Interspecific variation in xylem vulnerability to cavitation among tropical tree and shrub species

In tropical moist forests, seasonal drought limits plant survival, productivity and diversity. Drought-tolerance mechanisms of tropical species should reflect the maximum seasonal water deficits experienced in a particular habitat. We investigated stem xylem vulnerability to cavitation in nine tropical species with different life histories and habitat associations. Stem xylem vulnerability was scored as the xylem water potential causing 50 and 75% loss of hydraulic conductivity (P50 and P75, respectively). Four shade-tolerant shrubs ranged from moderately resistant (P50=-1.9 MPa for Ouratea lucens Kunth. Engl.) to highly resistant to cavitation (P50=-4.1 MPa for Psychotria horizontalis Sw.), with shallow-rooted species being the most resistant. Among the tree species, those characteristic of waterlogged soils, Carapa guianensis Aubl., Prioria copaifera Griseb. and Ficus citrifolia Mill., were the most vulnerable to cavitation (P50=-0.8 to -1.6 MPa). The wet-season, deciduous tree, Cordia alliodora (Ruiz and Pav.) Oken., had resistant xylem (P50=-3.2 MPa), whereas the dry-season, deciduous tree, Bursera simaruba (L.) Sarg. was among the most vulnerable to cavitation (P50=-0.8 MPa) of the species studied. For eight out of the nine study species, previously reported minimum seasonal leaf water potentials measured in the field during periods of drought correlated with our P50 and P75 values. Rooting depth, deciduousness, soil type and growth habit might also contribute to desiccation tolerance. Our results support the functional dependence of drought tolerance on xylem resistance to cavitation.     

Electric resistance as a measure of tree water status during seasonal drought in a tropical dry forest in Costa Rica

Variation in electric resistance of stem tissues was used to measure differences and changes in water status among trees in a tropical dry forest in Costa Rica during the dry season. For more than 30 tree species, stem water content (SWC), measured as electric resistance between nails driven 20 mm deep into tree trunks, correlated well with wood density, saturation water content, dehydration, measured with the pressure chamber, and tree development during drought. At dry sites, SWC was lowest in hardwood trees (characterized by high wood density) and highest in stem-succulent lightwood trees (characterized by low wood density). Among hardwood trees, SWC varied with soil water availability. During the dry season, SWC declined before leaf shedding and increased during rehydration preceding bud break. The time course of seasonal changes in SWC apparently constitutes an indirect measure of variation in the relative water content of outer stem tissues, which determines development of dry-forest trees during the dry season.     

Patterns of hydraulic architecture and water relations of two tropical canopy trees with contrasting leaf phenologies: Ochroma pyramidale and Pseudobombax septenatum

Many authors have attempted to explain the adaptive response of tropical plants to drought based on studies of water relations at the leaf level. Little attention has been given to the role of the xylem system in the control of plant water requirements. To evaluate this role, we studied the hydraulic architecture and water relations parameters of two tropical canopy trees with contrasting leaf phenologies: deciduous Pseudobombax septenatum (Jacq.) Dug and evergreen Ochroma pyramidale (Cav. ex lamb) Urban, both in the family Bombacaceae. The hydraulic architecture parameters studied include hydraulic conductivity, specific conductivity, leaf specific conductivity, and Huber value. Water relations parameters include leaf water potential, stem and leaf water storage capacitance, transpiration, stomatal conductance, and vulnerability of stems to cavitation and loss of hydraulic conductivity by embolisms. Compared to temperate trees, both species showed a pattern of highly vulnerable stems (50% loss of conductivity due to embolism at water potentials less than 1 MPa) with high leaf specific conductivities. The vulnerability of xylem to water-stress-induced embolism was remarkably similar for the two species but the leaf specific conductivity of petioles and leaf-bearing stems of the evergreen species, Ochroma (e.g., 9.08 and 11.4 x 10(-4) kg s(-1) m(-1) MPa(-1), respectively), were 3.4 and 2.3 times higher, respectively, than those of the deciduous species, Pseudobombax (e.g., 2.64 and 5.15 x 10(-4) kg s(-1) m(-1) MPa(-1), respectively). A runaway embolism model was used to test the ability of Ochroma and Pseudobombax stems to maintain elevated transpiration rates during the higher evaporative demand of the dry season. The percent loss of leaf area predicted by the runaway embolism model for stems of Pseudobombax ranged from 5 to 30%, not enough to explain the deciduous phenology of this tree species without analysis of root resistance or leaf and petiole vulnerability to embolism.     

