Loss Of Hydraulic Conductivity Research Articles

Distribution of pines in the Iberian Peninsula agrees with species differences in foliage frost tolerance, not with vulnerability to freezing-induced xylem embolism.

Drought and frosts are major determinants of plant functioning and distribution. Both stresses can cause xylem embolism and foliage damage. The objective of this study was to analyse if the distribution of six common pine species along latitudinal and altitudinal gradients in Europe is related to their interspecific differences in frost tolerance and to the physiological mechanisms underlying species-specific frost tolerance. We also evaluate if frost tolerance depends on plant water status. We studied survival to a range of freezing temperatures in 2-year-old plants and assessed the percentage loss of hydraulic conductivity (PLC) due xylem embolism formation and foliage damage determined by needle electrolyte leakage (EL) after a single frost cycle to -15 °C and over a range of predawn water potential (ψpd) values. Species experiencing cold winters in their range (Pinus nigra J.F. Arnold, Pinus sylvestris L. and Pinus uncinata Raymond ex A. DC.) had the highest frost survival rates and lowest needle EL and soluble sugar (SS) concentration. In contrast, the pines inhabiting mild or cool winter locations (especially Pinus halepensis Mill. and Pinus pinea L. and, to a lesser extent, Pinus pinaster Ait.) had the lowest frost survival and highest needle EL and SS values. Freezing-induced PLC was very low and differences among species were not related to frost damage. Reduction in ψpd decreased leaf frost damage in P. pinea and P. sylvestris, increased it in P. uncinata and had a neutral effect on the rest of the species. This study demonstrates that freezing temperatures are a major environmental driver for pine distribution and suggests that interspecific differences in leaf frost sensitivity rather than vulnerability to freezing-induced embolism or SS explain pine juvenile frost survival.

Pneumatic Method to Measure Plant Xylem Embolism.

Embolism, the formation of air bubbles in the plant water transport system, has a major impact on plant water relations. Embolism formation in the water transport system of plants disrupts plant water transport capacity, impairing plant functioning and triggering plant mortality. Measuring embolism with traditional hydraulic methods is both time-consuming and requires large amounts of plant material. While the stem hydraulic methods measure loss of xylem hydraulic conductance due to embolism formation, the pneumatic method directly quantifies the amount of emboli inside the xylem as changes in xylem air content. The pneumatic method is an easy and fast (8+ embolism curves per day) method to measure plant embolism requiring minimal plant material. Here, we provide detailed descriptions and recent technical improvements on the pneumatic method.

Variation in xylem vulnerability to embolism in European beech from geographically marginal populations.

Climate change is expected to increase the frequency and intensity of droughts and heatwaves in Europe, leading to effects on forest growth and major forest dieback events due to hydraulic failure caused by xylem embolism. Inter-specific variability in embolism resistance has been studied in detail, but little is known about intra-specific variability, particularly in marginal populations. We evaluated 15 European beech populations, mostly from geographically marginal sites of the species distribution range, focusing particularly on populations from the dry southern margin. We found small, but significant differences in resistance to embolism between populations, with xylem pressures causing 50% loss of hydraulic conductivity ranging from -2.84 to -3.55 MPa. Significant phenotypic clines of increasing embolism resistance with increasing temperature and aridity were observed: the southernmost beech populations growing in a warmer drier climate and with lower habitat suitability have higher resistance to embolism than those from Northern Europe growing more favourable conditions. Previous studies have shown that there is little or no difference in embolism resistance between core populations, but our findings show that marginal populations have developed ways of protecting their xylem based on either evolution or plasticity.

Acclimation of branch and leaf hydraulics in adult Fagus sylvatica and Picea abies in a forest through-fall exclusion experiment.

Decreasing water availability due to climate change poses the question of whether and to what extent tree species are able to hydraulically acclimate and how hydraulic traits of stems and leaves are coordinated under drought. In a through-fall exclusion experiment, hydraulic acclimation was analyzed in a mixed forest stand of Fagus sylvatica L. and Picea abies (L.) Karst. In drought-stressed (TE, through-fall exclusion over 2 years) and control (CO) trees, hydraulic vulnerability was studied in branches as well as in leaves (F. sylvatica) and end-twigs (P. abies, entirely formed during the drought period) sampled at the same height in sun-exposed portions of the tree crown. In addition, relevant xylem anatomical traits and leaf pressure-volume relations were analyzed. The TE trees reached pre-dawn water potentials down to -1.6 MPa. In both species, water potentials at 50% loss of xylem hydraulic conductivity were ~0.4 MPa more negative in TE than in CO branches. Foliage hydraulic vulnerability (expressed as water potential at 50% loss of leaf/end-twig hydraulic conductance) and water potential at turgor loss point were also, respectively, 0.4 and 0.5 MPa lower in TE trees. Minor differences were observed in conduit mean hydraulic diameter and cell wall reinforcement. Our findings indicate significant and fast hydraulic acclimation under relatively mild drought in both tree species. Acclimation was well coordinated between branches and foliage, which might be essential for survival and productivity of mature trees under future drought periods.

