Sap flow through partially embolized xylem vessel networks.

Sap is transported through numerous conduits in the xylem of woody plants along the path from the soil to the leaves. When all conduits are functional, vessel lumen diameter is a strong predictor of hydraulic conductivity. As vessels become embolized, sap movement becomes increasingly affected by factors operating at scales beyond individual conduits, creating resistances that result in hydraulic conductivity diverging from diameter-based estimates. These effects include pit resistances, connectivity, path length, network topology, and vessel or sector isolation. The impact of these factors varies with the level and distribution of emboli within the network, and manifest as alterations in the relationship between the number and diameter of embolized vessels with measured declines in hydraulic conductivity across vulnerability to embolism curves. Divergences between measured conductivity and diameter-based estimates reveal functional differences that arise because of species- and tissue-specific vessel network structures. Such divergences are not uniform, and xylem tissues may diverge in different ways and to differing degrees. Plants regularly operate under nonoptimal conditions and contain numerous embolized conduits. Understanding the hydraulic implications of emboli within a network and the function of partially embolized networks are critical gaps in our understanding of plants occurring within natural environments.

Drought resistance in two populations of invasive Tamarix compared using multiple methods.

An on-going question in plant hydraulic research is whether there is intra-specific variability and/or plasticity in xylem traits. Plasticity could be important in taxa that colonize diverse habitats. We used Tamarix, a non-native woody plant, to investigate population differences in hydraulic conductivity (Ks), vulnerability-to-embolism curves and vessel anatomy. We also conducted a season-long drought experiment to determine water potentials associated with crown dieback of field-grown plants. We measured vessel length and diameter, and compared visual (micro-computed tomography; microCT) and hydraulic methods to quantify percentage loss in hydraulic conductivity (PLC). Among plants grown in a common environment, we did not find differences in our measured traits between two populations of Tamarix that differ in salinity at their source habitats. This taxon is relatively vulnerable to embolism. Within samples, large diameter vessels displayed increased vulnerability to embolism. We found that the microCT method overestimated theoretical conductivity and underestimated PLC compared with the hydraulic method. We found agreement for water potentials leading to crown dieback and results from the hydraulic method. Saplings, grown under common conditions in the present study, did not differ in their xylem traits, but prior research has found difference among source-site grown adults. This suggests that plasticity may be key in the success of Tamarix occurring across a range of habits in the arid southwest USA.

Xylem structure and hydraulic function in roots and stems of chaparral shrub species from high and low elevation in the Sierra Nevada, California.

Xylem structure and hydraulics were compared between individuals at lower and upper elevation distribution limits for five chaparral shrub species along a steep transect in the southern Sierra Nevada, California, USA. Higher-elevation plants experienced frequent winter freeze-thaw events and increased precipitation. We hypothesized that environmental differences would lead to xylem trait differences between high and low elevations, but predictions were complicated because both water stress (low elevation) and freeze-thaw events (high elevation) may select for similar traits, such as narrow vessel diameter. We found significant changes in the ratio of stem xylem area to leaf area (Huber value) between elevations, with more xylem area required to support leaves at low elevations. Co-occurring species significantly differed in their xylem traits, suggesting diverse strategies to cope with the highly seasonal environment of this Mediterranean-type climate region. Roots were more hydraulically efficient and more vulnerable to embolism relative to stems, potentially due to roots being buffered from freeze-thaw stress, which allows them to maintain wider diameter vessels. Knowledge of the structure and function of both roots and stems is likely important in understanding whole-plant response to environmental gradients.

Functional response and resistance to drought in seedlings of six shrub species with contrasting leaf traits from the Mediterranean Basin and California.