Osmotic potential of several hardwood species as affected by manipulation of throughfall precipitation in an upland oak forest during a dry year

Components of dehydration tolerance, including osmotic potential at full turgor (Psi(pio)) and osmotic adjustment (lowering of Psi(pio)), of several deciduous species were investigated in a mature, upland oak forest in eastern Tennessee. Beginning July 1993, the trees were subjected to one of three throughfall precipitation treatments: ambient, ambient minus 33% (dry treatment), and ambient plus 33% (wet treatment). During the dry 1995 growing season, leaf water potentials of all species declined to between -2.5 and -3.1 MPa in the dry treatment. There was considerable variation in Psi(pio) among species (-1.0 to -2.0 MPa). Based on Psi(pio) values, American beech (Fagus grandifolia Ehrh.), dogwood (Cornus florida L.), and sugar maple (Acer saccharum Marsh.) were least dehydration tolerant, red maple (A. rubrum L.) was intermediate in tolerance, and white oak (Quercus alba L.) and chestnut oak (Quercus prinus L.) were most tolerant. During severe drought, overstory chestnut oak and understory dogwood, red maple and chestnut oak displayed osmotic adjustment (-0.12 to -0.20 MPa) in the dry treatment relative to the wet treatment. (No osmotic adjustment was evident in understory red maple and chestnut oak during the previous wet year.) Osmotic potential at full turgor was generally correlated with leaf water potential, with both declining over the growing season, especially in species that displayed osmotic adjustment. However, osmotic adjustment was not restricted to species considered dehydration tolerant; for example, dogwood typically maintained high Psi(pio) and displayed osmotic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings tended to have higher Psi(pio) than overstory trees when water availability was high, but Psi(pio) of understory trees declined to values observed for overstory trees during severe drought. We conclude that Psi(pio) varies among deciduous hardwood species and is dependent on canopy position and soil water potential in the rooting zone.     

Vulnerability of several conifers to air embolism

Hydraulic properties of xylem in seven species of conifer were studied during late winter and early spring 1991. Vulnerability to cavitation and air embolism was investigated using hydraulic conductivity and acoustic techniques. Embolisms were induced in branches excised from mature trees by air-drying them in the laboratory. Both techniques gave comparable results indicating that they both assess the same phenomenon. Within a tree, vulnerability was related to the permeability of the xylem, the largest stems tended to cavitate before the smallest ones when water deficits developed in a branch. Interspecific comparisons showed large differences in the xylem water potential needed to induce significant embolism, values ranged from -2.5 MPa in Pinus sylvestris to -4 MPa in Cedrus atlantica, but these differences did not correlate with differences in the xylem permeability of the species. The vulnerability of a species to air embolism was found to be consistent with its ecophysiological behavior in the presence of water stress, drought-tolerant species being less vulnerable than drought-avoiding species.     

