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Leaf biochemical traits indicating early symptoms of plant stress can be assessed using imaging spectroscopy combined with radiative transfer modelling (RTM). In this study, we assessed the potential applicability of the leaf radiative transfer model Fluspect-Cx to simulate optical properties and estimate leaf biochemical traits through inversion of two native Australian eucalypt species: Eucalyptus dalrympleana and E. delegetensis. The comparison of measured and simulated optical properties revealed the necessity to recalibrate the refractive index and specific absorption coefficients of the eucalypt leaves’ biochemical constituents. Subsequent validation of the modified Fluspect-Cx showed a closer agreement with the spectral measurements. The average root mean square error (RMSE) of reflectance, transmittance and absorptance values within the wavelength interval of 450–1600 nm was smaller than 1%. We compared the performance of both the original and recalibrated Fluspect-Cx versions through inversions aiming to simultaneously retrieve all model inputs from leaf optical properties with and without prior information. The inversion of recalibrated Fluspect-Cx constrained by laboratory-based measurements produced a superior accuracy in estimations of leaf water content (RMSE = 0.0013 cm, NRMSE = 6.55%) and dry matter content (RMSE = 0.0036 g·cm−2, NRMSE = 21.28%). The estimation accuracies of chlorophyll content (RMSE = 8.46 µg·cm−2, NRMSE = 24.73%), carotenoid content (RMSE = 3.83 µg·cm−2, NRMSE = 30.82%) and anthocyanin content (RMSE = 1.69 µg·cm−2, NRMSE = 37.12%) were only marginally better than for the inversion without any constraints. Additionally, we investigated the possibility to substitute the prior information derived in the laboratory by non-destructive reflectance-based spectral indices sensitive to the retrieved biochemical traits, resulting in the most accurate estimation of carotenoid content (RMSE = 3.65 µg·cm−2, NRMSE = 29%). Future coupling of the

We introduce the AusTraits database - a compilation of values of plant traits for taxa in the Australian flora (hereafter AusTraits). AusTraits synthesises data on 448 traits across 28,640 taxa from field campaigns, published literature, taxonomic monographs, and individual taxon descriptions. Traits vary in scope from physiological measures of performance (e.g. photosynthetic gas exchange, water-use efficiency) to morphological attributes (e.g. leaf area, seed mass, plant height) which link to aspects of ecological variation. AusTraits contains curated and harmonised individual- and species-level measurements coupled to, where available, contextual information on site properties and experimental conditions. This article provides information on version 3.0.2 of AusTraits which contains data for 997,808 trait-by-taxon combinations. We envision AusTraits as an ongoing collaborative initiative for easily archiving and sharing trait data, which also provides a template for other national or regional initiatives globally to fill persistent gaps in trait knowledge.

© 2017 The Author(s). Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere-atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.

The surge in global efforts to understand the causes and consequences of drought on forest ecosystems has tended to focus on specific impacts such as mortality. We propose an ecoclimatic framework that takes a broader view of the ecological relevance of water deficits, linking elements of exposure and resilience to cumulative impacts on a range of ecosystem processes. This ecoclimatic framework is underpinned by two hypotheses: (i) exposure to water deficit can be represented probabilistically and used to estimate exposure thresholds across different vegetation types or ecosystems; and (ii) the cumulative impact of a series of water deficit events is defined by attributes governing the resistance and recovery of the affected processes. We present case studies comprising Pinus edulis and Eucalyptus globulus, tree species with contrasting ecological strategies, which demonstrate how links between exposure and resilience can be examined within our proposed framework. These examples reveal how climatic thresholds can be defined along a continuum of vegetation functional responses to water deficit regimes. The strength of this framework lies in identifying climatic thresholds on vegetation function in the absence of more complete mechanistic understanding, thereby guiding the formulation, application and benchmarking of more detailed modelling.