Extreme drought events during post-fire regeneration are becoming increasingly frequent in Mediterranean-type ecosystems. Understanding how plants with different traits and origins respond to such conditions during early life stages is therefore critical for assessing the effect of climate change. Here, seedlings of three Cistus (semi-deciduous malacophylls from the Mediterranean Basin) and three Ceanothus (evergreen sclerophylls from California) species, two post-fire seeder genera with contrasting leaf traits, were subjected to complete water deprivation for 3 months in a common garden experiment. The leaf and plant structure and plant tissue water relations were characterized before the drought, and the functional responses (water availability, gas exchange and fluorescence) were monitored during the drought. Both genera exhibited contrasting leaf structure and tissue water relations traits, with higher leaf area and specific leaf area as well as higher osmotic potential at maximum turgor and turgor loss point in Cistus than Ceanothus. During drought, Ceanothus showed a more conservative use of water than Cistus, with a water potential less sensitive to decreasing soil moisture and a strong decline in photosynthesis and stomatal conductance in response to water deficit, but also a level of fluorescence more responsive to drought than Cistus. However, we could not find a different degree of drought resistance between the genera. This was particularly clear between Cistus ladanifer L. and Ceanothus pauciflorus DC., the two most functionally contrasting species, but at the same time, the two most drought-resistant. Our findings demonstrate that species with different leaf traits and functional responses to water stress may not differ in their degree of drought resistance, at least during the seedling stage. This underlines the need to take general categorizations by genus or functional types with caution and to deepen our knowledge about the Mediterranean-type species ecophysiology, especially during early life stages, in order to anticipate their vulnerability to climate change.

Seasonal patterns of increases in stem girth, vessel development, and hydraulic function in deciduous tree species.

BACKGROUND AND AIMS: The onset of spring growth and vessel formation were examined within three deciduous woody plant species, Acer rubrum, Populus balsamifera ssp. trichocarpa and Quercus rubra. We were broadly interested in the lag between the onset of girth expansion and the formation of mature and hydraulically conductive vessels within the new xylem. METHODS: Dendrometers were installed on 20 trees (6-7 per species), and expansion of both bole and distal stems was monitored throughout the growing season in a common garden. For each species, four to six distal stems were harvested every other week for anatomical examination of vessel formation. Additionally, for Populus and Quercus, hydraulic conductivity measurements and active xylem staining were completed on all stem samples. KEY RESULTS: For all three species, the timing of girth expansion was similar. Expansion of distal branches occurred 12-37 d earlier than that of the bole. Vessel formation initiated several weeks prior to leaf-out, but no new earlywood vessels were mature at the time of bud break for Acer and Populus and only a few were present in Quercus. Initial stem girth expansion occurred 2 to >6 weeks before the maturation of the first current-year vessels, and there was an additional delay of up to 4 weeks before mature vessels became hydraulically functional. Hydraulic conductivity was strongly correlated with the number and diameter of stained vessels. CONCLUSIONS: Bud break and leaf expansion relied predominantly on water supplied by vessels formed during prior seasons. Early-season activity is likely affected by the function of older xylem vessels and the environmental factors that influence their structure and function. Understanding the functional lifespan of vessels and the varying contributions of new and older vessels to conductivity are critical to understanding of the phenology and vascular function of long-lived woody plants in response to changing climates.

Xylem biomechanics, water storage, and density within roots and shoots of an angiosperm tree species.

Xylem is a complex tissue that forms the bulk of tree bodies and has several functions, including to conduct water, store water and nutrients, and biomechanically support the plant body. We examined how xylem functional traits varied at different positions within 9-year-old Populus balsamifera subsp. trichocarpa. Whole trees were excavated, and xylem samples were collected at 1-m increments along the main root-to-shoot axis of six trees, from root tip to shoot tip. We examined biomechanical and water-storage traits of the xylem, including using a non-invasive imaging technique to examine water content within long, intact branches (high-resolution computed tomography; microCT). Xylem density, strength, and stiffness were greater in shoots than roots. Along the main root-to-shoot axis, xylem strength and stiffness were greatest at shoot tips, and the tissue became linearly weaker and less stiff down the plant and through the root. Roots had greater water storage with lower biomechanical support, and shoots had biomechanically stronger and stiffer xylem with lower water storage. These findings support trade-offs among xylem functions between roots and shoots. Understanding how xylem functions differ throughout tree bodies is important in understanding whole-tree functioning and how terrestrial plants endure numerous environmental challenges over decades of growth.

Diversity in conduit and pit structure among extant gymnosperm taxa.

