Hydraulic architecture of deciduous and evergreen dry rainforest tree species from north-eastern Australia

Hydraulic conductivity and xylem anatomy were examined in stems of two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., and two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., from a seasonally dry rainforest in north Queensland, Australia. The deciduous species possessed hydraulic architecture typical of drought-sensitive plants, i.e. low wood density, wider xylem vessels, higher maximal rates of sapwood specific hydraulic conductivity (K_s) and high vulnerability to drought-induced embolism. In contrast, the evergreen species had lower rates of K_h and leaf specific conductivity (K_L) but were less susceptible to embolism. The evergreen species experienced leaf water potentials <–4.0 MPa during the dry season, while the deciduous species shed their leaves before leaf water potentials declined below –2.0 MPa. Thus, the hydraulic architecture of the evergreens allows them to withstand the greater xylem pressure gradients required to maintain water transport to the canopy during the dry season. Our results are consistent with observations made in neotropical dry forests and demonstrate that drought-deciduous species with low wood density and high water storage capacity are likely to be more hydraulically efficient, but more vulnerable to embolism, than coexisting evergreens.     

Hydraulic properties and photosynthetic rates in co-occurring lianas and trees in a seasonal tropical rainforest in southwestern China

In this study, we examined wood anatomy, hydraulic properties, photosynthetic rate, and water status and osmotic regulation in three liana species and three tree species co-occurring in a seasonal tropical rain forest. Our results showed that the three liana species had larger vessel diameter, lower sapwood density, and consequently higher branch sapwood specific hydraulic conductivity (K _S) than the three tree species. Across species, K _S was positively correlated with leaf nitrogen concentration and maximum net CO₂ assimilation rate. However, it was also positively correlated with xylem water potential at 50% loss of hydraulic conductivity, indicating a trade-off between hydraulic efficiency and safety. Compared to the tree species, the liana species had higher predawn leaf water potential and lower osmotic adjustment in the dry season. The combination of more efficient water transport, higher photosynthetic rates, and their ability to access to more reliable water source at deeper soil layers in the dry season in the lianas should contribute to their fast growth.     

Independence of stem and leaf hydraulic traits in six Euphorbiaceae tree species with contrasting leaf phenology

Hydraulic traits and hydraulic-related structural properties were examined in three deciduous (Hevea brasiliensis, Macaranga denticulate, and Bischofia javanica) and three evergreen (Drypetes indica, Aleurites moluccana, and Codiaeum variegatum) Euphorbiaceae tree species from a seasonally tropical forest in south-western China. Xylem water potential at 50% loss of stem hydraulic conductivity (P50_stem) was more negative in the evergreen tree, but leaf water potential at 50% loss of leaf hydraulic conductivity (P50_leaf) did not function as P50_stem did. Furthermore, P50_stem was more negative than P50_leaf in the evergreen tree; contrarily, this pattern was not observed in the deciduous tree. Leaf hydraulic conductivity overlapped considerably, but stem hydraulic conductivity diverged between the evergreen and deciduous tree. Correspondingly, structural properties of leaves overlapped substantially; however, structural properties of stem diverged markedly. Consequently, leaf and stem hydraulic traits were closely correlated with leaf and stem structural properties, respectively. Additionally, stem hydraulic efficiency was significantly correlated with stem hydraulic resistance to embolism; nevertheless, such a hydraulic pattern was not found in leaf hydraulics. Thus, these results suggest: (1) that the evergreen and deciduous tree mainly diverge in stem hydraulics, but not in leaf hydraulics, (2) that regardless of leaf or stem, their hydraulic traits result primarily from structural properties, and not from leaf phenology, (3) that leaves are more vulnerable to drought-induced embolism than stem in the evergreen tree, but not always in the deciduous tree and (4) that there exists a trade-off between hydraulic efficiency and safety for stem hydraulics, but not for leaf hydraulics.     