Ubiquitous, large diameter pores have not previously been adequately demonstrated to occur in leaf cuticles. Here we show conclusively that such structures occur in Eidothea zoexylocarya, a rainforest tree species of Proteaceae restricted to the Australian Wet Tropics. The pores are abundant, large-diameter apertures (∼1 lm), that extend perpendicularly most of the way through the cuticle from the inside. They occur on both sides of the leaf, but are absent from the cuticle associated with stomatal complexes on the abaxial side. No such pores were found in any other species, including the only other species of Eidothea, E. hardeniana from New South Wales, and other species that have previously been purported to possess cuticular pores. To determine whether these pores made the cuticles more leaky to water vapor, we measured astomatous cuticular conductances to water vapor for E. zoexylocarya and seven other Proteaceae species of the Wet Tropics. Cuticular conductance for E. zoexylocarya was relatively low, indicating that the prominent pores do not increase conductance. The function of the pores is currently obscure, but the presence of both pores and an adaxial hypodermis in E. zoexylocarya but not E. hardeniana suggests evolution in response to greater environmental stresses in the tropics.

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Ψleaf). Manipulating the transpiration rate in excised leaves enabled us to vary Ψleaf in the range −0.1 MPa to less than −1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Ψleaf remained above −1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Ψleaf led to a proportional decrease in Kleaf, such that midday Ψleaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Ψleaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Ψleaf at turgor loss and Ψleaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero.

The effect of freezing on stem xylem hydraulic conductivity and leaf chlorophyll a fluorescence was measured in 12 tree and shrub species from a treeline heath in Tasmania, Australia. Reduction in stem hydraulic conductivity after a single freeze-thaw cycle was minimal in conifers and the vessel-less angiosperm species Tasmannia lanceolata (Winteraceae), whereas mean loss of conductivity in vessel-forming angiosperms fell in the range 17-83%. A positive linear relationship was observed between percentage loss of hydraulic conductivity by freeze-thaw and the average conduit diameter across all 12 species. This supports the hypothesis that large-diameter vascular conduits have a greater likelihood of freeze-thaw cavitation because larger bubbles are produced, which are more likely to expand under tension. Leaf frost tolerances, as measured by a 50% loss of maximum PSII quantum yield, varied from -6 to -13°C, indicating that these species were more frost-sensitive than plants from northern hemisphere temperate forest and treeline communities. There was no evidence of a relationship between frost tolerance of leaves and the resilience of stem water transport to freezing, suggesting that low temperature survival and the resistance of stem water transport to freezing are independently evolving traits. The results of this study bear on the ecological importance of stem freezing in the southern hemisphere treeline zones.

1. Recent work on the hydraulic conductance of leaves suggests that maximum photosynthetic performance of a leaf is defined largely by its plumbing. Pursuing this idea, we tested how the diurnal course of gas exchange of trees in a dry tropical forest was affected by artificially depressing the hydraulic conductance of leaves (Kleaf). 2. Individual leaves from four tropical tree species were exposed to a brief episode of forced evaporation by blowing warm air over leaves in situ. Despite humid soil and atmospheric conditions, this caused leaf water potential (Ψleaf) to fall sufficiently to induce a 50–74% drop in Kleaf. 3. Two of the species sampled proved highly sensitive to artificially depressed Kleaf, leading to a marked and sustained decline in the instantaneous rate of CO2 uptake, stomatal conductance and transpiration. Leaves of these species showed a depression of hydraulic and photosynthetic capacity in response to the ‘blow-dry’ treatment similar to that observed when major veins in the leaf were severed. 4. By contrast, the other two species sampled were relatively insensitive to Kleaf manipulation; photosynthetic rates were indistinguishable from control (untreated) leaves 4 h after treatment. These insensitive species demonstrate a linear decline of Kleaf with Ψleaf, while Kleaf in the two sensitive species falls precipitously at a critical water deficit. 5. We propose that a sigmoidal Kleaf vulnerability enables a high diurnal yield of CO2 at the cost of exposing leaves to the possibility of xylem cavitation. Linear Kleaf vulnerability leads to a relatively lower CO2 yield, while providing better protection against cavitation.