Post-drought hydraulic recovery is accompanied by non-structural carbohydrate depletion in the stem wood of Norway spruce saplings

Hydraulic failure and carbon starvation are recognized as main causes of drought-induced forest decline. As water transport and carbon dynamics are strictly interdependent, it is necessary to clarify how dehydration-rehydration cycles are affecting the relations between stem embolism and non-structural carbohydrates (NSC). This is particularly needed for conifers whose embolism repair capability is still controversial. Potted Norway spruce saplings underwent two drought-re-irrigation cycles of same intensity, but performed in two consecutive summers. During the second cycle, stem percent loss of hydraulic conductivity (PLC) and NSC content showed no carry-over effects from the previous drought, indicating complete long-term recovery. The second drought treatment induced moderate PLC (20%) and did not affect total NSCs content, while starch was converted to soluble sugars in the bark. After one week of re-irrigation, PLC recovered to pre-stress values (0%) and NSCs were depleted, only in the wood, by about 30%. Our data suggest that spruce can repair xylem embolism and that, when water is newly available, NSCs stored in xylem parenchyma can be mobilized over short term to sustain respiration and/or for processes involved in xylem transport restoration. This, however, might imply dependency on sapwood NSC reserves for survival, especially if frequent drought spells occur.

Impact of brackishwater shrimp farming at the interface of rice growing areas and the prospects for improvement in coastal India

The coastal plain of Odisha in the northeastern region of India is designated the “rice bowl” of the state and is vulnerable to the impact of brackishwater shrimp farming, a prominent livelihood in southeastern Asia. Shrimp farming is highly profitable. However, owing to plentiful resources, shrimp farming has encroached on several rice-growing areas and a decline in the quality of natural resources in the coastal neighborhood has since been reported. This paper aims to study the effects of the unplanned expansion of brackishwater shrimp farming on natural resources and to provide a pathway to suitable utilization in order to improve the livelihood security of marginal shrimp farming communities in coastal Odisha, India. The practice of brackishwater shrimp farming has been determined to induce salt stress at 341–9387 ppt ha−1 crop−1 with a soil EC ranging from 0.3 to 3.4 and 0.5 to 9.5 dSm−1 under the Scientific Extensive Traditional (SET) method and 0.1 to 1.4 and 0.2 to 3.6 dSm−1 under the Traditional/Improved Traditional (IT) practice during post– and pre–farming periods, respectively. Soil with ≥35% clay content underwent a severe loss of saturated hydraulic conductivity (Ks), and soil with a low exchangeable sodium percentage (6.09–8.03%) showed more susceptibility towards Na saturation than did soil with a high exchangeable sodium percentage (>10%) after brackishwater shrimp farming. Growing salt-tolerant rice in shrimp ponds during non–farming periods was observed to reduce soil Ks by only 1.2–1.3-fold compared to a reduction of 22–40-fold under shrimp farming. The paper concludes that by promoting salt washing and alleviating salinity hazards, the shrimp–rice sequence has shown promise to restore soil quality, reduce vulnerability, enhance resilience in brackishwater shrimp farming where the farms interface with rice-growing coastal areas, and provide support to conserve the coastal environment.

Xylem Sap Surface Tension May Be Crucial for Hydraulic Safety.