Hydraulic properties and freezing-induced cavitation in sympatric evergreen and deciduous oaks with contrasting habitats

We investigated the hydraulic properties in relation to soil moisture, leaf habit, and phylogenetic lineage of 17 species of oaks (Quercus) that occur sympatrically in northern central Florida (USA). Leaf area per shoot increased and Huber values (ratio of sapwood area to leaf area) decreased with increasing soil moisture of species’ habitats. As a result, maximum hydraulic conductance and maximum transpiration were positively correlated with mean soil moisture when calculated on a sapwood area basis, but not when calculated on a leaf area basis. This reveals the important role that changes in allometry among closely related species can play in co‐ordinating water transport capacity with soil water availability. There were significant differences in specific conductivity between species, but these differences were not explained by leaf habit or by evolutionary lineage. However, white oaks had significantly smaller average vessel diameters than red oaks or live oaks. Due to their lower Huber values, maximum leaf specific conductivity (K_L) was higher in evergreen species than in deciduous species and higher in live oaks than in red oaks or white oaks. There were large differences between species and between evolutionary lineages in freeze–thaw‐induced embolism. Deciduous species, on average, showed greater vulnerability to freezing than evergreen species. This result is strongly influenced by evolutionary lineage. Specifically, white oaks, which are all deciduous, had significantly higher vulnerability to freezing than live oaks (all evergreen) and red oaks, which include both evergreen and deciduous species. These results highlight the importance of taking evolutionary lineage into account in comparative physiological studies.     

Phenology and water relations of tree sprouts and seedlings in a tropical deciduous forest of South India

The phenology of sprouts (>1 year old, up to 1.5 m in height) and seedlings (<1 year old) of six woody species (four deciduous, one brevi-deciduous, and one evergreen) was examined during the dry season in a tropical deciduous forest of South India. Xylem water potential (_x), leaf relative water content (RWC; % turgid weight), and xylem specific conductivity (K _S; kg s^–1 m^–1 MPa^–1) of sprouts were measured on two occasions during the dry season. In addition, K _S of seedlings (<1 year old) of one deciduous and one evergreen species was determined to allow comparison with sprouts. _x of deciduous species was significantly higher at the second sampling date and was accompanied by a significant increase in K _S and RWC, while the brevi-deciduous and evergreen species did not show any difference in _x. Seedlings of Terminalia crenulata (deciduous) and Ixora parviflora (evergreen) had significantly lower K _S compared to sprouts, while seedlings of all four deciduous species shed their leaves much earlier in the dry season than did conspecific sprouts. More favorable water relations of sprouts compared to seedlings during the peak of the dry season may explain the lower rates of die-back and mortality of sprouts observed in dry deciduous forests of India.

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This revised version was published online in May 2005 with corrections to Received-/Accepted-dates.     

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s) and leaf hydraulic conductance (K_Leaf) across species. All species had similar maximum photosynthetic rates (A_max). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.     

Midday stomatal conductance is more related to stem rather than leaf water status in subtropical deciduous and evergreen broadleaf trees

Midday depressions in stomatal conductance (g_s) and photosynthesis are common in plants. The aim of this study was to understand the hydraulic determinants of midday g_s, the coordination between leaf and stem hydraulics and whether regulation of midday g_s differed between deciduous and evergreen broadleaf tree species in a subtropical cloud forest of Southwest (SW) China. We investigated leaf and stem hydraulics, midday leaf and stem water potentials, as well as midday g_s of co‐occurring deciduous and evergreen tree species. Midday g_s was correlated positively with midday stem water potential across both groups of species, but not with midday leaf water potential. Species with higher stem hydraulic conductivity and greater daily reliance on stem hydraulic capacitance were able to maintain higher stem water potential and higher g_s at midday. Deciduous species exhibited significantly higher stem hydraulic conductivity, greater reliance on stem capacitance, higher stem water potential and g_s at midday than evergreen species. Our results suggest that midday g_s is more associated with midday stem than with leaf water status, and that the functional significance of stomatal regulation in these broadleaf tree species is probably for preventing stem xylem dysfunction.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

A Cost-Benefit Analysis of Leaves of Eight Australian Savanna Tree Species of Differing Leaf Life-Span