The Tasmanian montane and rainforest conifer genus, Athrotaxis, provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, degree of imbricacy and stomatal distribution. Hybrid swarms in the field and glasshouse grown hybrid progeny are highly variable for these traits. In glasshouse grown plants of the true species and a diverse hybrid progeny, stomatal conductivity and density varied by about 200% among individuals with a strong correlation between these traits. Hybrids were found to produce lower stomatal densities and less discrimination of 13C than either species, leading to a negative relationship between stomatal density and delta 13C. In contrast to the highly variable stomatal densities and delta 13C in glasshouse plants, field plants were very conservative in both characters. Relatively low stomatal density and high water use efficiency in field plants suggests strong natural selection to optimize the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A. selaginoides was only 4-6 times as great as, and was strongly correlated with, conductance in the dark. This suggests either incomplete stomatal closure or greater cuticular conductance. In contrast, A. cupressoides appeared to be less "leaky", which may reflect adaptation to its more exposed habitat.

Leaf veins are almost ubiquitous across the range of terrestrial plant diversity, yet their influence on leaf photosynthetic performance remains uncertain. We show here that specific physical attributes of the vascular plumbing network are key limiters of the hydraulic and photosynthetic proficiency of any leaf. Following the logic that leaf veins evolved to bypass inefficient water transport through living mesophyll tissue, we examined the hydraulic pathway beyond the distal ends of the vein system as a possible limiter of water transport in leaves. We tested a mechanistic hypothesis that the length of this final traverse, as water moves from veins across the mesophyll to where it evaporates from the leaf, governs the hydraulic efficiency and photosynthetic carbon assimilation of any leaf. Sampling 43 species across the breadth of plant diversity from mosses to flowering plants, we found that the post-vein traverse as determined by characters such as vein density, leaf thickness, and cell shape, was strongly correlated with the hydraulic conductivity and maximum photosynthetic rate of foliage. The shape of this correlation provided clear support for the a priori hypothesis that vein positioning limits photosynthesis via its influence on leaf hydraulic efficiency.

The Tasmanian montane and rainforest conifer genus, Athrotaxis, provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, degree of imbricacy and stomatal distribution. Hybrid swarms in the field and glasshouse grown hybrid progeny are highly variable for these traits. In glasshouse grown plants of the true species and a diverse hybrid progeny, stomatal conductivity and density varied by about 200% among individuals with a strong correlation between these traits. Hybrids were found to produce lower stomatal densities and less discrimination of 13C than either species, leading to a negative relationship between stomatal density and delta 13C. In contrast to the highly variable stomatal densities and delta 13C in glasshouse plants, field plants were very conservative in both characters. Relatively low stomatal density and high water use efficiency in field plants suggests strong natural selection to optimize the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A. selaginoides was only 4-6 times as great as, and was strongly correlated with, conductance in the dark. This suggests either incomplete stomatal closure or greater cuticular conductance. In contrast, A. cupressoides appeared to be less "leaky", which may reflect adaptation to its more exposed habitat.

Leaf veins are almost ubiquitous across the range of terrestrial plant diversity, yet their influence on leaf photosynthetic performance remains uncertain. We show here that specific physical attributes of the vascular plumbing network are key limiters of the hydraulic and photosynthetic proficiency of any leaf. Following the logic that leaf veins evolved to bypass inefficient water transport through living mesophyll tissue, we examined the hydraulic pathway beyond the distal ends of the vein system as a possible limiter of water transport in leaves. We tested a mechanistic hypothesis that the length of this final traverse, as water moves from veins across the mesophyll to where it evaporates from the leaf, governs the hydraulic efficiency and photosynthetic carbon assimilation of any leaf. Sampling 43 species across the breadth of plant diversity from mosses to flowering plants, we found that the post-vein traverse as determined by characters such as vein density, leaf thickness, and cell shape, was strongly correlated with the hydraulic conductivity and maximum photosynthetic rate of foliage. The shape of this correlation provided clear support for the a priori hypothesis that vein positioning limits photosynthesis via its influence on leaf hydraulic efficiency.