Effects of ring-porous and diffuse-porous stem wood anatomy on the hydraulic parameters used in a water flow and storage model

Calibration of a recently developed water flow and storage model based on experimental data for a young diffuse-porous beech tree (Fagus sylvatica L.) and a young ring-porous oak tree (Quercus robur L.) revealed that differences in stem wood anatomy between species strongly affect the calibrated values of the hydraulic model parameters. The hydraulic capacitance (C) of the stem storage tissue was higher in oak than in beech (939.8 versus 212.3 mg MPa(-1)). Model simulation of the elastic modulus (epsilon) revealed that this difference was linked to the higher elasticity of the stem storage tissue of oak compared with beech. Furthermore, the hydraulic resistance (R (x)) of beech was about twice that of oak (0.1829 versus 0.1072 MPa s mg(-1)). To determine the physiological meaning of the R (x) parameter identified by model calibration, we analyzed the stem wood anatomy of the beech and oak trees. Calculation of stem specific hydraulic conductivity (k (s)) of beech and oak with the Hagen-Poiseuille equation confirmed the differences in R (x) predicted by the model. The contributions of different vessel diameter classes to the total hydraulic conductivity of the xylem were calculated. As expected, the few big vessels contributed much more to total conductivity than the many small vessels. Compared with beech, the larger vessels of oak resulted in a higher k (s) (10.66 versus 4.90 kg m(-1) s(-1) MPa(-1)). The calculated ratio of k (s) of oak to beech was 2, confirming the R (x) ratio obtained by model calibration. Thus, validation of the R (x) parameter of the model led to identification of its physiological meaning.     

Hydraulic efficiency and safety of vascular and non-vascular components in Pinus pinaster leaves

Leaves, the distal section of the soil-plant-atmosphere continuum, exhibit the lowest water potentials in a plant. In contrast to angiosperm leaves, knowledge of the hydraulic architecture of conifer needles is scant. We investigated the hydraulic efficiency and safety of Pinus pinaster needles, comparing different techniques. The xylem hydraulic conductivity (k(s)) and embolism vulnerability (P(50)) of both needle and stem were measured using the cavitron technique. The conductance and vulnerability of whole needles were measured via rehydration kinetics, and Cryo-SEM and 3D X-ray microtomographic observations were used as reference tools to validate physical measurements. The needle xylem of P. pinaster had lower hydraulic efficiency (k(s)?=?2.0?×?10(-4) m(2) MPa(-1) s(-1)) and safety (P(50)?=?-?1.5 MPa) than stem xylem (k(s)?=?7.7?×?10(-4) m(2) MPa(-1) s(-1); P(50)?=?-?3.6 to?-?3.2 MPa). P(50) of whole needles (both extra-vascular and vascular pathways) was?-?0.5 MPa, suggesting that non-vascular tissues were more vulnerable than the xylem. During dehydration to?-?3.5 MPa, collapse and embolism in xylem tracheids, and gap formation in surrounding tissues were observed. However, a discrepancy in hydraulic and acoustic results appeared compared with visualizations, arguing for greater caution with these techniques when applied to needles. Our results indicate that the most distal parts of the water transport pathway are limiting for hydraulics of P. pinaster. Needle tissues exhibit a low hydraulic efficiency and low hydraulic safety, but may also act to buffer short-term water deficits, thus preventing xylem embolism.     

Contrasting seasonal leaf habits of canopy trees between tropical dry-deciduous and evergreen forests in Thailand

We compared differences in leaf properties, leaf gas exchange and photochemical properties between drought-deciduous and evergreen trees in tropical dry forests, where soil nutrients differed but rainfall was similar. Three canopy trees (Shorea siamensis Miq., Xylia xylocarpa (Roxb.) W. Theob. and Vitex peduncularis Wall. ex Schauer) in a drought-deciduous forest and a canopy tree (Hopea ferrea Lanessan) in an evergreen forest were selected. Soil nutrient availability is lower in the evergreen forest than in the deciduous forest. Compared with the evergreen tree, the deciduous trees had shorter leaf life spans, lower leaf masses per area, higher leaf mass-based nitrogen (N) contents, higher leaf mass-based photosynthetic rates (mass-based P(n)), higher leaf N-based P(n), higher daily maximum stomatal conductance (g(s)) and wider conduits in wood xylem. Mass-based P(n) decreased from the wet to the dry season for all species. Following onset of the dry season, daily maximum g(s) and sensitivity of g(s) to leaf-to-air vapor pressure deficit remained relatively unchanged in the deciduous trees, whereas both properties decreased in the evergreen tree during the dry season. Photochemical capacity and non-photochemical quenching (NPQ) of photosystem II (PSII) also remained relatively unchanged in the deciduous trees even after the onset of the dry season. In contrast, photochemical capacity decreased and NPQ increased in the evergreen tree during the dry season, indicating that the leaves coped with prolonged drought by down-regulating PSII. Thus, the drought-avoidant deciduous species were characterized by high N allocation for leaf carbon assimilation, high water use and photoinhibition avoidance, whereas the drought-tolerant evergreen was characterized by low N allocation for leaf carbon assimilation, conservative water use and photoinhibition tolerance.     