Cost-benefit analysis of foliar construction and maintenance costs and of carbon assimilation of leaves of differing life-span were conducted using two evergreen, three semi-deciduous, and three deciduous tree species of savannas of north Australia. Rates of radiant-energy-saturated CO₂ assimilation (P _max) and dark respiration were measured and leaves were analysed for total nitrogen, fat, and ash concentrations, and for heat of combustion. Specific leaf area, and leaf N and ash contents were significantly lower in longer-lived leaves (evergreen) than shorter-lived leaves (deciduous) species. Leaves of evergreen species also had significantly higher heat of combustion and lower crude fat content than leaves of deciduous species. On a leaf area basis, P _max was highest in leaves of evergreen species, but on a leaf dry mass basis it was highest in leaves of deciduous species. P _max and total Kieldahl N content were linearly correlated across all eight species, and foliar N content was higher in leaves of deciduous than evergreen species. Leaf construction cost was significantly higher and maintenance costs were lower for leaves of evergreen than deciduous species. Maintenance and construction costs were linearly related to each other across all species. Leaves of evergreen species had a higher cost-benefit ratio compared to leaves of deciduous species but with longer lived leaves, the payback interval was longer in evergreen than deciduous species. These results support the hypotheses that longer lived leaves are more expensive to construct than short-lived leaves, and that a higher investment of N into short-lived leaves occurs which supports a higher P _max over a shorter payback interval. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydraulic and photosynthetic co-ordination in seasonally dry tropical forest trees

In the present study the linkage between hydraulic, photosynthetic and phenological properties of tropical dry forest trees were investigated. Seasonal patterns of stem‐specific conductivity (K_SP) described from 12 species, including deciduous, brevi‐deciduous and evergreen species, indicated that only evergreen species were consistent in their response to a dry‐to‐wet season transition. In contrast, K_SP in deciduous and brevi‐deciduous species encompassed a range of responses, from an insignificant increase in K_SP following rains in some species, to a nine‐fold increase in others. Amongst deciduous species, the minimum K_SP during the dry season ranged from 6 to 56% of wet season K_SP, indicating in the latter case that a significant portion of the xylem remained functional during the dry season. In all species and all seasons, leaf‐specific stem conductivity (K_L) was strongly related to the photosynthetic capacity of the supported foliage, although leaf photosynthesis became saturated in species with high K_L. The strength of this correlation was surprising given that much of the whole‐plant resistance appears to be in the leaves. Hydraulic capacity, defined as the product of K_L and the soil–leaf water potential difference, was strongly correlated with the photosynthetic rate of foliage in the dry season, but only weakly correlated in the wet season.     

Leaf phenology is associated with soil water availability and xylem traits in a tropical dry forest

In tropical dry forests, spatial heterogeneity in soil water availability is thought to determine interspecific differences in key components of resource use strategies, such as leaf phenology and xylem function. To understand the environmental drivers of variation in leaf phenology and xylem function, we explored the relation of soil water potential to topographic metrics derived from a digital elevation model. Subsequently, we compared nine xylem hydraulic, mechanical and storage traits in 18 species in three phenological classes (readily deciduous, tardily deciduous, and evergreen) in the dry tropical forest of Chamela, Mexico. Soil water potential was negatively correlated with elevation, insolation and water flow accumulation. Evergreen species characterized low-elevation moist sites, whereas deciduous species dominated hills and dry sites. Overall, evergreen species had lower xylem specific conductivity than deciduous species, and tardily deciduous species were different from readily deciduous and evergreen species in five of eight xylem traits. In dry tropical forests, water availability promotes divergence in leaf phenology and xylem traits, ranging from low wood density, evergreen species in moist sites to a combination of low wood density, readily deciduous species plus high wood density, tardily deciduous species in dry sites.     

Functional coordination between branch hydraulic properties and leaf functional traits in miombo woodlands: implications for water stress management and species habitat preference

We investigated functional coordination between branch hydraulic properties and leaf functional traits among nine miombo woodlands canopy tree species differing in habitat preference and phenology. Specifically, we were seeking to answer the question: are branch hydraulic properties coordinated with leaf functional traits linked to plant drought tolerance in seasonally dry tropical forests and what are the implications for species habitat preference? The hydraulic properties investigated in this study were stem area specific hydraulic conductivity (K _S), Huber value (H _v), and xylem cavitation vulnerability (??₅₀). The leaf functional traits measured were specific leaf area (SLA), leaf dry matter content (LDMC), and mean leaf area (MLA). Generalists displayed significantly (P?<?0.05) higher cavitation resistance (??₅₀) and SLA, but lower sapwood specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), MLA, and LDMC than mesic specialists. Although MLA was uncorrelated with ??₅₀, we found significant (P?<?0.05) positive and negative correlations between plant hydraulic properties and leaf functional traits linked to plant drought tolerance ability, indicating that the interactions between branch hydraulics and leaf functional traits related to plant drought tolerance ability may influence tree species habitat preference in water-limited ecosystems.     

A Cost-Benefit Analysis of Leaves of Eight Australian Savanna Tree Species of Differing Leaf Life-Span

Cost-benefit analysis of foliar construction and maintenance costs and of carbon assimilation of leaves of differing life-span were conducted using two evergreen, three semi-deciduous, and three deciduous tree species of savannas of north Australia. Rates of radiant-energy-saturated CO₂ assimilation (P _max) and dark respiration were measured and leaves were analysed for total nitrogen, fat, and ash concentrations, and for heat of combustion. Specific leaf area, and leaf N and ash contents were significantly lower in longer-lived leaves (evergreen) than shorter-lived leaves (deciduous) species. Leaves of evergreen species also had significantly higher heat of combustion and lower crude fat content than leaves of deciduous species. On a leaf area basis, P _max was highest in leaves of evergreen species, but on a leaf dry mass basis it was highest in leaves of deciduous species. P _max and total Kieldahl N content were linearly correlated across all eight species, and foliar N content was higher in leaves of deciduous than evergreen species. Leaf construction cost was significantly higher and maintenance costs were lower for leaves of evergreen than deciduous species. Maintenance and construction costs were linearly related to each other across all species. Leaves of evergreen species had a higher cost-benefit ratio compared to leaves of deciduous species but with longer lived leaves, the payback interval was longer in evergreen than deciduous species. These results support the hypotheses that longer lived leaves are more expensive to construct than short-lived leaves, and that a higher investment of N into short-lived leaves occurs which supports a higher P _max over a shorter payback interval.     

Photosynthetic pathway alters hydraulic structure and function in woody plants

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (K_s ) was 16%–98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher K_s values compared with three C₄ species. Leaf specific conductivity (K_L ; hydraulic conductivity per leaf area) was 60%–98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO₂ variation, may also affect the evolution of wood structure and function.     

哀牢山中山湿性常绿阔叶林木径向生长季节动态及其对气候因子的响应

研究采用树木生长环在哀牢山中山湿性常绿阔叶林持续9年(2009—2017年)监测了2个常绿树种(厚皮香,Ternstroemia gymnanthera;南亚枇杷,Eriobotrya bengalensis)和2个落叶树种(西桦,Betula alnoides;珍珠花,Lyonia ovalifolia)的树干月生长量,采用逻辑斯蒂生长模型(Logistic model)模拟树木径向生长量和物候参数,并分析了年、季尺度上径向生长与主要气候因子的关系。结果表明:1)4个树种年平均生长量为6.3 mm,落叶树种年平均生长量(10.6 mm/a)显著高于常绿树种(3.0 mm/a);2)雨季(5—10月)是哀牢山中山湿性常绿阔叶林树木生长的主要时期,4个树种雨季平均生长量为5.9 mm,占全年总生长量的93%,其中落叶树种雨季生长量占全年的96%,而常绿树种雨季生长量占全年的86%;3)常绿树种生长季长度为169天,长于落叶树种(137天),而落叶树种最大生长速率(0.14 mm/d)显著高于常绿树种(0.03 mm/d),最大径向生长速率能很好地预测树种年生长量;4)低温、雾日和光合有效...     

Non-structural carbon compounds in temperate forest trees

The current carbon supply status of temperate forest trees was assessed by analysing the seasonal variation of non‐structural carbohydrate (NSC) concentrations in leaves, branch wood and stem sapwood of 10 tree species (six deciduous broad‐leafed, one deciduous conifer and three evergreen conifer trees) in a temperate forest that is approximately 100 years old. In addition, all woody tissue was analysed for lipids (acylglycerols). The major NSC fractions were starch, sucrose, glucose and fructose, with other carbohydrates (e.g. raffinose and stachyose) and sugar alcohols (cyclitols and sorbitol) playing only a minor quantitative role. The radial distribution of NSC within entire stem cores, assessed here for the first time in a direct interspecific comparison, revealed large differences in the size of the active sapwood fraction among the species, reflecting the specific wood anatomy (ring‐porous versus diffuse‐porous xylem). The mean minimum NSC concentrations in branch wood during the growing season was 55% of maximum, and even high NSC concentrations were maintained during times of extensive fruit production in masting Fagus sylvestris. The NSC in stem sapwood varied very little throughout the season (cross species mean never below 67% of maximum), and the small reductions observed were not significant for any of the investigated species. Although some species contained substantial quantities of lipids in woody tissues (‘fat trees’; Tilia, Pinus, Picea, Larix), the lipid pools did not vary significantly across the growing season in any species. On average, the carbon stores of deciduous trees would permit to replace the whole leave canopy four times. These data imply that there is not a lot of leeway for a further stimulation of growth by ongoing atmospheric CO₂ enrichment. The classical view that deciduous trees rely more on C‐reserves than evergreen trees, seems unwarranted or has lost its justification due to the greater than 30% increase in atmospheric CO₂ concentrations over the last 150 years.     

Xylem cavitation vulnerability influences tree species’ habitat preferences in miombo woodlands

Although precipitation plays a central role in structuring Africa’s miombo woodlands, remarkably little is known about plant-water relations in this seasonally dry tropical forest. Therefore, in this study, we investigated xylem vulnerability to cavitation for nine principal tree species of miombo woodlands, which differ in habitat preference and leaf phenology. We measured cavitation vulnerability (Ψ₅₀), stem-area specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), seasonal variation in predawn water potential (Ψ_PD) and xylem anatomical properties [mean vessel diameter, mean hydraulic diameter, mean hydraulic diameter accounting for 95 % flow, and maximum vessel length (V _L)]. Results show that tree species with a narrow habitat range (mesic specialists) were more vulnerable to cavitation than species with a wide habitat range (generalists). Ψ₅₀ for mesic specialists ranged between ?1.5 and ?2.2 MPa and that for generalists between ?2.5 and ?3.6 MPa. While mesic specialists exhibited the lowest seasonal variation in Ψ_PD, generalists displayed significant seasonal variations in Ψ_PD suggesting that the two miombo habitat groups differ in their rooting depth. We observed a strong trade-off between K _S and Ψ₅₀ suggesting that tree hydraulic architecture is one of the decisive factors setting ecological boundaries for principal miombo species. While vessel diameters correlated weakly (P > 0.05) with Ψ₅₀, V _L was positively and significantly correlated with Ψ₅₀. Ψ_PD was significantly correlated with Ψ₅₀ further reinforcing the conclusion that tree hydraulic architecture plays a significant role in species’ habitat preference in miombo woodlands.     

Coordination of leaf and stem water transport properties in tropical forest trees

Stomatal regulation of transpiration constrains leaf water potential (Ψ_L) within species-specific ranges that presumably avoid excessive tension and embolism in the stem xylem upstream. However, the hydraulic resistance of leaves can be highly variable over short time scales, uncoupling tension in the xylem of leaves from that in the stems to which they are attached. We evaluated a suite of leaf and stem functional traits governing water relations in individuals of 11 lowland tropical forest tree species to determine the manner in which the traits were coordinated with stem xylem vulnerability to embolism. Stomatal regulation of Ψ_L was associated with minimum values of water potential in branches (Ψ_br) whose functional significance was similar across species. Minimum values of Ψ_br coincided with the bulk sapwood tissue osmotic potential at zero turgor derived from pressure–volume curves and with the transition from a linear to exponential increase in xylem embolism with increasing sapwood water deficits. Branch xylem pressure corresponding to 50% loss of hydraulic conductivity (P ₅₀) declined linearly with daily minimum Ψ_br in a manner that caused the difference between Ψ_br and P ₅₀ to increase from 0.4 MPa in the species with the least negative Ψ_br to 1.2 MPa in the species with the most negative Ψ_br. Both branch P ₅₀ and minimum Ψ_br increased linearly with sapwood capacitance (C) such that the difference between Ψ_br and P ₅₀, an estimate of the safety margin for avoiding runaway embolism, decreased with increasing sapwood C. The results implied a trade-off between maximizing water transport and minimizing the risk of xylem embolism, suggesting a prominent role for the buffering effect of C in preserving the integrity of xylem water transport. At the whole-tree level, discharge and recharge of internal C appeared to generate variations in apparent leaf-specific conductance to which stomata respond dynamically.     

Ecological differentiation in xylem cavitation resistance is associated with stem and leaf structural traits

Cavitation resistance is a critical determinant of drought tolerance in tropical tree species, but little is known of its association with life history strategies, particularly for seasonal dry forests, a system critically driven by variation in water availability. We analysed vulnerability curves for saplings of 13 tropical dry forest tree species differing in life history and leaf phenology. We examined how vulnerability to cavitation (P₅₀) related to dry season leaf water potentials and stem and leaf traits. P₅₀‐values ranged from ?0.8 to ?6.2 MPa, with pioneers on average 38% more vulnerable to cavitation than shade‐tolerants. Vulnerability to cavitation was related to structural traits conferring tissue stress vulnerability, being negatively correlated with wood density, and surprisingly maximum vessel length. Vulnerability to cavitation was negatively related to the Huber‐value and leaf dry matter content, and positively with leaf size. It was not related to SLA. We found a strong trade‐off between cavitation resistance and hydraulic efficiency. Most species in the field were operating at leaf water potentials well above their P₅₀, but pioneers and deciduous species had smaller hydraulic safety margins than shade‐tolerants and evergreens. A trade‐off between hydraulic safety and efficiency underlies ecological differentiation across these tropical dry forest tree species.     

Photosynthetic pathway alters xylem structure and hydraulic function in herbaceous plants

Plants using the C₄ photosynthetic pathway have greater water use efficiency (WUE) than C₃ plants of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C₄ plants do not need to acquire and transport as much water as C₃ species. Because the structure of xylem tissue reflects hydraulic demand by the leaf canopy, a reduction in water transport requirements due to C₄ photosynthesis should affect the evolution of xylem characteristics in C₄ plants. In a comparison of stem hydraulic conductivity and vascular anatomy between eight C₃ and eight C₄ herbaceous species, C₄ plants had lower hydraulic conductivity per unit leaf area (K_L) than C₃ species of similar life form. When averages from all the species were pooled together, the mean K_L for the C₄ species was 1.60 × 10^?4 kg m^?1 s^?1 MPa^?1, which was only one‐third of the mean K_L of 4.65 × 10^?4 kg m^?1 s^?1 MPa^?1 determined for the C₃ species. The differences in K_L between C₃ and C₄ species corresponded to the two‐ to three‐fold differences in WUE observed between C₃ and C₄ plants. In the C₄ species from arid regions, the difference in K_L was associated with a lower hydraulic conductivity per xylem area, smaller and shorter vessels, and less vulnerable xylem to cavitation, indicating the C₄ species had evolved safer xylem than the C₃ species. In the plants from resource‐rich areas, such as the C₄ weed Amaranthus retroflexus, hydraulic conductivity per xylem area and xylem anatomy were similar to that of the C₃ species, but the C₄ plants had greater leaf area per xylem area. The results indicate the WUE advantage of C₄ photosynthesis allows for greater flexibility in hydraulic design and potential fitness. In resource‐rich environments in which competition is high, an existing hydraulic design can support greater leaf area, allowing for higher carbon gain, growth and competitive potential. In arid regions, C₄ plants evolved safer xylem, which can increase survival and performance during drought events.