• The hydraulic conductance of leaves ( k leaf ) was examined to determine whether this little understood component of the water transport pathway plays a role in governing leaf phenology of two deciduous dry forest trees ( Calycophyllum candidissimum and Rhedera trinervis ). • k leaf was monitored in parallel with stem hydraulic conductivity (K stem ) during the transition from wet to dry season. The relationships between declining photosynthetic quantum yield during senescence and declining k leaf and K stem were compared. • Divergent patterns were observed in the response of K stem to seasonal drying; however, the behaviour of k leaf was essentially similar in both species. Large (five- to ten-fold) decreases in leaf hydraulic conductance occurred before, and during the later stages of leaf senescence. During senescence, declining k leaf , which continued until leaves were ultimately shed, was associated with a concomitant decline in quantum yield. • We conclude that, in these species, the loss of hydraulic conductance of the leaf vascular system is linked to, and possibly responsible for, the loss of photosynthetic capacity during leaf senescence.

We examined the leaf hydraulic design in 10 species based on their rehydration kinetics. In all cases, a biphasic response described the temporal pattern of water uptake, with time constants of ~30 to 800 s and ~800 to 8000 s. The time constants of the fast phase were significantly shorter in the six angiosperms (30 to 110 s) compared with the two singleveined conifer species (>400 s) examined, while the two multi-veined gymnosperm species, Gnetum gnemon and Ginkgo biloba, had time constants for the fast phase of ~150 s. Among angiosperm species, the fast phase constituted 50–90% of the total water absorbed, whereas in gymnosperms 70–90% of the water uptake could be assigned to the slow phase. In the four gymnosperms, the relative water uptake corresponding to the fast phase matched to a good degree the relative volume of the venation and bundle sheath extension; whereas in the angiosperm species, the relatively larger water influx during the fast phase was similar in relative volume to the combined venation, bundle sheath extension, epidermis and (in four species) the spongy mesophyll. This suggests a general trend from a design in which the epidermis is weakly connected to the veins (all four gymnosperms), to a design with good hydraulic connection between epidermis and veins that largely bypasses the mesophyll (four of six angiosperms), to a design in which almost the entire leaf appears to function as a single pool.

The hydraulic plumbing of vascular plant leaves varies considerably between major plant groups both in the spatial organization of veins, as well as their anatomical structure. Five conifers, three ferns and 12 angiosperm trees were selected from tropical and temperate forests to investigate whether the profound differences in foliar morphology of these groups lead to correspondingly profound differences in leaf hydraulic efficiency. We found that angiosperm leaves spanned a range of leaf hydraulic conductance from 3.9 to 36 mmol m 2 s − 1 MPa − 1 , whereas ferns (5.9–11.4 mmol m − 2 s − 1 MPa − 1 ) and conifers (1.6–9.0 mmol m − 2 s − 1 MPa − 1 ) were uniformly less conductive to liquid water. Leaf hydraulic conductance (K leaf ) correlated strongly with stomatal conductance indicating an internal leaf-level regulation of liquid and vapour conductances. Photosynthetic capacity also increased with K leaf , however, it became saturated at values of K leaf over 20 mmol m − 2 s − 1 MPa − 1 . The data suggest that vessels in the leaves of the angiosperms studied provide them with the flexibility to produce highly conductive leaves with correspondingly high photosynthetic capacities relative to tracheid-bearing species. .

Leaf structure and physiology are thought to be closely linked to leaf longevity and leaf habit. Here we compare the seasonal variation in leaf hydraulic conductance (kleaf ) andwater potential of two evergreen tree species with contrasting leaf life spans, and two species with similar leaf longevity but contrasting leaf habit, one being deciduous and the other evergreen. One of the evergreen species, Simarouba glauca, produced relatively short-lived leaves that maintained high hydraulic conductance year round by periodic flushing. The other evergreen species, Quercus oleoides, produced longer-lived leaves with lower kleaf and as a result minimum leaf water potential was much lower than in S. glauca (−2.8 MPa vs −1.6 MPa). Associated with exposure to lower water potentials, Q. oleoides leaves were harder, had a higher modulus of elasticity, and were less vulnerable to cavitation than S. glauca leaves. Both species operate at water potentials capable of inducing 20 (S. glauca) to 50% (Q. oleoides) loss of kleaf during the dry season although no evidence of cumulative losses in kleaf were observed in either species suggesting regular repair of embolisms. Leaf longevity in the deciduous species Rhedera trinervis is similar to that of S. glauca, although maximum kleaf was lower. Furthermore, a decline in leaf water potential at the onset of the dry season led to cumulative losses in kleaf in R. trinervis that culminated in leaf shedding.