Contrasting distribution and seasonal dynamics of carbohydrate reserves in stem wood of adult ring-porous sessile oak and diffuse-porous beech trees

We tested the hypothesis that broad-leaved forest species with contrasting wood anatomy and hydraulic system (ring-porous versus diffuse-porous) also differ in distribution and seasonal dynamics of carbohydrate reserves in stem wood. Total nonstructural carbohydrate (TNC) reserves (starch and sugars) were measured enzymatically in the 10 youngest stem xylem rings of adult oak (Quercus petraea (Matt.) Liebl.) and beech (Fagus sylvatica L.) trees during an annual cycle. Radial distribution of carbohydrates was investigated according to ring age. On all dates, oak trees had twofold higher TNC concentration than beech trees (41 versus 23 mg g(DM)(-1)), with starch accounting for the high TNC concentration in oak. Seasonal dynamics of TNC concentration were significantly (P < 0.05) more pronounced in oak (20-64 mg TNC g(DM)(-1)) than in beech (17-34 mg TNC g(DM)(-1)). A marked decrease in TNC concentration was observed in oak trees during bud burst and early wood growth, whereas seasonal fluctuations in TNC concentrations in beech trees were small. The radial distribution of TNC based on ring age differed between species: TNC was restricted to the sapwood rings in oak, whereas in beech, it was distributed throughout the wood from the outermost sapwood ring to the pith. Although the high TNC concentrations in the outermost rings accounted for most of the observed seasonal pattern, all of the 10 youngest xylem rings analyzed participated in the seasonal dynamics of TNC in beech trees. The innermost sapwood rings of oak trees had low TNC concentrations. Stem growth and accumulation of carbon reserves occurred concomitantly during the first part of the season, when there was no soil water deficit. When soil water content was depleted, stem growth ceased in both species, whereas TNC accumulation was negligibly affected and continued until leaf fall. The contrasting dynamics and distribution of carbohydrate reserves in oak and beech are discussed with reference to differences in phenology, early spring growth and hydraulic properties between ring-porous trees and diffuse-porous trees.     

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.     

Seasonal patterns of leaf gas exchange and water relations in dry rain forest trees of contrasting leaf phenology

Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.     

Xylem cavitation in roots and stems of Douglas-fir and white fir

Roots of hardwoods have been shown to be more vulnerable to xylem cavitation than stems. This study examined whether this pattern is also observed in a conifer species. Vulnerability to cavitation was determined from the pressure required to inject air into the vascular system of hydrated roots and stems, and reduce hydraulic conductance of the xylem. According to the air-seeding hypothesis for the cavitation mechanism, these air pressures predict the negative xylem pressure causing cavitation in dehydrating stems. This was evaluated for stems of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and white fir (Abies concolor (Gord. & Glend.) Lindl.). The air-injection method was applied to roots and stems of different sizes and positions in Douglas-fir trees. Roots, especially smaller roots with a xylem diameter < 5 mm, were more vulnerable to cavitation than stems. Mean cavitation pressure for smaller roots was -2.09 +/- 0.42 versus -3.80 +/- 0.19 MPa for larger roots (> 8 mm diameter). Within the shoot system, smaller stems (< 5 mm diameter) were most vulnerable to cavitation, having a mean cavitation pressure of -4.23 +/- 0.565 versus -5.27 +/- 0.513 MPa for large stems (> 8 mm diameter). There was no correlation between tracheid diameter and mean cavitation pressure within root or stem systems, despite larger tracheid diameters in roots (23.3 +/- 3.9 micro m) than in stems (9.2 +/- 1.6 micro m). Smaller safety margins from cavitation in roots may be beneficial in limiting water use during mild drought, and in protecting the stem from low xylem pressures during extreme drought.     