Gymnosperm taxa are incredibly diverse in many traits, including taxa with evergreen or deciduous leaves; broad, compound, needle-like or scale-like leaves; trees, shrubs, and lianas; and taxa with seed cones that range from fleshy to woody. Although less appreciated, xylem conduits are also diverse in structure among extant gymnosperm taxa. Within the xylem of gymnosperms, axial transport occurs predominantly via tracheids, although 10-40% of gymnosperm taxa, particularly within the Gnetophyta and Cycadophyta, also contain vessels. Gymnosperm taxa vary greatly in their inter-conduit pit structure, with different types of pit membranes and pitting arrangements. While some taxa display torus-margo bordered pits (60%), many others do not contain tori (40%), and at least some taxa without tori occur within each of the four extant phyla (Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta). Pit membrane types vary within families but appear relatively conserved within genera. Woody species with torus-bearing pit membranes occur in colder environments (lower mean annual temperature) compared to those without tori; but occurrence does not differ with mean annual precipitation. Detailed descriptions of pit membrane types are lacking for many species and genera, indicating a need for increased anatomical study. Increased knowledge of these traits could provide a unique experimental context in which to study the evolution of conduit networks, the hydraulic implications of conduit and pit structure, and the diverse structural and functional strategies utilized by gymnosperms. There are myriad potential study questions and research opportunities within this unique and diverse group of plants.

Starch storage capacity of sapwood is related to dehydration avoidance during drought.

PREMISE: The xylem tissue of plants performs three principal functions: transport of water, support of the plant body, and nutrient storage. Tradeoffs may arise because different structural requirements are associated with different functions or because suites of traits are under selection that relate to resource acquisition, use, and turnover. The structural and functional basis of xylem storage is not well established. We hypothesized that greater starch storage would be associated with greater sapwood parenchyma and reduced fibers, which would compromise resistance to xylem tensions during dehydration. METHODS: We measured cavitation resistance, minimum water potential, starch content, and sapwood parenchyma and fiber area in 30 species of southern California chaparral shrubs (evergreen and deciduous). RESULTS: We found that species storing greater starch within their xylem tended to avoid dehydration and were less cavitation resistant, and this was supported by phylogenetic independent contrasts. Greater sapwood starch was associated with greater parenchyma area and reduced fiber area. For species without living fibers, the associations with parenchyma were stronger, suggesting that living fibers may expand starch storage capacity while also contributing to the support function of the vascular tissue. Drought-deciduous species were associated with greater dehydration avoidance than evergreens. CONCLUSIONS: Evolutionary forces have led to an association between starch storage and dehydration resistance as part of an adaptive suite of traits. We found evidence for a tradeoff between tissue mechanical traits and starch storage; moreover, the evolution of novel strategies, such as starch-storing living fibers, may mitigate the strength of this tradeoff.

Forest and woodland replacement patterns following drought-related mortality.

Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

Factors controlling drought resistance in grapevine (Vitis vinifera, chardonnay): application of a new microCT method to assess functional embolism resistance.

PREMISE: Quantifying resistance to embolism in woody plants is important for understanding their drought response. Methods to accurately quantify resistance to embolism continue to be debated. METHODS: We used a new microCT-based approach that quantifies embolized conduits and also analyzes conductive conduits by using an x-ray-dense, iodine-rich tracer that moves though the vascular system and can easily be observed in microCT images. Many previous microCT studies assumed that all conduits were initially conductive, which may not be the case if there are developing or occluded conduits. We compared microCT results to a standard benchtop dehydration method and a centrifuge method. During dehydration, we measured gas exchange and quantified water potential at mortality. RESULTS: Our microCT curves agreed with previously published microCT curves from the same greenhouse-grown cultivar. We found a significant difference in embolism estimates if we assumed that all water-filled conduits were functional rather than only those containing tracer. Embolism estimates from microCT differed from both the benchtop and centrifuge methods. The benchtop and centrifuge methods did not differ from one another. CONCLUSIONS: The new microCT method presented here is valuable in sampling species that may contain nonconductive conduits. Disagreement between microCT and two other methods was likely due to differences in the ways they quantify embolism. MicroCT assess the theoretical effect of embolism, whereas benchtop and centrifuge methods directly measure hydraulic conductivity. The theoretical approach does not fully account for the resistances of flow through a complex 3D vascular network.