Competitive interaction between conifers and angiosperms has moulded the structure of global vegetation since the Cretaceous. Angiosperms appear to enjoy their greatest advantage in the lowland tropics, an advantage often attributed to the presence of vessels in their xylem tissue. By monitoring the seedling growth of three members of the pan-tropical conifer family Podocarpaceae and three tropical angiosperm tree species, our aim was to determine whether these conifer and angiosperm seedlings showed distinct patterns of growth and light adaptation that might be attributed to the presence/absence of vessels. Angiosperm seedlings were consistently more efficient in terms of leaf area carried per unit stem investment, as well as more responsive to light climate than the conifer seedlings. Apparently linked to this were larger growth rate, stem hydraulic conductivity and stomatal conductance in the angiosperm sample. Stem hydraulic conductivity and maximum stomatal conductance were highly correlated among species and light treatments explaining the association between highly conductive vessel-bearing wood and high rates of gas exchange. We conclude that xylem vessels contribute to higher rates of gas exchange and more efficient production of leaf area in our sample angiosperms than in conifers. However, this advantage is limited by shade.

• Hydraulic characteristics of pteridophyte (fern and Selaginella ) foliage were investigated to determine whether the processes of water conduction and water loss are coordinated in these early vascular plants similarly to angiosperms. • Eight species of pteridophytes and associated woody angiosperms were examined from the sun and shade in a seasonally dry tropical forest. • Maximum leaf hydraulic conductivity ( K leaf ) in the four pteridophytes was within the range of the sampled shade angiosperms but much lower than that of the sundwelling angiosperms. Hydraulic conductivity of both angiosperm and pteridophyte leaves showed a similar response to desiccation, with K leaf becoming rapidly depressed once leaf water potential fell below a threshold. Stomatal closure in angiosperms corresponded closely with the water potential responsible for 50% loss of K leaf while pteridophytes were found to close stomata before K leaf depression. • The contrasting behaviour of stomata in this sample of pteridophytes suggest that this may be an intrinsic difference between pteridophytes and angiosperms, with lower safety margins in angiosperms possibly enhancing both optimization of gas exchange and xylem investment.

Gas exchange measurements were made on saplings of Southern Beech, Nothofagus cunninghamii (Hook.) Oerst. collected from three altitudes (350, 780 and 1100 m above sea level) and grown in a common glasshouse trial. Plants were grown from cuttings taken 2 years earlier from a number of plants at each altitude in Mt Field National Park, Tasmania. Stomatal density increased with increasing altitude of origin, and stomatal conductance and carbon assimilation rate were linearly related across all samples. The altitude of origin influenced the stomatal conductance and therefore carbon assimilation rate, with plants from 780 m having a greater photosynthetic rate than those from 350 m. The intercellular concentration of CO2 as a ratio of external CO2 concentration (ci/ca) was similar in all plants despite the large variation in maximum stomatal conductance. Carboxylation efficiency was greater in plants from 780 m than in plants from 350 m. Altitude of origin has a strong influence on the photosynthetic performance of N. cunninghamii plants even when grown under controlled conditions, and this influence is expressed in both leaf biochemistry (carboxylation efficiency) and leaf morphology (stomatal density).

Diurnal patterns of hydraulic conductance of the leaf lamina ( K leaf ) were monitored in a field-grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance ( g s ) and CO 2 uptake ( A leaf ) were associated with short-term changes in K leaf . On days of high evaporative demand mid-day depression of K leaf to between 40 and 50% of pre-dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than - 1 MPa implying a refilling mechanism, or that loss of K leaf was not linked to cavitation. Laboratory measurement of the response of K leaf to Y leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K leaf and Y leaf predicted from measured transpiration, xylem water potential and the K leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K leaf , g s and A leaf suggests that xylem dysfunction in the leaf may lead to mid-day depression of gas exchange in this species.