Contrasting physiological responses of two co-occurring eucalypts to seasonal drought at restored bauxite mine sites

This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.     

Effects of summer drought and winter freezing on stem hydraulic conductivity of Rhododendron species from contrasting climates

We studied the limits to maximum water transport in three diffuse-porous evergreen shrubs exposed to frequent winter freeze-thaw events (Rhododendron maximum L. and R. catawbiense Michaux from the Appalachian Mountains) and to a severe summer drought (R. macrophyllum G. Don. from the Oregon Cascades). Percent loss of hydraulic conductivity (PLC), vulnerability curves to xylem embolism and freezing point temperatures of stems were measured over 2 years. Controlled freeze-thaw experiments were also conducted to determine the effect of thaw rate on PLC. During both years, native PLC was significantly higher in winter than in summer for R. macrophyllum. Seasonal changes in PLC were variable in both R. catawbiense and R. maximum. Only R. maximum plants growing in gaps or clearings showed higher PLC than understory plants. A rapid (2-4 day) natural recovery of high native PLC during the winter was observed in both R. maximum and R. macrophyllum. Compared with the bench-dehydration method, vulnerability curves based on the air-injection method consistently had less negative slopes and greater variation. Fifty percent PLC (PLC(50)) obtained from vulnerability curves based on the dehydration method occurred at -1.75, -2.42 and -2.96 MPa for R. catawbiense, R. maximum and R. macrophyllum, respectively. Among the study species, R. macrophyllum, which commonly experiences a summer drought, had the most negative water potential at PLC(50). In all species, stem freezing point temperatures were not consistently lower in winter than in summer. A single fast freeze-thaw event significantly increased PLC, and R. catawbiense had the highest PLC in response to freezing treatments. Recovery to control PLC values occurred if a low positive hydraulic pressure was maintained during thawing. Rhododendron macrophyllum plants, which commonly experience few freeze-thaw events, had large stem diameters, whereas plants of R. catawbiense, which had small stem diameters, suffered high embolism in response to a single freeze-thaw event. Both drought-induced and winter-induced embolism caused a significant reduction in hydraulic conductivity in all species during periods when drought or freeze-thaw events occurred in their native habitats. However, rapid recovery of PLC following freezing or drought maintained the species above their relatively low margins of safety for complete xylem dysfunction.     

Diversity of deciduousness and phenological traits of key Indian dry tropical forest trees

? In seasonally dry tropical forests deciduousness (leaflessness) is an important strategy of trees to survive in water stress period during summer. Deciduousness is a reflection of interacted effect of seasonal drought, tree characteristics and soil moisture conditions.

? The present study aims to document the diversity in leaf pheno-phases in terms of duration of deciduousness (which is reciprocal to growing season length), wood density, leaf mass per area (LMA) and leaf strategy index in 24 important tree species growing in the Vindhyan dry tropical forest in India.

? On the basis of phenological observations, the tree species were categorized into two main groups: leaf exchanging species exhibiting overlapping periods of leaf fall and leaf flush, and deciduous species whose timings of leaf flush and leaf fall differ resulting in a time lag (deciduousness) between the completion of leaf fall and initiation of leaf flush. Presence of wide range of deciduousness duration (from ca. a week to 7 months) among dry tropical trees indicates large variations in their growing season length. In the tree species studied, as the duration of deciduousness increased, leaf flushing period decreased significantly but leaf fall period showed little variation.

? Differing deciduousness in tree species exhibited substantial differences in their leafing (vegetative growth) pattern, as reflected by ratio of durations of leaf flush to leaf fall (leaf strategy index). Across different species, duration of deciduousness was significantly positively correlated with leaf strategy index, and significantly negatively correlated with both wood density and LMA.