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

Vulnerability of xylem to embolism is an important trait related to drought resistance of plants. Methods continue to be developed and debated for measuring embolism. We tested three methods (benchtop dehydration/hydraulic, micro-computed tomography (microCT) and optical) for assessing the vulnerability to embolism of a native California oak species (Quercus douglasii Hook. & Arn.), including an analysis of three different stem ages. All three methods were found to significantly differ in their estimates, with a greater resistance to embolism as follows: microCT > optical > hydraulic. Careful testing was conducted for the hydraulic method to evaluate multiple known potential artifacts, and none was found. One-year-old stems were more resistant than older stems using microCT and optical methods, but not hydraulic methods. Divergence between the microCT and optical methods from the standard hydraulic method was consistent with predictions based on known errors when estimating theoretical losses in hydraulic function in both microCT and optical methods. When the goal of a study is to describe or predict losses in hydraulic conductivity, neither the microCT nor optical methods are reliable for accurately constructing vulnerability curves of stems; nevertheless, these methods may be useful if the goal of a study is to identify embolism events irrespective of hydraulic conductivity or hydraulic function.

High-resolution computed tomography reveals dynamics of desiccation and rehydration in fern petioles of a desiccation-tolerant fern.

Desiccation-tolerant (DT) plants can dry past -100 MPa and subsequently recover function upon rehydration. Vascular DT plants face the unique challenges of desiccating and rehydrating complex tissues without causing structural damage. However, these dynamics have not been studied in intact DT plants. We used high resolution micro-computed tomography (microCT), light microscopy, and fluorescence microscopy to characterize the dynamics of tissue desiccation and rehydration in petioles (stipes) of intact DT ferns. During desiccation, xylem conduits in stipes embolized before cellular dehydration of living tissues within the vascular cylinder. During resurrection, the chlorenchyma and phloem within the stipe vascular cylinder rehydrated before xylem refilling. We identified unique stipe traits that may facilitate desiccation and resurrection of the vascular system, including xylem conduits containing pectin (which may confer flexibility and wettability); chloroplasts within the vascular cylinder; and an endodermal layer impregnated with hydrophobic substances that impede apoplastic leakage while facilitating the upward flow of water within the vascular cylinder. Resurrection ferns are a novel system for studying extreme dehydration recovery and embolism repair in the petioles of intact plants. The unique anatomical traits identified here may contribute to the spatial and temporal dynamics of water movement observed during desiccation and resurrection.

Direct comparison of four methods to construct xylem vulnerability curves: Differences among techniques are linked to vessel network characteristics.

During periods of dehydration, water transport through xylem conduits can become blocked by embolism formation. Xylem embolism compromises water supply to leaves and may lead to losses in productivity or plant death. Vulnerability curves (VCs) characterize plant losses in conductivity as xylem pressures decrease. VCs are widely used to characterize and predict plant water use at different levels of water availability. Several methodologies for constructing VCs exist and sometimes produce different results for the same plant material. We directly compared four VC construction methods on stems of black cottonwood (Populus trichocarpa), a model tree species: dehydration, centrifuge, X-ray-computed microtomography (microCT), and optical. MicroCT VC was the most resistant, dehydration and centrifuge VCs were intermediate, and optical VC was the most vulnerable. Differences among VCs were not associated with how cavitation was induced but were related to how losses in conductivity were evaluated: measured hydraulically (dehydration and centrifuge) versus evaluated from visual information (microCT and optical). Understanding how and why methods differ in estimating vulnerability to xylem embolism is important for advancing knowledge in plant ecophysiology, interpreting literature data, and using accurate VCs in water flux models for predicting plant responses to drought.

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current-year growth.

Xylem vessel structure changes as trees grow and mature. Age- and development-related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water-stress-induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current-year growth of young (1-4 years), intermediate (2-7 years), and older (3-10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple-aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.

Extensive drought-associated plant mortality as an agent of type-conversion in chaparral shrublands.

Contents Summary 498 I. Introduction 498 II. Ecological drought and vegetation type-conversion 499 III. Chaparral mortality during extreme drought events 501 IV. Some species survive drought and others do not 501 V. Recovery potential 502 VI. Conclusions 503 Acknowledgements 503 References 503 SUMMARY: California experienced an intense drought from 2012 to 2015, with southern California remaining in drought to the present. Widespread chaparral shrub mortality was observed during the peak of the drought in 2014. Some species were more impacted than others and shallow-rooted shrub species were the most vulnerable to drought-associated mortality. This type of drought represents what is termed an 'ecological drought' during which an ecosystem is driven beyond thresholds of vulnerability, triggering impairment of ecosystem services and feedbacks that may result in long-term type-conversion of natural communities. The ability of shrublands to recover will depend on the timing, intensity and seasonality of future extreme climate events, post-fire recruitment potential of species with obligate fire-associated recruitment, and interactions with other stresses.

Identifying which conduits are moving water in woody plants: a new HRCT-based method.

In vivo imaging methods are useful for examination of plant vascular tissues, particularly in the identification of fluid vs gas-filled conduits; however, these methods may not allow for the simple identification of conductive conduits. Our aim in the present study was to develop a method that would allow for the in vivo identification of conductive conduits. Intact plants and segments of grapevine (Vitis vinifera L.) and intact American chestnut (Castanea dentata (Marshall) Borkh.) saplings were examined. We found that iohexol, a water soluble iodine-rich molecule, was a useful contrast agent. We also stained the xylem of segments and gas- dried samples to compare between intact scans and excised segments. Iohexol could be readily fed through cut roots or stems into the transpiration stream, was successfully transported through the xylem and marked conductive vessels within high-resolution computed tomography (HRCT) scans. Iohexol results were comparable to those obtained by staining cut segments, with iohexol detecting greater numbers of smaller conduits in some samples. Samples contained gas-filled conduits, as well as both conductive (containing iohexol tracer) and non-conductive (no iohexol tracer) fluid-filled vessels. Fluid-filled non-conductive vessels were likely still developing or were not connected to the sap stream by a low resistance pathway. We found minimal differences between intact and excised segments other than excision-related dilution of iohexol. Both vessels and vasicentric tracheids were filled with iohexol in chestnut, providing a new tool to study the functions of these different cell types. The use of iohexol as a tracer to identify conductive vessels may greatly improve the utility of HRCT as a tool in the study of plant hydraulic function. Future studies using HRCT will likely need to incorporate conductive vessel markers or controls into experiments due to the presence of non-conductive fluid-filled vessels within the xylem.

Functional lifespans of xylem vessels: Development, hydraulic function, and post-function of vessels in several species of woody plants.

PREMISE OF THE STUDY: Xylem vessels transition through different stages during their functional lifespan, including expansion and development of vessel elements, transition to vessel hydraulic functionality, and eventual transition to post-functionality. We used information on vessel development and function to develop a model of vessel lifespan for woody plants. METHODS: We examined vessel functional lifespan using repeated anatomical sampling throughout the growing season, combined with active-xylem staining to evaluate vessel hydraulic transport functionality. These data were combined with a literature review. The transitions between vessel functional lifespans for several species are illustrated, including grapevine (Vitis vinifera L., Vitaceae), English oak (Quercus robur L., Fagaceae), American chestnut [Castanea dentata (Marshall) Borkh.; Fagaceae], and several arid and semi-arid shrub species. KEY RESULTS: In intact woody plants, development and maturation of vessel elements may be gradual. Once hydraulically functional, vessel elements connect to form a vessel network that is responsible for bulk hydraulic flow through the xylem. Vessels become nonfunctional due to the formation of gas emboli. In some species and under some conditions, vessel functionality of embolized conduits may be restored through refilling. Blockages, such as tyloses, gels, or gums, indicate permanent losses in hydraulic functional capacity; however, there may be some interesting exceptions to permanent loss of functionality for gel-based blockages. CONCLUSIONS: The gradual development and maturation of vessel elements in woody plants, variation in the onset of functionality between different populations of vessels throughout the growing season, and differences in the timing of vessel transitions to post-functionality are important aspects of plant hydraulic function.

Conflicting demands on angiosperm xylem: Tradeoffs among storage, transport and biomechanics.

The secondary xylem of woody plants transports water mechanically supports the plant body and stores resources. These three functions are interdependent giving rise to tradeoffs in function. Understanding the relationships among these functions and their structural basis forms the context in which to interpret xylem evolution. The tradeoff between xylem transport efficiency and safety from cavitation has been carefully examined with less focus on other functions, particularly storage. Here, we synthesize data on all three xylem functions in angiosperm branch xylem in the context of tradeoffs. Species that have low safety and efficiency, examined from a resource economics perspective, are predicted to be adapted for slow resource acquisition and turnover as characterizes some environments. Tradeoffs with water storage primarily arise because of differences in fibre traits, while tradeoffs in carbohydrate storage are driven by parenchyma content of tissue. We find support for a tradeoff between safety from cavitation and storage of both water and starch in branch xylem tissue and between water storage capacity and mechanical strength. Living fibres may facilitate carbohydrate storage without compromising mechanical strength. The division of labour between different xylem cell types allows for considerable functional and structural diversity at multiple scales.