EFFECTS OF VINE COMPETITION ON AVAILABILITY OF LIGHT,WATER, AND NITROGEN TO A TREE HOST (LIQUIDAMBAR STYRACIFLUA)

Competitive effects of vines on their tree hosts are well documented, but the mechanisms involved in these interactions are poorly understood. The objectives of this study were to measure the effects of below- and/or aboveground competition from the vines Lonicera japonica and Parthenocissus quinquefolia on availability of light, water, and nitrogen to the host tree Liquidambar styraciflua, and to examine the relationship between resource availability and tree growth. Light penetration through tree canopies, pre-dawn leaf water potential, and leaf nitrogen concentration were used as predictors of light, water, and nitrogen availability to the tree, respectively. Vine presence significantly reduced light penetration through the tree canopies, but this reduction was not clearly related to the growth responses of trees. Vines did not reduce the pre-dawn leaf water potential of competing trees, which was consistently above -0.5 MPa for the duration of the study. Leaf nitrogen concentration of trees, on the other hand, was significantly reduced by belowground competition with L. japonica. The positive correlation between the annual average leaf nitrogen concentration and tree diameter growth suggested that competition for nitrogen mediated the effects of belowground competition of vines on tree growth.     

Effect of water availability and fertilization on water status,growth, vigour and the resistance of Scots pine to fungal mass inoculation with Ophiostoma ips

Abstract

The effect of water and nutrient availability on the performance of Scots pine (Pinus sylvestris L.) against Ophiostoma ips (Rumb.), a bark beetle-associated phytopathogenic blue-stain fungus, was investigated. Field-grown trees were subjected for 18 months to water-stress and/or fertilization, and the effects of such treatments on the needle nutrient status, tree vegetative growth and vigour were examined. At the end of the experimental period, the trees were mass-inoculated (800 inocula m^?2) with the fungus, and the relationship between resource availability and tree performance against pathogen attack was also tested. Predawn shoot water potential (Ψ_PD) of irrigated trees was significantly higher than that of water-stressed trees, and fertilized trees had a significantly lower C/N ratio. The Ψ_PD values and needle nitrogen content suggest that resource-limited trees were under moderate stress. Improved nutrient availability significantly increased tree growth and tree vigour. However, no evidence for an effect of improved nutrient availability on tree fungal resistance was found in our study.     

Pollination and resource limitation as interacting constraints on almond fruit set

• Pollination and resource availability are factors determining reproductive success of plants, and in agriculture these factors influence yield of fruit‐bearing crops. Our understanding of the importance of crop pollination is fast improving, but less is known about how the interaction between pollination and resources constrains fruit production.
• We conducted an experiment with almond trees (Prunus dulcis) to examine how the number of flowers, light availability and competition for resources affected nut (fruit) production on individual spurs (fruit‐bearing structures) exposed to open‐pollination or hand‐pollination.
• We found a positive relationship between flower number and nut number on spurs with up to four flowers, but no further benefit after four flowers, suggesting a resource threshold expressed by individual spurs. Spurs with few flowers increased the conversion rate of flowers to nuts when supplemented with hand‐pollination, but spurs with more flowers were more likely to achieve the threshold number of nuts even under open‐pollination. Our experiment included a further treatment involving spraying whole trees with pollen. This treatment reduced nut production by spurs with many flowers and high light availability, suggesting competition is experienced by well‐resourced spurs when resources need to be shared among developing nuts across the whole tree.
• Our study supports the hypothesis that excess flower production in fruit trees increases the potential for fruit production when pollinator and resource availability is variable (bet‐hedging). Spurs with more flowers typically produce more nuts (within a limited range), but only if both resources and pollen supply increase with flower number. For almond growers, a focus on maintaining high flower numbers, especially in high light regions of the canopy, is the foundation for high levels of production. Strategies to lift flower number and light are complicated by trade‐offs inherent in tree architecture and orchard design. However, fruit set would be lifted above that achieved by current practice by an increase in the pollination rate of flowers.

     

Vertical distribution of Hymenophyllaceae species among host tree microhabitats in a temperate rain forest in Southern Chile

Question: Are differences in microhabitat preferences of co‐occurring epiphytic Hymenophyllaceae species (filmy ferns) correlated with differences in ecophysiological responses to light availability and humidity in the host tree? Location: The Andean foothills in south‐central Chile. Methods: We evaluated the distribution pattern of nine filmy fern species in microhabitats that differ in light availability and humidity in four host tree species. A DCA was developed to assess Hymenophyllaceae species microhabitat preference in terms of canopy openness (CO) and relative humidity. We assessed whether differences in chlorophyll content, maximum photochemical efficiency (Fv/Fm), photosynthetic capacity (A_max), evapotranspiration (E) and instantaneous water use efficiency (WUE) are consistent with any pattern. Results: CO and relative humidity differed significantly with height in the host trees. While CO increased with height in a host tree, relative humidity decreased. DCA analysis showed that filmy fern species distribution within and among trees was mainly explained by the relative humidity of the microhabitat. Chlorophyll content, chlorophyll a/b ratio, A_max and E differed significantly among filmy fern species. A_max and E were correlated with axis 1 scores from the DCA analysis. Conclusions: The vertical distribution and abundance of filmy fern species in Chilean temperate rain forest seems to be closely related to the different microhabitats offered by host trees. This pattern may reflect interspecific differences in ecophysiological traits related both to light availability and humidity. Our results suggest that humidity is the main environmental factor driving functional responses and habitat preferences of these filmy fern species.     

Enantiomeric NMR signal separation behavior and mechanism of samarium(III) and neodymium(III) complexes with (S,S)‐ethylenediamine‐N,N'‐disuccinate

Enantiomeric ¹H and ¹³C NMR signal separation behaviors of various α‐amino acids and DL‐tartarate were investigated by using the samarium(III) and neodymium(III) complexes with (S ,S )‐ethylenediamine‐N ,N' ‐disuccinate as chiral shift reagents. A relatively smaller concentration ratio of the lanthanide(III) complex to substrates was suitable for the neodymium(III) complex compared with the samarium(III) one, striking a balance between relatively greater signal separation and broadening. To clarify the difference in the signal separation behavior, the chemical shifts of β‐protons for fully bound D‐ and L‐alanine (δ_b(D) and δ_b(L)) and their adduct formation constants (K s) were obtained for both metal complexes. Preference for D‐alanine was similarly observed for both complexes, while it was revealed that the difference between the δ_b(D) and δ_b(L) values is the significant factor to determine the enantiomeric signal separation. The neodymium(III) and samarium(III) complexes can be used complementarily for higher and smaller concentration ranges of substrates, respectively, because the neodymium(III) complex gives the larger difference between the δ_b(D) and δ_b(L) values with greater signal broadening compared to the samarium(III) complex.     

Long‐term mortality patterns of the deep‐rooted Acacia erioloba: The middle class shall die!

Question: Is there a relationship between size and death in the long‐lived, deep‐rooted tree, Acacia erioloba, in a semi‐arid savanna? What is the size‐class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments? Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa. Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long‐term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001. Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov‐Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 ‐ 30 m², and declined in larger size‐classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too. Conclusion: A. erioloba mortality was size‐dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi‐arid environment.     

In Vitro and In Vivo Evaluation of N‐{2‐[4‐(3‐Cyanopyridin‐2‐yl)piperazin‐1‐yl]ethyl}‐3‐[11C]methoxybenz­amide,a Positron Emission Tomography (PET) Radioligand for Dopamine D4 Receptors,in Rodents

The D₄ dopamine receptor belongs to the D₂‐like family of dopamine receptors, and its exact regional distribution in the central nervous system is still a matter of considerable debate. The availability of a selective radioligand for the D₄ receptor with suitable properties for positron emission tomography (PET) would help resolve issues of D₄ receptor localization in the brain, and the presumed diurnal change of expressed protein in the eye and pineal gland. We report here on in vitro and in vivo characteristics of the high‐affinity D₄ receptor‐selective ligand N‐{2‐[4‐(3‐cyanopyridin‐2‐yl)piperazin‐1‐yl]ethyl}‐3‐[¹¹C]methoxybenzamide ([¹¹C] 2 ) in rat. The results provide new insights on the in vitro properties that a brain PET dopamine D₄ radioligand should possess in order to have improved in vivo utility in rodents.     

Increased forest carbon storage with increased atmospheric CO2 despite nitrogen limitation: a game‐theoretic allocation model for trees in competition for nitrogen and light

Changes in resource availability often cause competitively driven changes in tree allocation to foliage, wood, and fine roots, either via plastic changes within individuals or through turnover of individuals with differing strategies. Here, we investigate how optimally competitive tree allocation should change in response to elevated atmospheric CO₂ along a gradient of nitrogen and light availability, together with how those changes should affect carbon storage in living biomass. We present a physiologically‐based forest model that includes the primary functions of wood and nitrogen. From a tree's perspective, wood is an offensive and defensive weapon used against neighbors in competition for light. From a biogeochemical perspective, wood is the primary living reservoir of stored carbon. Nitrogen constitutes a tree's photosynthetic machinery and the support systems for that machinery, and its limited availability thus reduces a tree's ability to fix carbon. This model has been previously successful in predicting allocation to foliage, wood, and fine roots along natural productivity gradients. Using game theory, we solve the model for competitively optimal foliage, wood, and fine root allocation strategies for trees in competition for nitrogen and light as a function of CO₂ and nitrogen mineralization rate. Instead of down‐regulating under nitrogen limitation, carbon storage under elevated CO₂ relative to carbon storage at ambient CO₂ is approximately independent of the nitrogen mineralization rate. This surprising prediction is a consequence of both increased competition for nitrogen driving increased fine root biomass and increased competition for light driving increased allocation to wood under elevated CO₂.     

Drought‐related tree death of savanna eucalypts: Species susceptibility,soil conditions and root architecture

Questions: For eucalypt savanna in northeast Australia subject to multi‐year rainfall deficits this paper asks whether (1) dominant tree species (Ironbarks, Boxes) are more drought susceptible than the sub‐dominant Bloodwoods; (2) whether soil moisture is beyond wilting point in surface soil layers but available at depth; (3) soil conditions (moisture availability and texture) are related to tree death during drought; (4) the root systems of the Boxes and Ironbarks are shallower than the Bloodwoods; and the survivors of drought within species have deeper root systems than those that died. Location: Central Queensland, Australia. Methods: Patterns of tree death between eucalypt species were compared from field data collected after drought. Soil conditions during drought were described and compared with patterns of tree death for the Ironbark Eucalyptus melanophloia. The basal area and orientation of coarse roots were measured on upturned trees after broad‐scale tree clearing, and compared between species, and between live and dead trees with tree size as a covariate. Results: Drought‐induced tree death was higher for dominant Ironbark‐Box than for sub‐dominant Bloodwoods. During a moderate to severe drought in 2004, 41% of 100 cm deep subsoils had soil matric potential less than‐5600 kPa. The drought hardy Bloodwoods had a greater root basal area and particularly so for vertical roots compared to the drought sensitive Ironbark‐Box. Within species there was no significant difference in root basal area characteristics between trees that were recently killed by drought and those that remained relatively healthy. Surface soil moisture availability was lower where tree densities were high, and tree death increased as surface soil moisture became less available. Tree death was also greater as the clay content of sub‐soils increased. Discussion: The study suggests species with roots confined to upper soil layers will suffer severe water stress. The results strongly indicate that root architecture, and the way it facilitates water use during drought, is important for the relative dominance of the tree species. Patchiness in drought‐induced tree death seems to be at least partially a product of heterogeneity in sub‐soil conditions and competition for soil moisture.     

Tree shape plasticity in relation to crown exposure

Trees outside closed forest stands differ in the relation between stem diameter, height and crown volume from trees that grew with neighbours close by. Whether this plasticity in tree shape varies between species in relation to their light requirement is unknown. We purposefully sampled 528 trees ranging 5–100?cm diameter at breast height growing in a range of light conditions. Across ten broad-leaved species observed in Sumatra or Kalimantan, a generic relationship was found between light exposure of the crown and a light-dependent a _l parameter that modifies the height–diameter allometric equation (H?=?a _l D ^b ) from those for closed stands. In our results, vertical stretching is well predicted by light availability. In fully open conditions, trees are on average 31% shorter for the same diameter than under (partial) shade. Most of the stretching response occurs in all species as soon as some degree of lateral shading occurs. The response, however, varies by species (8–44% reduction) in a way apparently unrelated to species’ successional status. Crown volume varied less than stem height in its relationship with stem diameter across all light conditions tested. The scaling of crown volume with stem diameter, however, differed markedly between tree species.     

Strong overestimation of water‐use efficiency responses to rising CO2 in tree‐ring studies

The carbon isotope ratio (δ¹³C) in tree rings is commonly used to derive estimates of the assimilation‐to‐stomatal conductance rate of trees, that is, intrinsic water‐use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO₂ concentrations (C_a), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree‐ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C_a. We found that increases in tree size and stand age greatly influenced iWUE. The effect of C_a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C_a, mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO₂ effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO₂ effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.     

Disparate effects of global‐change drivers on mountain conifer forests: warming‐induced growth enhancement in young trees vs. CO2 fertilization in old trees from wet sites

Theory predicts that the postindustrial rise in the concentration of CO₂ in the atmosphere (c_a) should enhance tree growth either through a direct fertilization effect or indirectly by improving water use efficiency in dry areas. However, this hypothesis has received little support in cold‐limited and subalpine forests where positive growth responses to either rising c_a or warmer temperatures are still under debate. In this study, we address this issue by analyzing an extensive dendrochronological network of high‐elevation Pinus uncinata forests in Spain (28 sites, 544 trees) encompassing the whole biogeographical extent of the species. We determine if the basal area increment (BAI) trends are linked to climate warming and increased c_a by focusing on region‐ and age‐dependent responses. The largest improvement in BAI over the past six centuries occurred during the last 150 years affecting young trees and being driven by recent warming. Indeed, most studied regions and age classes presented BAI patterns mainly controlled by temperature trends, while growing‐season precipitation was only relevant in the driest sites. Growth enhancement was linked to rising c_a in mature (151–300 year‐old trees) and old‐mature trees (301–450 year‐old trees) from the wettest sites only. This finding implies that any potential fertilization effect of elevated c_a on forest growth is contingent on tree features that vary with ontogeny and it depends on site conditions (for instance water availability). Furthermore, we found widespread growth decline in drought‐prone sites probably indicating that the rise in c_a did not compensate for the reduction in water availability. Thus, warming‐triggered drought stress may become a more important direct driver of growth than rising c_a in similar subalpine forests. We argue that broad approaches in biogeographical and temporal terms are required to adequately evaluate any effect of rising c_a on forest growth.     

The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G_Si) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G_Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G_Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q_o). Under conditions of D = 1 kPa and saturating Q_o, G_Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood‐to‐leaf area ratio (A_S:A_L) doubled. A simple hydraulic model explained the variation in G_Si based on an inverse relationship with height, and a linear relationship with A_S:A_L. Thus, in F. sylvatica, adjustments in A_S:A_L partially compensate for the negative effect of increased flow‐path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.     

Different patterns of biomass allocation of mature and sapling host tree in response to liana competition in the southern temperate rainforest

Lianas can negatively affect their host tree. The evidence comes from studies in tropical forests, where lianas decrease the growth rate and reproduction of their host tree. This is primarily a consequence of water and nutrient competition, two limiting factors in tropical forests. In contrast, for some areas of southern temperate rainforests the competition for these resources could be less severe, because of the high rainfall and fertile soils. But so far, no study has determined the effect of liana competition over their host tree in southern temperate rainforests. The aim of this study was to evaluate, in field conditions, the effect of liana Cissus striata (Ruiz & Pavon) competition over the growth rate of mature Nothofagus obliqua (Bidr Egefam) host tree. In an experimental approach, we determined whether above‐ and/or below‐ground competition is more important in this interaction. We also looked for compensatory strategies that would allow to trees to respond to liana competition. In field conditions, we found that infested trees have a decrease in their relative growth rate of 26% and a reduction of the leaf area index (LAI) of 20% compared with control trees. In the greenhouse experiment, we found that saplings were water stressed and that there was light competition. Neither competition for water nor light had a significant effect on the growth rate of infested saplings. This could be explained because saplings showed compensatory strategies in response to competition. These strategies were based in the biomass distribution between organs (leaf area, slenderness index) and within leaves (LMA). In conclusion, we found that C. striata has a negative effect over the growth of mature and sapling N. obliqua host trees. This was a consequence of above‐ground and below‐ground competition, but we cannot disentangle which type of competition is more important. Trees respond to liana competition, mature host trees change the canopy architecture and saplings allocate resource between and within organs, which allows them to optimize resource capture.     

Colonization of disturbed Scots pine trees by bark‐ and wood‐boring beetles

相似文献   

Distinct seasonal dynamics of responses to elevated CO2 in two understorey grass species differing in shade‐tolerance

相似文献   

Growth, carbon-isotope discrimination, and drought-associated mortality across a Pinus ponderosa elevational transect

Drought‐ and insect‐associated tree mortality at low‐elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine (Pinus ponderosa) elevation transect that experienced low‐elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole‐crown gas exchange was indexed via estimates of the CO₂ partial pressure difference between leaf and atmosphere (p_a?p_c) derived from tree ring carbon isotope ratios (δ¹³C), corrected for temporal trends in atmospheric CO₂ and δ¹³C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, p_a?p_c, and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of p_a?p_c to climate, and a steep relationship between p_a?p_c and BAI/BA. The p_a?p_c results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low‐elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid‐latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands.     

Differential effects of emergent Nothofagus dombeyi on growth and basal area of canopy species in an old‐growth temperate rainforest

Question: Does overyielding of tree species mixtures in vertically stratified forests depend on complementary light use? Location: Andes of south‐central Chile. Methods: Basal area data were obtained from 80 circular plots distributed regularly throughout old‐growth stands with an emergent Nothofagus dombeyi tier over a canopy composed mainly of Laureliopsis philippiana and Saxegothaea conspicua. Radial growth was measured from cores obtained from trees at the centre of each plot. The effects of competition on growth were evaluated through a competition index (CI) based on distances to and diameters of the two nearest neighbours. Results: Overall, basal area of the canopy species was only weakly affected by the number of N. dombeyi per plot, and with basal area of N. dombeyi. However, the two main canopy species responded differently: whereas basal area of S. conspicua was negatively correlated with that of N. dombeyi, that of L. philippiana showed no response. Radial growth of S. conspicua was negatively correlated with CI calculated from canopy trees and more weakly so from emergent N. dombeyi. In contrast, radial growth of L. philippiana was not affected by competition with either canopy or emergent neighbours. Conclusions: Results indicate that emergent N. dombeyi tend to depress growth and basal area of S. conspicua, but not of the more shade‐tolerant L. philippiana. This supports the proposal that enhancement of wood production in stratified mixtures will be greatest when component species have strongly contrasting light use traits.     

The role of microsite sunlight environment on growth,architecture, and resource allocation in dominant Acacia tree seedlings,in Serengeti,East Africa

Seedling establishment is a critical life history stage for savanna tree recruitment due to variability in resource availability. While tree–grass competition for water is recognized as an important driver of tree seedling mortality, the importance of sunlight exposure on tree seedling performance has received little attention in savanna ecosystems despite variable seedling light environments caused by heterogeneity in biomass of the grass canopy. We studied the seasonal sunlight micro-environment for two dominant East African tree species (Acacia?=?Vachellia) robusta (Burch) and A. tortilis (Forssk) under natural field conditions. In the Serengeti National Park, Tanzania, A. robusta trees occur in tall grasslands of the north (shady) and A. tortilis in the southern short grasslands (less shaded). We also designed a greenhouse experiment to quantify sunlight effects on seedling growth, architecture, and resource allocation traits. In the field, A. robusta seedlings were associated with lower understorey sunlight during the wet season compared to A. tortilis, with this trend switching during the dry season. In the greenhouse experiments, under low sunlight (25% radiation), A. robusta gained height faster than A. tortilis and self-shading among canopy leaves was evident in A. tortilis but not A. robusta. Biomass allocation to leaves, stems, and roots differed between species under different light environments suggesting phenotypic plasticity in response to variable light availability. Our study suggests that microsite light variability should be incorporated in models of the spatial and temporal variability of savanna tree recruitment.

     

The impact of vessel size on vulnerability curves: data and models for within‐species variability in saplings of aspen,Populus tremuloides Michx

The objective of this study was to quantify the relationship between vulnerability to cavitation and vessel diameter within a species. We measured vulnerability curves (VCs: percentage loss hydraulic conductivity versus tension) in aspen stems and measured vessel‐size distributions. Measurements were done on seed‐grown, 4‐month‐old aspen (Populus tremuloides Michx) grown in a greenhouse. VCs of stem segments were measured using a centrifuge technique and by a staining technique that allowed a VC to be constructed based on vessel diameter size‐classes (D). Vessel‐based VCs were also fitted to Weibull cumulative distribution functions (CDF), which provided best‐fit values of Weibull CDF constants (c and b) and P₅₀ = the tension causing 50% loss of hydraulic conductivity. We show that P₅₀ = 6.166D^?0.3134 (R² = 0.995) and that b and 1/c are both linear functions of D with R² > 0.95. The results are discussed in terms of models of VCs based on vessel D size‐classes and in terms of concepts such as the ‘pit area hypothesis’ and vessel pathway redundancy.