? Wide variations in deciduousness, leaf strategy index, wood density and LMA in the 24 species investigated indicate considerable functional diversity in tree species growing in Vindhyan dry tropical region. Variation in seasonal duration of deciduousness among species is reflections of differences in tree functional traits like stem wood density, leaf strategy index and LMA.

     

Shoot and root vulnerability to xylem cavitation in four populations of Douglas-fir seedlings

The objectives of this study were to assess the range of genotypic variation in the vulnerability of the shoot and root xylem of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings to water-stress-induced cavitation, and to assess the trade-off between vulnerability to cavitation and conductivity per unit of stem cross-sectional area (k(s)), both within a species and within an individual tree. Douglas-fir occupies a broad range of environments and exhibits considerable genetic variation for growth, morphology, and drought hardiness. We chose two populations from each of two varieties (the coastal var. menziesii and the interior var. glauca) to represent environmental extremes of the species. Vulnerability curves were constructed for shoots and roots by plotting the percentage loss in conductivity versus water potential. Vulnerability in shoot and root xylem varied genetically with source climate. Stem xylem differed in vulnerability to cavitation between populations; the most mesic population, coastal wet (CW), was the most susceptible of the four populations. In the roots, the most vulnerable population was again CW; the interior wet (IW) population was moderately susceptible compared with the two dry populations, coastal dry (CD) and interior dry (ID). Root xylem was more susceptible to cavitation than stem xylem and had significantly greater k(s). The trade-off between vulnerability to cavitation and k(s), however, was not evident across populations. The most vulnerable population (CW) had a shoot k(s) of 0.534 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1), compared with 0.734 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1) for the less vulnerable CD stems. In the roots, IW was more vulnerable than ID, but had the same k(s).     

Cavitation in dehydrating xylem of Picea abies: energy properties of ultrasonic emissions reflect tracheid dimensions

Ultrasonic emission (UE) testing is used to analyse the vulnerability of xylem to embolism, but the number of UEs often does not sufficiently reflect effects on hydraulic conductivity. We monitored the absolute energy of UE signals in dehydrating xylem samples hypothesizing that (i) conduit diameter is correlated with UE energy and (ii) monitoring of UE energy may enhance the utility of this technique for analysis of xylem vulnerability. Split xylem samples were prepared from trunk wood of Picea abies, and four categories of samples, derived from mature (I: earlywood, II: 30-50% latewood, III: >50% latewood) or juvenile wood (IV: earlywood) were used. Ultrasonic emissions during dehydration were registered and anatomical parameters (tracheid lumen area, number per area) were analysed from cross-sections. Attenuation of UE energy was measured on a dehydrating wood beam by repeated lead breaks. Vulnerability to drought-induced embolism was analysed on dehydrating branches by hydraulic, UE number or UE energy measurements. In split samples, the cumulative number of UEs increased linearly with the number of tracheids per cross-section, and UE energy was positively correlated with the mean lumen area. Ultrasonic emission energies of earlywood samples (I and IV), which showed normally distributed tracheid lumen areas, increased during dehydration, whereas samples with latewood (II and III) exhibited a right-skewed distribution of lumina and UE energies. Ultrasonic emission energy was hardly influenced by moisture content until ～40% moisture loss, and decreased exponentially thereafter. Dehydrating branches showed a 50% loss of conductivity at -3.6 MPa in hydraulic measurements and at -3.9 and -3.5 MPa in UE analysis based on cumulative number or energy of signals, respectively. Ultrasonic emission energy emitted by cavitating conduits is determined by the xylem water potential and by the size of element. Energy patterns during dehydration are thus influenced by the vulnerability to cavitation, conduit size distribution as well as attenuation properties. Measurements of UE energy may be used as an alternative to the number of UEs in vulnerability analysis.     