Just as a soggy paper straw is prone to yielding under the applied suction of a thirsty drinker, the xylem tracheids in leaves seem prone to collapse as water potential declines, impeding their function. Here we describe the collapse, under tension, of lignified cells peripheral to the leaf vein of a broad-leaved rainforest conifer, Podocarpus grayi de Laub. Leaves of Podocarpus are characterized by an array of cylindrical tracheids aligned perpendicular to the leaf vein, apparently involved in the distribution of water radially through the mesophyll. During leaf desiccation the majority of these tracheids collapsed from circular to flat over the water potential range 21.5 to 22.8 MPa. An increase in the percentage of tracheids collapsed during imposed water stress was mirrored by declining leaf hydraulic conductivity (Kleaf), implying a direct effect on water transport efficiency. Stomata responded to water stress by closing at 22.0 MPa when 45% of cells were collapsed and Kleaf had declined by 25%. This was still substantially before the initial indications of cavitation-induced loss of hydraulic conductance in the leaf vein, at 23 MPa. Plants droughted until 49% of tracheids had collapsed were found to fully recover tracheid shape and leaf function 1 week after rewatering. A simple mechanical model of tracheid collapse, derived from the theoretical buckling pressure for pipes, accurately predicted the collapse dynamics observed in P. grayi, substantiating estimates of cell wall elasticity and measured leaf water potential. The possible adaptive advantages of collapsible vascular tissue are discussed.

This study examined the linkage between xylem vulnerability, stomatal response to leaf water potential ( Y L ), and loss of leaf turgor in eight species of seasonally dry tropical forest trees. In order to maximize the potential variation in these traits species that exhibit a range of leaf habits and phenologies were selected. It was found that in all species stomatal conductance was responsive to Y L over a narrow range of water potentials, and that Y L inducing 50% stomatal closure was correlated with both the Y L inducing a 20% loss of xylem hydraulic conductivity and leaf water potential at turgor loss in all species. In contrast, there was no correlation between the water potential causing a 50% loss of conductivity in the stem xylem, and the water potential at stomatal closure ( Y SC ) amongst species. It was concluded that although both leaf and xylem characters are correlated with the response of stomata to Y L, there is considerable flexibility in this linkage. The range of responses is discussed in terms of the differing leaf-loss strategies exhibited by these species.

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR(max)) at saturating quantum flux (PPFDsat), and decreasing thereafter. ETR(max) ranged from 217 mu mol electrons . m(-2). s(-1) at a PPFDsat of 1725 mu mol photons . m(-2). s(-1) in Actinostrobus acuminatus to an ETR of 60 mu mol electrons . m(-2). s(-1) at a PPFDsat of 745 mu mol electrons . m(-2). s(-1) in Podocarpus dispermis. Good correlations were observed between ETR(max) and both PPFDsat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 +/- 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q(n)), with q(n) saturating faster in species with low PPFDsat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFDsat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity.

The Winteraceae are traditionally regarded as the least-specialized descendents of the first flowering plants, based largely on their lack of xylem vessels. Since vessels have been viewed as a key innovation for angiosperm diversification, Winteraceae have been portrayed as declining relicts, limited to wet forest habitats where their tracheidbased wood does not impose a significant hydraulic constraints. In contrast, phylogenetic analyses place Winteraceae among angiosperm clades with vessels, indicating that their vesselless wood is derived rather than primitive, whereas extension of the Winteraceae fossil record into the Early Cretaceous suggests a more complex ecological history than has been deduced from their current distribution. However, the selective regime and ecological events underlying the possible loss of vessels in Winteraceae have remained enigmatic. Here we examine the hypothesis that vessels were lost as an adaptation to freezing-prone environments in Winteraceae by measuring the responses of xylem water transport to freezing for a diverse group of Winteraceae taxa as compared to Canella winterana (Canellaceae, a close relative with vessels) and sympatric conifer taxa. We found that mean percent loss of xylem water transport capacity following freeze-thaw varied from 0% to 6% for Winteraceae species from freezing-prone temperate climates and approximately 20% in those taxa from tropical (nonfreezing) climates. Similarly, conifers exhibit almost no decrease in xylem hydraulic conductivity following freezing. In contrast, water transport in Canella stems is nearly 85% blocked after freeze-thaw. Although vessel-bearing wood of Canella possesses considerably greaterhydraulic capacity than Winteraceae, nearly 20% of xylem hydraulic conductance remains, a value that is comparable to the hydraulic capacity of vesselless Winteraceae xylem, if the proportion of hydraulic flow through vessels (modeled as ideal capillaries) is removed. Thus, the evolutionary

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.