The surface tension (γ) of xylem sap plays a key role in stabilizing air-water interfaces at the pits between water- and gas-filled conduits to avoid air seeding at low water potentials. We studied seasonal changes in xylem sap γ in Picea abies and Pinus mugo growing at the alpine timberline. We analyzed their vulnerability to drought-induced embolism using solutions of different γ and estimated the potential effect of seasonal changes in γ on hydraulic vulnerability. In both species, xylem sap γ showed distinct seasonal courses between about 50 and 68 mn m-1 Solutions with low γ caused higher vulnerability to drought-induced xylem embolism. The water potential at 50% loss of hydraulic conductivity in P. abies and P. mugo was -3.35 and -3.86 MPa at γ of 74 mn m-1 but -2.11 and -2.09 MPa at 45 mn m-1 This indicates up to about 1 MPa seasonal variation in 50% loss of hydraulic conductivity. The results revealed pronounced effects of changes in xylem sap γ on the hydraulic safety of trees in situ. These effects also are relevant in vulnerability analyses, where the use of standard solutions with high γ overestimates hydraulic safety. Thus, γ should be considered carefully in hydraulic studies.

Drought-induced embolism in stems of sunflower: A comparison of in vivo micro-CT observations and destructive hydraulic measurements

Vulnerability curves (VCs) are a useful tool to investigate the susceptibility of plants to drought-induced hydraulic failure, and several experimental techniques have been used for their measurement. The validity of the bench dehydration method coupled to hydraulic measurements, considered as a 'golden standard', has been recently questioned calling for its validation with non-destructive methods. We compared the VCs of a herbaceous crop plant (Helianthus annuus) obtained during whole-plant dehydration followed by i) hydraulic flow measurements in stem segments (classical destructive method) or by ii) in vivo micro-CT observations of stem xylem conduits in intact plants. The interpolated P50 values (xylem water potential inducing 50% loss of hydraulic conductance) were −1.74 MPa and −0.87 MPa for the hydraulic and the micro-CT VC, respectively. Interpolated P20 values were similar, while P50 and P80 were significantly different, as evidenced by non-overlapping 95% confidence intervals. Our results did not support the tension-cutting artefact, as no overestimation of vulnerability was observed when comparing the hydraulic VC to that obtained with in vivo imaging. After one scan, 25% of plants showed signs of x-ray induced damage, while three successive scans caused the formation of a circular brownish scar in all tested plants. Our results support the validity of hydraulic measurements of samples excised under tension provided standard sampling and handling protocols are followed, but also show that caution is needed when investigating vital plant processes with x-ray imaging.

A multi-species synthesis of physiological mechanisms in drought-induced tree mortality.

Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.

Gas exchange recovery following natural drought is rapid unless limited by loss of leaf hydraulic conductance: evidence from an evergreen woodland.

Drought can cause major damage to plant communities, but species damage thresholds and postdrought recovery of forest productivity are not yet predictable. We used an El Niño drought event as a natural experiment to test whether postdrought recovery of gas exchange could be predicted by properties of the water transport system, or if metabolism, primarily high abscisic acid concentration, might delay recovery. We monitored detailed physiological responses, including shoot sapflow, leaf gas exchange, leaf water potential and foliar abscisic acid (ABA), during drought and through the subsequent rehydration period for a sample of eight canopy and understory species. Severe drought caused major declines in leaf water potential, elevated foliar ABA concentrations and reduced stomatal conductance and assimilation rates in our eight sample species. Leaf water potential surpassed levels associated with incipient loss of leaf hydraulic conductance in four species. Following heavy rainfall gas exchange in all species, except those trees predicted to have suffered hydraulic impairment, recovered to prestressed rates within 1d. Recovery of plant gas exchange was rapid and could be predicted by the hydraulic safety margin, providing strong support for leaf vulnerability to water deficit as an index of damage under natural drought conditions.

Xylem resistance to embolism: presenting a simple diagnostic test for the open vessel artefact

Xylem vulnerability to embolism represents an essential trait for the evaluation of the impact of hydraulics in plant function and ecology. The standard centrifuge technique is widely used for the construction of vulnerability curves, although its accuracy when applied to species with long vessels remains under debate. We developed a simple diagnostic test to determine whether the open-vessel artefact influences centrifuge estimates of embolism resistance. Xylem samples from three species with differing vessel lengths were exposed to less negative xylem pressures via centrifugation than the minimum pressure the sample had previously experienced. Additional calibration was obtained from non-invasive measurement of embolism on intact olive plants by X-ray microtomography. Results showed artefactual decreases in hydraulic conductance (k) for samples with open vessels when exposed to a less negative xylem pressure than the minimum pressure they had previously experienced. X-Ray microtomography indicated that most of the embolism formation in olive occurs at xylem pressures below -4.0 MPa, reaching 50% loss of hydraulic conductivity at -5.3 MPa. The artefactual reductions in k induced by centrifugation underestimate embolism resistance data of species with long vessels. A simple test is suggested to avoid this open vessel artefact and to ensure the reliability of this technique in future studies.

Plant water variables and reproductive traits are influenced by seasonal climatic variables in Prosopis burkartii (Fabaceae) at Northern Chile

Prosopis burkartii is a critically endangered phreatophyte legume endemic to the Atacama Desert of northern Chile where only 50 individuals remain. We studied seasonal water variables and flower parameters in a population of 15P. burkartii individuals. For each tree, xylem water potential (Ψ) was estimated by measuring two twigs with a Scholander pressure bomb. Each month, we made 3 measurements of xylem Ψ in each tree: pre-dawn (Ψpd), midday (Ψmd), and late-afternoon (Ψla). We also measured pre-dawn hydraulic conductivity (K) and the percent loss of hydraulic conductivity (PLC) on the same trees. Flower parameters were estimated using tagged twigs. The total number of flowers produced by each plant, the number of pods per plants, number of fertile pods and the percentage of abortion of flowers and pods were estimated. Values of Ψ ranged from −2.7±0.1 (July) to −0.8±0.2MPa (September). On a daily basis, individuals had maximal Ψ at pre-dawn and minimal Ψ at midday, with a recovery of Ψ during the late-afternoon likely due to the low evaporative demand at that time. Furthermore, the native conductance of the stems (Ki) had inverse trends in comparison with PLC data. Number of flowers per plant was strongly correlated with Ψmd estimated during October. During the study period, we observed that Ψmd had higher values during the spring and at the beginning of summer, indicating maintenance of a favorable plant water status during the season of flowering at the time of day when thermal stress is greatest. The differences in PLC suggest that individuals of P. burkartii can maintain functional xylem conduits during summer droughts and extract water from the soil while maintaining high values of midday Ψ, the period of the day with the greatest stress.

Physiological Response of Garry Oak (Quercus garryana) Seedlings to Drought

The purpose of this study was to determine physiological differences in drought response among Garry oak (Quercus garryana Douglas ex Hook. [Fagaceae]) seedlings from acorns collected at seven acorn collection sites in Washington state. Our objective was to aid in Garry oak restoration efforts by investigating potential differences in drought tolerance for seedlings grown from acorns collected at different sites. Acorns were collected from six sites east of the Cascades, and one site on Whidbey Island (Oak Harbor) west of the Cascades. Differences in morphological and photosynthetic characteristics observed for seedlings grown from acorns collected at these different sites became more pronounced after drought stress was induced by withholding water. Seedlings from acorns collected at the northernmost east-side site (Swauk Creek) were most susceptible to drought and had photosynthetic rates that were 35% lower under drought conditions than when well-watered. Seedlings from acorns collected at the site west of the cascades (Oak Harbor) were the least susceptible to drought and had assimilation rates under drought conditions that were similar to those when well-watered. Differences were also observed among seedlings from different sites in the occurrence of drought-induced loss of stem hydraulic conductivity and changes in leaf water potential and relative water content in response to drought. We conclude that seedlings grown from Garry oak trees at different sites across Washington State have distinct physiological responses to drought that may lead to differing survivorship when used in reforestation efforts and exposed to drought stress in the field.

Effects of soil nutrient conditions on water transport properties and recovery from severe drought stress in Pinus densiflora saplings

ABSTRACTTo evaluate the acclimation of water transport properties to soil nutrient conditions, morphological and hydraulic traits were measured in 4-year-old potted saplings of Japanese red pine that were grown under differing nutrient conditions for 3 years. To compare recovery from severe drought stress under differing nutrient conditions, the well-watered saplings were subjected to drought treatments of almost Ψ50 by stopping irrigation; this is equivalent to the xylem water potential at which 50% loss of hydraulic conductivity occurs. After resuming daily watering, the xylem water potential and gas exchange rate were measured. Whole-plant transpirational demand from large total-leaf area tended to be high for plants under high-nutrient conditions. However, no differences were observed in water transport properties, such as the conducting area, hydraulic efficiency, and cavitation resistance, suggesting that acclimation of the hydraulic architecture did not occur when plants were grown under sufficient soil moisture conditions. The root dry weight tended to be large and the belowground dry mass to root dry mass (T/R ratio) tended to be low for plants under high-nutrient conditions. Plants grown under high-nutrient conditions recovered their gas exchange rate and leaf-specific hydraulic conductance slightly during the month following severe drought stress by irrigation. The recovery of Japanese red pine appears to vary under different levels of nutrient availability probably because of the influence of root water absorption capacity.

Contrasting Hydraulic Architectures of Scots Pine and Sessile Oak at Their Southernmost Distribution Limits.

Many temperate European tree species have their southernmost distribution limits in the Mediterranean Basin. The projected climatic conditions, particularly an increase in dryness, might induce an altitudinal and latitudinal retreat at their southernmost distribution limit. Therefore, characterizing the morphological and physiological variability of temperate tree species under dry conditions is essential to understand species’ responses to expected climate change. In this study, we compared branch-level hydraulic traits of four Scots pine and four sessile oak natural stands located at the western and central Mediterranean Basin to assess their adjustment to water limiting conditions. Hydraulic traits such as xylem- and leaf-specific maximum hydraulic conductivity (KS-MAX and KL-MAX), leaf-to-xylem area ratio (AL:AX) and functional xylem fraction (FX) were measured in July 2015 during a long and exceptionally dry summer. Additionally, xylem-specific native hydraulic conductivity (KS-N) and native percentage of loss of hydraulic conductivity (PLC) were measured for Scots pine. Interspecific differences in these hydraulic traits as well as intraspecific variability between sites were assessed. The influence of annual, summer and growing season site climatic aridity (P/PET) on intraspecific variability was investigated. Sessile oak displayed higher values of KS-MAX, KL-MAX, AL:AX but a smaller percentage of FX than Scots pines. Scots pine did not vary in any of the measured hydraulic traits across the sites, and PLC values were low for all sites, even during one of the warmest summers in the region. In contrast, sessile oak showed significant differences in KS-MAX, KL-MAX, and FX across sites, which were significantly related to site aridity. The striking similarity in the hydraulic traits across Scots pine sites suggests that no adjustment in hydraulic architecture was needed, likely as a consequence of a drought-avoidance strategy. In contrast, sessile oak displayed adjustments in the hydraulic architecture along an aridity gradient, pointing to a drought-tolerance strategy.

The root cambium ultrastructure during drought stress in Corylus avellana

The activity of the vascular cambium, which plays an important role in plant growth and development, is well known to respond to environmental changes such as drought. Although plant death is suggested to be tightly associated with the death of cambium and other meristematic tissues, direct observations of the cambium ultrastructure during drought-induced mortality remain poorly documented. Here, seedlings of Corylus avellana were drought stressed over various days in autumn to reach xylem water potentials (Ψx, -MPa) between -1 and -5.2 MPa. Light and transmission electron microscopy was applied to visualize the ultrastructure of the root cambial cells, while stem vulnerability curves were obtained to quantify embolism resistance. Seedlings that were moderately water stressed with Ψx less negative than P50 (-2.0 MPa, i.e., the xylem water potential at 50% loss of hydraulic conductivity) showed intact cambium cells. In contrast, the cell membrane of fusiform cambial cells became detached, the tonoplast was shrunken or damaged and cell organelles started to disintegrate in severely water stressed plants with Ψx more negative than P50. Ray cambial cells, however, remained intact and alive for a longer period than fusiform cells. Death of the cambial fusiform cells corresponded more or less to the lethal water potential of this species (Ψlethal = -4.83 MPa). These observations suggest that drought-induced mortality of C. avellana is closely associated with cambial death.

Mexican conifers differ in their capacity to face climate change

The recent massive dieback of forest trees due to drought stress makes assessment of the variability of physiological traits that might be critical for predicting forest response and adaptation to climate change even more urgent. We investigated xylem vulnerability to cavitation and xylem specific hydraulic conductivity in seven species of three principal conifer genera (Juniperus monticola, Juniperus deppeana, Juniperus flaccida, Pinus pseudostrobus, Pinus leiophylla, Pinus devoniana, and the endangered Picea chihuahuana) of the Mexican mountains in order to identify the species most vulnerable to future warmer and drier climates. Hydraulic traits were examined using the in situ flow centrifuge technique (Cavitron) on branches collected from adult trees of natural populations and seedlings growing in a common garden. We found evidence of significant differences in xylem safety between genera (P50: pressure inducing 50% loss of hydraulic conductance): the three juniper species exhibited low P50 values (ranging from -9.9 to -10.4 MPa), relative to the much more vulnerable pine and spruce species (P50 ranging between - 2.9 to - 3.3 MPa). Our findings also revealed no variation in P50 between adult trees assessed in the field and seedlings growing in a common garden. We therefore propose that if, as projected, climate change makes their natural habitats much warmer and drier, populations of Mexican pines and the studied spruce will be likely to decline severely as a result of drought-stress induced cavitation, while the juniper species will survive.

Divergent Hydraulic Safety Strategies in Three Co-occurring Anacardiaceae Tree Species in a Chinese Savanna.

Vulnerability segmentation, the condition under which plant leaves are more vulnerable to drought-induced cavitation than stems, may act as a “safety valve” to protect stems from hydraulic failure. Evergreen, winter-deciduous, and drought-deciduous tree species co-occur in tropical savannas, but there have been no direct studies on the role of vulnerability segmentation and stomatal regulation in maintaining hydraulic safety in trees with these three leaf phenologies. To this end, we selected three Anacardiaceae tree species co-occurring in a Chinese savanna, evergreen Pistacia weinmanniifolia, drought-deciduous Terminthia paniculata, and winter-deciduous Lannea coromandelica, to study inter-species differentiation in leaf and stem hydraulic safety. We found that the two deciduous species had significantly higher sapwood-specific hydraulic conductivity and leaf-specific hydraulic conductance than the evergreen species. Moreover, two deciduous species were more vulnerable to stem cavitation than the evergreen species, although both drought-deciduous species and evergreen species had drought-resistance leaves. The evergreen species maintained a wide hydraulic safety margin (HSM) in stems and leaves; which was achieved by embolism resistance of both stems and leaves and isohydric stomatal control. Both deciduous species had limited HSMs in stems and leaves, being isohydric in the winter-deciduous species and anisohydric in drought-deciduous species. The difference in water potential at 50% loss of hydraulic conductivity between the leaves and the terminal stems (P50leaf−stem) was positive in P. weinmanniifolia and L. coromandelica, whereas, T. paniculata exhibited a lack of vulnerability segmentation. In addition, differences in hydraulic architecture were found to be closely related to other structural traits, i.e., leaf mass per area, wood density, and sapwood anatomy. Overall, the winter-deciduous species exhibits a drought-avoidance strategy that maintains the hydraulic safety of the more carbon-costly stems by sacrificing cheaper and more vulnerable leaves, while the evergreen species exhibits a hydraulic strategy of drought tolerance with strong stomatal regulation. In contrast, the drought-deciduous species lacks vulnerability segmentation and sheds leaves at the expense of top shoots during peak drought. This study demonstrates that even sympatric tree species that differ in leaf phenology can exhibit divergent adaptive hydraulic safety strategies.

Fitplc - an R package to fit hydraulic vulnerability curves

We describe a toolkit to fit hydraulic vulnerability curves, such as the percent loss of xylem hydraulic conductivity ('PLC curves') as a function of the water potential. The toolkit is implemented as an R package, and is thus free to use and open source. The package fits the Weibull or sigmoidal function to measurements of PLC, conductance or conductivity, at corresponding leaf or stem water potentials. From the fitted curve, estimates of Px (the water potential at which x% conductivity is lost, e.g. the P50), and slope parameter (Sx) are provided together with confidence intervals (CI) around the fitted line. The CIs are estimated with the bootstrap. We also demonstrate the advantages of using mixed-effects models in situations where multiple individuals are measured on a species, as compared to the more traditional approach of fitting curves separately and averaging the parameters. We demonstrate the use of the new package with example data on seven species measured with two different techniques.

Wood density as a proxy for vulnerability to cavitation: Size matters

In this study, vulnerability to cavitation, P50 (i.e. the water potential causing 50 % loss of hydraulic conductivity), of Norway spruce trunkwood at different cambial age was related to wood density. Wood density was calculated from mass in the oven dry state related either to volume at the oven-dry state (dry wood density) or to volume at full saturation (basic wood density). Dry wood density and basic wood density were strongly linearly related (r² = 0.99); there was however a shift from the 1:1 reference line with increasing dry wood density. Dry wood density as well as basic wood density had similar high predictive qualities for P50 (r² = 0.85). The quadratic regression lines took however a quite different course below -4 MPa because volume shrinkage increased with increasing dry wood density. For Norway spruce sapwood with high dry wood density, mixing up different wood density traits would thus result in a predicted overestimation or underestimation of vulnerability to cavitation. Gravimetrically wood density measurements at different moisture contents (starting at full saturation) are easily to achieve on standard size specimens and the conversion curves obtained will be of high value for future ecological studies on other species and across species.