI of detached branches. For Pn, we also compared optical data with direct micro-computed tomography (micro-CT) imaging in both intact saplings and cut shoots subjected to BD. Based on the BD procedure, Ac, Oc and Pn had P50 values of -2.87, -2.47 and-2.11MPa, respectively, while the GI procedure overestimated the leaf vulnerability (-2.68, -2.04 and -1.54MPa for Ac, Oc and Pn, respectively). The overestimation was higher for Oc and Pn than for Ac, likely reflecting the species-specific vessel lengths. According to micro-CT observations performed on Pn, the leaf midrib showed none or very few embolized conduits at -1.2MPa, consistent with the OVcs obtained with the BD procedure but at odds with that derived on the basis of GI. Overall, our data suggest that coupling the optical method with GI might not be a reliable technique to quantify leaf hydraulic vulnerability since it could be affected by the 'open-vessel' artifact. Accurate detection of xylem embolism in the leaf vein network should be based on BD, preferably of intact up-rooted plants.