The question as to what triggers stomatal closure during leaf desiccation remains controversial. This paper examines characteristics of the vascular and photosynthetic functions of the leaf to determine which responds most similarly to stomata during desiccation. Leaf hydraulic conductance (Kleaf) was measured from the relaxation kinetics of leaf water potential (l), and a novel application of this technique allowed the response of Kleaf to l to be determined. These “vulnerability curves” show that Kleaf is highly sensitive to l and that the response of stomatal conductance to l is closely correlated with the response of Kleaf to l. The turgor loss point of leaves was also correlated with Kleaf and stomatal closure, whereas the decline in PSII quantum yield during leaf drying occurred at a lower l than stomatal closure. These results indicate that stomatal closure is primarily coordinated with Kleaf. However, the close proximity of l at initial stomatal closure and initial loss of Kleaf suggest that partial loss of Kleaf might occur regularly, presumably necessitating repair of embolisms.

Recent phylogenetic analyses have placed the root of flowering plants near Amborella trichopoda, a woody plant restricted to cloud forest habitats in New Caledonia. A distinctive feature of A. trichopoda is its reported lack of xylem vessels. Here we present observations of pit membrane structure and end wall morphology for primary and secondary tracheary cells of A. trichopoda as well as field measurements of stem hydraulic properties of A. trichopoda compared with five cloud forest species from New Caledonia. Observations of stem radial sections revealed that the primary wall material in the protoxylem and metaxylem elements was intact. No large porosities (such as those that have been observed in the pit membranes of Nymphaeales) were observed. However, a few elliptical pits of tracheary cells in the secondary xylem appeared to lack pit membranes. These observations are consistent with our measurements of functional conduit length, which indicate that the longest open conduits are equal to the length of two secondary xylem elements joined end to end. Thus, the xylem of A. trichopoda appears to be functionally vesselless, with the caveat that connections between individual vascular elements may occasionally be open (i.e., lacking in at least one pit membrane). Sapwood area and leaf area–specific hydraulic conductivities of A. trichopoda are similar to those of conifers and angiosperms, with and without xylem vessels, growing in understory cloud forest environments. These findings bear on discussions of the morphology and ecology of the first flowering plants as well as on the possible causes of their diversification.

The instantaneous and integrated leaf gas exchange of 13 species of southern hemisphere conifers grown under identical glasshouse conditions were examined to determine whether there was any correlation between the characteristics of water use at the leaf level and environmental water availability. In the conifer species examined, the minimum ratio of internal to ambient CO2 measured in leaves during artificially imposed drought [(C-i/C-a)(min)] was strongly correlated with the minimum rainfall observed within the natural range of each species. This suggests that the distributions of these species are constrained by the drought tolerance of their photosynthetic apparatus. A good correlation was found between the ratio of internal to ambient CO2 measured in leaves under optimal conditions (C-i/C-a)(max) and leaf delta(13)C (and hence inferred integral [c(i)/c(a)]). Neither of these, however, correlated with the environmental parameters considered most likely to be limiting species distribution, i.e, precipitation and altitude. These data suggest that decreasing water availability may have been the major factor responsible for the restriction and extinction of conifers in the southern hemisphere.

Recent phylogenetic analyses place Amborella trichopoda, a semiclimbing premontane rain forest shrub endemic to New Caledonia, as sister to all other extant flowering plants. Here we present new observations on leaf anatomy and morphology, leaf photosynthetic physiology, and xylem hydraulic function for Amborella in the field. We focus on experimental measurements of the degree of photosynthetic and xylem hydraulic acclimation to sunlight by examining Amborella plants growing in deeply shaded understories (0.7% full sunlight) compared with individuals from exposed montane ridge crests in New Caledonia. Amborella leaves exhibit few anatomical differences between sun and shade. Instead of primarily adjusting leaf anatomical characters to irradiance, A. trichopoda leaves exposed periodically to full sunlight modify their orientation, becoming folded up along the midvein, presumably to avoid increases in light interception. Chlorophyll a fluorescence measurements demonstrate that the photosynthetic apparatus of A. trichopoda is shade adapted, based on a low capacity for excess light energy dissipation and low photosynthetic electron transport rate. In addition, these characteristics exhibit limited upregulation in response to increased irradiance. In parallel, there are small or no differences in leaf area-specific hydraulic conductivity between sun and shade shoots of A. trichopoda. Taken together, A. trichopoda appears to possess limited developmental and physiological flexibility to light flux density. The bearing of these observations, in the context of other early-diverging clades of angiosperms identified by recent phylogenetic studies, on the physiological ecology of the first angiosperms is discussed.

Almost all parasitic plants, including more than 3000 species, are angiosperms. The only suggested gymnosperm exception is the New Caledonian conifer, Parasitaxus ustus , which forms a bizarre graft-like attachment to the roots of another conifer Falcatifolium taxoides . Yet, the degree of resource dependence of Parasitaxus on Falcatifolium has remained speculative. Here we show that Parasitaxus is definitively parasitic, but it displays a physiological habit unlike any known angiosperm parasite. Despite possessing chloroplasts, it was found that the burgundy red shoots of Parasitaxus lack significant photosynthetic electron transport. However unlike nonphotosynthetic angiosperm parasites (holoparasites), tissues of Parasitaxus are considerably enriched in 13 carbon relative to its host. In line with anatomical observations of fungal hyphae embedded in the parasite/host union, stable carbon isotopic measurements indicate that carbon transport from the host to Parasitaxus most likely involves a fungal partner. Therefore, Parasitaxus parallels fungusfeeding angiosperms (mycoheterotrophs) that steal carbon from soil mycorrhizal fungi. Yet with its tree-like habit, association with fungi residing within the host union, high stomatal conductance, and low water potential, it is demonstrated that Parasitaxus functions unlike any known angiosperm mycoheterotroph or holoparasite. Parasitaxus appears to present a unique physiological chimera of mistletoe- like water relations and fungal-mediated carbon trafficking from the host.

The Tasmanian montane and rainforest conifer genus, Athrotaxis, provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, degree of imbricacy and stomatal distribution. Hybrid swarms in the field and glasshouse grown hybrid progeny are highly variable for these traits. In glasshouse grown plants of the true species and a diverse hybrid progeny, stomatal conductivity and density varied by about 200% among individuals with a strong correlation between these traits. Hybrids were found to produce lower stomatal densities and less discrimination of 13C than either species, leading to a negative relationship between stomatal density and delta 13C. In contrast to the highly variable stomatal densities and delta 13C in glasshouse plants, field plants were very conservative in both characters. Relatively low stomatal density and high water use efficiency in field plants suggests strong natural selection to optimize the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A. selaginoides was only 4-6 times as great as, and was strongly correlated with, conductance in the dark. This suggests either incomplete stomatal closure or greater cuticular conductance. In contrast, A. cupressoides appeared to be less "leaky", which may reflect adaptation to its more exposed habitat.

The effect of freezing on stem xylem hydraulic conductivity and leaf chlorophyll a fluorescence was measured in 12 tree and shrub species from a treeline heath in Tasmania, Australia. Reduction in stem hydraulic conductivity after a single freeze-thaw cycle was minimal in conifers and the vessel-less angiosperm species Tasmannia lanceolata (Winteraceae), whereas mean loss of conductivity in vessel-forming angiosperms fell in the range 17-83%. A positive linear relationship was observed between percentage loss of hydraulic conductivity by freeze-thaw and the average conduit diameter across all 12 species. This supports the hypothesis that large-diameter vascular conduits have a greater likelihood of freeze-thaw cavitation because larger bubbles are produced, which are more likely to expand under tension. Leaf frost tolerances, as measured by a 50% loss of maximum PSII quantum yield, varied from -6 to -13°C, indicating that these species were more frost-sensitive than plants from northern hemisphere temperate forest and treeline communities. There was no evidence of a relationship between frost tolerance of leaves and the resilience of stem water transport to freezing, suggesting that low temperature survival and the resistance of stem water transport to freezing are independently evolving traits. The results of this study bear on the ecological importance of stem freezing in the southern hemisphere treeline zones.