Energy dissipation in drought-avoiding and drought-tolerant tree species at midday during the Mediterranean summer

Photosynthetic performance was monitored during two consecutive summers in four co-occurring evergreen Mediterranean tree species growing on a south-facing rocky slope. In response to midday water stress, the drought-avoiding species Pinus halepensis Mill. exhibited marked stomatal closure (g(s)) but no changes in stem water potential (Psi(s)), whereas the drought-tolerant species Quercus coccifera L., Q. ilex ssp. ballota (Desf.) Samp. and Juniperus phoenicea L. displayed declines in midday g(s) and Psi(s). The higher resistance to CO(2) influx in needles of P. halepensis compared with the other species did not result in either a proportional increase in non-radiative dissipation of excess energy or photo-inactivation of photosystem II (PSII). No significant differences were found among species either in the de-epoxidation state of the xanthophyll cycle (DPS) or in the pool of its components on a total chlorophyll basis (VAZ). Despite contrasting midday assimilation rates, the three drought-tolerant species all exhibited a pronounced drop in photochemical efficiency at midday that was characterized by a decrease in the excitation capture efficiency of the open PSII centers. Although photoinhibition was not fully reversed before dawn, it apparently did not result in cumulative photo-damage. Thus, the drought-avoiding and drought-tolerant species employed different mechanisms for coping with excess light during the midday depression in photosynthesis that involved contrasting midday photochemical efficiencies of PSII and different degrees of dynamic photoinhibition as a photo-protective mechanism. These behaviors may be related to the different mechanisms employed by drought-avoiding and drought-tolerant species to withstand water deficit.     

Hydraulic differences along the water transport system of South American Nothofagus species: do leaves protect the stem functionality?

Hydraulic traits were studied for six Nothofagus species from South America (Argentina and Chile), and for three of these species two populations were studied. The main goal was to determine if properties of the water conductive pathway in stems and leaves are functionally coordinated and to assess if leaves are more vulnerable to cavitation than stems, consistent with the theory of hydraulic segmentation along the vascular system of trees in ecosystems subject to seasonal drought. Vulnerability to cavitation, hydraulic conductivity of stems and leaves, leaf water potential, wood density and leaf water relations were examined. Large variations in vulnerability to cavitation of stems and leaves were observed across populations and species, but leaves were consistently more vulnerable than stems. Water potential at 50% loss of maximum hydraulic efficiency (P(50)) ranged from -0.94 to -2.44 MPa in leaves and from -2.6 to -5.3 MPa in stems across species and populations. Populations in the driest sites had sapwood and leaves more vulnerable to cavitation than those grown in the wettest sites. Stronger diurnal down-regulation in leaf hydraulic conductance compared with stem hydraulic conductivity apparently has the function to slow down potential water loss in stems and protect stem hydraulics from cavitation. Species-specific differences in wood density and leaf hydraulic conductance (K(Leaf)) were observed. Both traits were functionally related: species with higher wood density had lower K(Leaf). Other stem and leaf hydraulic traits were functionally coordinated, resulting in Nothofagus species with an efficient delivery of water to the leaves. The integrity of the more expensive woody portion of the water transport pathway can thus be maintained at the expense of the replaceable portion (leaves) of the stem-leaf continuum under prolonged drought. Compensatory adjustments between hydraulic traits may help to decrease the rate of embolism formation in the trees more vulnerable to cavitation.     

Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars

We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (Ks) or leaf specific conductivity (Kl) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in Ks, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (PsiPLC50) occurred at a water potential of -4.4 MPa in the well-watered treatment, and at -6.0 MPa in the moderate drought treatment. In the other species, PsiPLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between -6.4 and -7.5 MPa in both irrigation treatments. During severe drought, Ks of twigs and stomatal conductance (g(s)) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of Ks was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum g(s)) was reached at a predawn water potential (Psi(pd)) of -2.5 MPa in C. atlantica (Luberon) and at -3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Psi(pd) values. Cedrus brevifolia, with a Psi(pd) (-3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed.