Comparative venation costs of monocotyledon and dicotyledon species in the eastern Colorado steppe

MAIN CONCLUSION

Leaf vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles did not differ between monocot and dicot species in 21 species from the eastern Colorado steppe. Dicots possessed significantly larger minor vein networks than monocots. Across the tree of life, there is evidence that dendritic vascular transport networks are optimized, balancing maximum speed and integrity of resource delivery with minimal resource investment in transport and infrastructure. Monocot venation, however, is not dendritic, and remains parallel down to the smallest vein orders with no space-filling capillary networks. Given this departure from the "optimized" dendritic network, one would assume that monocots are operating at a significant energetic disadvantage. In this study, we investigate whether monocot venation networks bear significantly greater carbon/construction costs per leaf volume than co-occurring dicots in the same ecosystem, and if so, what physiological or ecological advantage the monocot life form possesses to compensate for this deficit. Given that venation networks could also be optimized for leaf mechanical support or provide herbivory defense, we measured the vascular system of both monocot and dicots at three scales to distinguish between leaf investment in mechanical support (macroscopic vein), total transport and capacitance (vascular bundle), or exclusively water transport (xylem) for both parallel and dendritic venation networks. We observed that vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles was not significantly different between monocot species and dicot species. Dicots, however, possess significantly larger minor vein networks than monocots. The 19 species subjected to gas-exchange measurement in the field displayed a broad range of Amax and but demonstrated no significant relationships with any metric of vascular network size in major or minor vein classes. Given that monocots do not seem to display any leaf hydraulic disadvantage relative to dicots, it remains an important research question why parallel venation (truly parallel, down to the smallest vessels) has not arisen more than once in the history of plant evolution.

Searching for mechanisms driving root pressure in Zea mays-a transcriptomic approach

While there are many theories and a variety of innovative datasets contributing to our understanding of the mechanism generating root pressure in vascular plants, we are still unable to produce a specific cellular mechanism for any species. To discover these mechanisms, we used RNA-Seq to explore differentially expressed genes in three different tissues between individual Zea mays plants expressing root pressure and those producing none. Working from the perspective that roots cells are utililizing a combination of osmotic exudation and hydraulic pressure mechanisms to generate positively-pressured flow of water into the xylem from the soil, we hypothesized that differential expression analysis would yield candidate genes coding for membrane transporters, ion channels, ATPases, and hormones with clear relevance to root pressure generation. In basal stem and coarse root tissue, we observed these classes of differentially expressed genes and more, including a strong cytoskeletal remodeling response. Fine roots displayed remarkably little differential expression relevant to root pressure, leading us to conclude that they either do not contribute to root pressure generation or are constitutively expressing root pressure mechanisms regardless of soil water content.

Development and application of an inexpensive open-source dendrometer for detecting xylem water potential and radial stem growth at high spatial and temporal resolution

There is currently a need for inexpensive, continuous, non-destructive water potential measurements at high temporal resolution (<1 min). We describe here the development and testing of an entirely open-source dendrometer that, when combined with periodic Scholander pressure chamber measurements, provides sub-minute resolution estimates of water potential when placed on tissues exhibiting little or no secondary growth (petioles, monocotyledon stems). The dendrometer can also be used to measure radial growth of stems and branches when placed on dicotyledon and gymnosperm species. The dendrometer can be interfaced directly with a computer in real time in the lab or greenhouse, or connected to a datalogger for long periods of use in the field on batteries. We tested this device on a herbaceous dicotyledon (Helianthus annuus) (petioles and stems) and a monocotyledon (Zea mays) species (stems) for 1 week during dehydration and re-watering treatments under laboratory conditions. We also demonstrated the ability of the device to record branch and trunk diameter variation of a woody dicotyledon (Rhus typhina) in the field. Under laboratory conditions, we compared our device (hereafter 'contact' dendrometer) with modified versions of another open-source dendrometer (the 'optical' dendrometer). Overall, contact and optical dendrometers were well aligned with one another, with Pearson correlation coefficients ranging from 0.77 to 0.97. Both dendrometer devices were well aligned with direct measurements of xylem water potential, with calibration curves exhibiting significant non-linearity, especially at water potentials near the point of incipient plasmolysis, with pseudo R2 values (Efron) ranging from 0.89 to 0.99. Overall, both dendrometers were comparable and provided sufficient resolution to detect subtle differences in stem water potential (ca. 50 kPa) resulting from light-induced changes in transpiration, vapour pressure deficit and drying/wetting soils. All hardware designs, alternative configurations, software and build instructions for the contact dendrometers are provided.

Grass veins are leaky pipes: vessel widening in grass leaves explain variation in stomatal conductance and vessel diameter among species

The widening of xylem vessels from tip to base of trees is an adaptation to minimize the hydraulic resistance of a long pathway. Given that parallel veins of monocot leaves do not branch hierarchically, vessels should also widen basipetally but, in addition to minimizing resistance, should also account for water volume lost to transpiration since they supply water to the lamina along their lengths, that is 'leakiness'. We measured photosynthesis, stomatal conductance, and vessel diameter at five locations along each leaf of five perennial grass species. We found that the rate of conduit widening in grass leaves was larger than the widening exponent required to minimize pathlength resistance (0.35 vs c. 0.22). Furthermore, variation in the widening exponent among species was positively correlated with maximal stomatal conductance (r2  = 0.20) and net CO2 assimilation (r2  = 0.45). These results suggest that faster rates of conduit widening (> 0.22) were associated with higher rates of water loss. Taken together, our results show that the widening exponent is linked to plant function in grass leaves and that natural selection has favored parallel vein networks that are constructed to meet transpiration requirements while minimizing hydraulic resistance within grass blades.

Soil-plant hydraulics explain stomatal efficiency-safety tradeoff

The efficiency-safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency-safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax ) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50 ). However, the underlying mechanism of this gmax -ψgs50 tradeoff remains unknown. Here, we utilized a soil-plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil-plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50 . Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil-root interface, and therefore in the leaves. We demonstrated that the gmax -ψgs50 tradeoff can be predicted based on soil-plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions.

The effects of copper deficiency on lignification, xylem vessel structure, and hydraulic traits in hybrid poplar

Copper (Cu) homeostasis is integral to many plant physiological processes, including lignification of plant cell walls. This link occurs through Cu's role as a cofactor in the apoplastic laccase enzymes that oxidize monolignols that then polymerize to form the hydrophobic lignin polymer, which provides rigidity and strength to the water transport system. In this study, we investigated the effect of Cu deficiency on lignin content and chemistry in poplar stems. We also examined the effect of Cu deficiency on the stiffness of stem wood and the hydraulic properties of leaves. Cu deficiency resulted in a significant reduction in lignin content, an increase in the syringyl to guaiacyl monomer ratio of stem xylem, and no change to stem modulus of elasticity. Accompanying these stem traits, Cu-deficient leaves had higher (less negative) turgor loss points and markedly stiffer mesophyll cell walls. Our results may reflect a novel response in poplar whereby structural stiffness and mechanical stability are maintained in the face of Cu deficiency and reduction in the guaiacyl lignin monomer content.

Coordination of photosynthetic traits across soil and climate gradients

"Least-cost theory" posits that C₃ plants should balance rates of photosynthetic water loss and carboxylation in relation to the relative acquisition and maintenance costs of resources required for these activities. Here we investigated the dependency of photosynthetic traits on climate and soil properties using a new Australia-wide trait dataset spanning 528 species from 67 sites. We tested the hypotheses that plants on relatively cold or dry sites, or on relatively more fertile sites, would typically operate at greater CO₂ drawdown (lower ratio of leaf internal to ambient CO₂ , C_i :C_a ) during light-saturated photosynthesis, and at higher leaf N per area (N_area ) and higher carboxylation capacity (V_{cmax 25} ) for a given rate of stomatal conductance to water vapour, g_sw . These results would be indicative of plants having relatively higher water costs than nutrient costs. In general, our hypotheses were supported. Soil total phosphorus (P) concentration and (more weakly) soil pH exerted positive effects on the N_area -g_sw and V_{cmax 25} -g_sw slopes, and negative effects on C_i :C_a . The P effect strengthened when the effect of climate was removed via partial regression. We observed similar trends with increasing soil cation exchange capacity and clay content, which affect soil nutrient availability, and found that soil properties explained similar amounts of variation in the focal traits as climate did. Although climate typically explained more trait variation than soil did, together they explained up to 52% of variation in the slope relationships and soil properties explained up to 30% of the variation in individual traits. Soils influenced photosynthetic traits as well as their coordination. In particular, the influence of soil P likely reflects the Australia's geologically ancient low-relief landscapes with highly leached soils. Least-cost theory provides a valuable framework for understanding trade-offs between resource costs and use in plants, including limiting soil nutrients.

Functional xylem characteristics associated with drought-induced embolism in angiosperms

Hydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to achieve at scale, leading to alternative metrics and correlations with more easily measured traits. These efforts have led to the longstanding and pervasive assumed mechanistic link between vessel diameter and vulnerability in angiosperms. However, there are at least two problems with this assumption that requires critical re-evaluation: (1) our current understanding of drought-induced embolism does not provide a mechanistic explanation why increased vessel width should lead to greater vulnerability, and (2) the most recent advancements in nanoscale embolism processes suggest that vessel diameter is not a direct driver. Here, we review data from physiological and comparative wood anatomy studies, highlighting the potential anatomical and physicochemical drivers of embolism formation and spread. We then put forward key knowledge gaps, emphasising what is known, unknown and speculation. A meaningful evaluation of the diameter-vulnerability link will require a better mechanistic understanding of the biophysical processes at the nanoscale level that determine embolism formation and spread, which will in turn lead to more accurate predictions of how water transport in plants is affected by drought.

Leaf nitrogen from the perspective of optimal plant function

Leaf dry mass per unit area (LMA), carboxylation capacity (V _cmax) and leaf nitrogen per unit area (N_area) and mass (N_mass) are key traits for plant functional ecology and ecosystem modelling. There is however no consensus about how these traits are regulated, or how they should be modelled. Here we confirm that observed leaf nitrogen across species and sites can be estimated well from observed LMA and V _cmax at 25°C (V _cmax25). We then test the hypothesis that global variations of both quantities depend on climate variables in specific ways that are predicted by leaf-level optimality theory, thus allowing both N_area to be predicted as functions of the growth environment.A new global compilation of field measurements was used to quantify the empirical relationships of leaf N to V _cmax25 and LMA. Relationships of observed V _cmax25 and LMA to climate variables were estimated, and compared to independent theoretical predictions of these relationships. Soil effects were assessed by analysing biases in the theoretical predictions.LMA was the most important predictor of N_area (increasing) and N_mass (decreasing). About 60% of global variation across species and sites in observed N_area, and 31% in N_mass, could be explained by observed LMA and V _cmax25. These traits, in turn, were quantitatively related to climate variables, with significant partial relationships similar or indistinguishable from those predicted by optimality theory. Predicted trait values explained 21% of global variation in observed site-mean V _cmax25, 43% in LMA and 31% in N_area. Predicted V _cmax25 was biased low on clay-rich soils but predicted LMA was biased high, with compensating effects on N_area. N_area was overpredicted on organic soils. Synthesis. Global patterns of variation in observed site-mean N_area can be explained by climate-induced variations in optimal V _cmax25 and LMA. Leaf nitrogen should accordingly be modelled as a consequence (not a cause) of V _cmax25 and LMA, both being optimized to the environment. Nitrogen limitation of plant growth would then be modelled principally via whole-plant carbon allocation, rather than via leaf-level traits. Further research is required to better understand and model the terrestrial nitrogen and carbon cycles and their coupling.

Recovery after deficiency: systemic copper prioritization and partitioning in the leaves and stems of hybrid poplar

Copper (Cu) is important for many aspects of plant function including photosynthesis. It has been suggested that photosynthesis, especially in young leaves, is prioritized for Cu delivery after deficiency in hybrid poplar. To determine relative Cu delivery prioritization, we enriched hydroponic plant growth media of Cu-deficient poplar with 98% 65Cu and tracked Cu delivery after deficiency to young leaves, mature leaves and stems. Young leaves acquired ~58% more 65Cu on Day 1 and ~65% more 65Cu by Day 3 compared with mature leaves. Additionally, stomatal conductance (gs) was measured on leaves for 6 weeks and during a 3-day 65Cu pulse resupply period. During deficiency, mature leaves maintained a higher gs than younger leaves but 3 days after Cu resupply the younger leaves that had recovered showed the highest gs. In conclusion, these results provide a quantitative understanding of how Cu is systemically transported and distributed to photosynthetic and stem tissues.

Physiological trait networks enhance understanding of crop growth and water use in contrasting environments

Plant function arises from a complex network of structural and physiological traits. Explicit representation of these traits, as well as their connections with other biophysical processes, is required to advance our understanding of plant-soil-climate interactions. We used the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to evaluate physiological trait networks in maize. Net primary productivity (NPP) and grain yield were simulated across five contrasting climate scenarios. Simulations achieving high NPP and grain yield in high precipitation environments featured trait networks conferring high water use strategies: deep roots, high stomatal conductance at low water potential ("risky" stomatal regulation), high xylem hydraulic conductivity and high maximal leaf area index. In contrast, high NPP and grain yield was achieved in dry environments with low late-season precipitation via water conserving trait networks: deep roots, high embolism resistance and low stomatal conductance at low leaf water potential ("conservative" stomatal regulation). We suggest that our approach, which allows for the simultaneous evaluation of physiological traits, soil characteristics and their interactions (i.e., networks), has potential to improve our understanding of crop performance in different environments. In contrast, evaluating single traits in isolation of other coordinated traits does not appear to be an effective strategy for predicting plant performance.

Age Dependent Partitioning Patterns of Essential Nutrients Induced by Copper Feeding Status in Leaves and Stems of Poplar

Copper (Cu) is an essential micronutrient, and its deficiency can cause plants to undergo metabolic changes at several levels of organization. It has been shown that leaf age can play a role in nutrient partitioning along the shoot axis of poplar. In this study, we investigated the effect of Cu deficiency on the altered partitioning of essential macro and micronutrients in leaves and stems of different age. Cu deficiency was associated with higher concentrations of calcium, magnesium, sulfur, iron, zinc, manganese, and molybdenum in leaves and relatively higher concentrations of calcium, phosphorus, iron, and zinc in stems. Leaf and stem age had significant effects on nutrient partitioning. Principal component analyses revealed patterns that point to inverse influences in leaves and stems on nutrient partitioning. Specifically, these analyses revealed that nutrient partitioning in leaves was influenced by Cu feeding status more than developmental stage, whereas nutrient partitioning in stems was influenced by developmental stage more than Cu feeding status. These results suggest that Cu deficiency and developmental stage can significantly influence the partitioning and homeostasis of macro and micronutrients in poplar organs.

AusTraits, a curated plant trait database for the Australian flora

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.

Growth and grain yield of eight maize hybrids are aligned with water transport, stomatal conductance, and photosynthesis in a semi-arid irrigated system

There is increasing interest in understanding how trait networks can be manipulated to improve the performance of crop species. Working towards this goal, we have identified key traits linking the acquisition of water, the transport of water to the sites of evaporation and photosynthesis, stomatal conductance, and growth across eight maize hybrid lines grown under well-watered and water-limiting conditions in Northern Colorado. Under well-watered conditions, hybrids with higher end-of-season growth and grain yield exhibited higher leaf-specific conductance, lower operating water potentials, higher rates of midday stomatal conductance, higher rates of net CO₂ assimilation, and greater leaf osmotic adjustment. This trait network was similar under water-limited conditions with the notable exception that linkages between water transport, midday stomatal conductance, and growth were even stronger than under fully watered conditions. The maintenance of high leaf-specific conductance throughout the day was achieved via higher maximal conductance rates rather than lower susceptibility to conductance loss. Our results suggest that efforts to improve maize performance in well-watered and water-limiting conditions would benefit from considering the physiological trait networks governing water and carbon flux rather than focusing on single traits independently of one another.

Drought-Induced Root Pressure in Sorghum bicolor

Root pressure, also manifested as profusive sap flowing from cut stems, is a phenomenon in some species that has perplexed biologists for much of the last century. It is associated with increased crop production under drought, but its function and regulation remain largely unknown. In this study, we investigated the initiation, mechanisms, and possible adaptive function of root pressure in six genotypes of Sorghum bicolor during a drought experiment in the greenhouse. We observed that root pressure was induced in plants exposed to drought followed by re-watering but possibly inhibited by 100% re-watering in some genotypes. We found that root pressure in drought stressed and re-watered plants was associated with greater ratio of fine: coarse root length and shoot biomass production, indicating a possible role of root allocation in creating root pressure and adaptive benefit of root pressure for shoot biomass production. Using RNA-Seq, we identified gene transcripts that were up- and down-regulated in plants with root pressure expression, focusing on genes for aquaporins, membrane transporters, and ATPases that could regulate inter- and intra-cellular transport of water and ions to generate positive xylem pressure in root tissue.

Weak tradeoff between xylem hydraulic efficiency and safety: climatic seasonality matters

A classic theory proposes that plant xylem cannot be both highly efficient in water transport and resistant to embolism, and therefore a hydraulic efficiency-safety trade-off should exist. However, the trade-off is weak, and many species exhibit both low efficiency and low safety, falling outside of the expected trade-off space. It remains unclear under what climatic conditions these species could maintain competitive fitness. We compiled hydraulic efficiency and safety traits for 682 observations of 499 woody species from 178 sites world-wide and measured the position of each observation within the proposed trade-off space. For both angiosperms and gymnosperms, observations from sites with high climatic seasonality, especially precipitation seasonality, tended to have higher hydraulic safety and efficiency than observations from sites with low seasonality. Specifically, high vapour pressure deficit, high solar radiation, and low precipitation during the wet season were driving factors. Strong climatic seasonality and drought in both dry and wet seasons appear to be ecological filters that select for species with co-optimized safety and efficiency, whereas the opposite environmental conditions may allow the existence of plants with low efficiency and safety.

Tip-to-base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities

In the stems of terrestrial vascular plants studied to date, the diameter of xylem water-conducting conduits D widens predictably with distance from the stem tip L approximating D ∝ L^b , with b ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip-to-base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip-to-base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip-to-base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.

Increasing axial parenchyma fraction in the Malagasy Magnoliids facilitated the co-optimisation of hydraulic efficiency and safety

The evolution of angiosperms was accompanied by the segregation and specialisation of their xylem tissues. This study aimed to determine whether the fraction and arrangement of parenchyma tissue influence the hydraulic efficiency-safety trade-off in the basal angiosperms. We examined xylem anatomical structure and hydraulic functioning of 28 woody species of Magnoliids in a tropical rainforest of Madagascar and reported, for the first time, quantitative measurements that support the relationship between vessel-to-xylem parenchyma connectivity and the hydraulic efficiency-safety trade-off. We also introduced a new measurement - the distance of species from the trade-off limit - to quantify the co-optimisation of hydraulic efficiency and safety. Although the basal angiosperms in this study had low hydraulic conductivity and safety, species with higher axial parenchyma fraction (APf) had significantly higher hydraulic conductivity. Hydraulic efficiency-safety optimisation was accompanied by higher APf and vessel-to-axial parenchyma connectivity. Conversely, species exhibiting high ray parenchyma fraction and high vessel-to-ray connectivity had lower K_s and were further away from the hydraulic trade-off limit line. Our results provide evidence that axial parenchyma fraction and paratracheal arrangement are associated with both enhanced hydraulic efficiency and safety.

Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel-tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWC_L-pd ) together were the strongest predictors of day capacitance (r²_adj = .44). Vessel-tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse-porous species, VWC_L-pd and vessel-ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring-porous species, VWC_L-pd , WD and vessel-fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.

What do you mean "functional" in ecology? Patterns versus processes

Use of the term "functional" trait has increased exponentially in ecology. Although accounting for numerous ecological questions, this concept raises several issues. We propose that the term "functional" could be misleading because (1) no rigorous criteria exist to identify "functional" traits and (2) it suggests that only some traits ("functional" ones) can inform our understanding of species functioning, whatever the scale or discipline. Hence, the concept of "functional" trait in ecology is starting to be challenged and it remains unclear why some traits should be considered functional, whereas other traits should not. We argue that the most used "functional" traits are meaningful because they reflect important differences between populations or species, based on synchronic comparisons, that is, irrespective of time (hereafter "pattern" traits). Hence, they are useful for identifying trade-offs and strategies across large numbers of observations, usually at rather coarse scales, and are most often used in analyses of "big data." However, given that many ecological processes occur across short time scales and narrow gradients of climate and resource availability, the efficacy of these traits to inform us about these ecological processes appears questionable. We show that trait measurements that take time explicitly into account (hereafter "process" traits) differ from pattern traits because they quantify the flows of material and energy within a given environment across a defined period of time. Although pattern traits and process traits are both functional, it is important to understand the differences between the approaches. Moreover, better accounting of ontogeny, life form, plasticity, and genetic variability is required to enhance the convergence between pattern and process approaches. This revised framework allows more explicit connections between trait ecology and other biological sciences. It should enhance the study of processes at all scales in order to investigate efficiently the adaptive responses of biological organisms to climate change.

Growing-season temperature and precipitation are independent drivers of global variation in xylem hydraulic conductivity

Stem xylem-specific hydraulic conductivity (K_S ) represents the potential for plant water transport normalized by xylem cross section, length, and driving force. Variation in K_S has implications for plant transpiration and photosynthesis, growth and survival, and also the geographic distribution of species. Clarifying the global-scale patterns of K_S and its major drivers is needed to achieve a better understanding of how plants adapt to different environmental conditions, particularly under climate change scenarios. Here, we compiled a xylem hydraulics dataset with 1,186 species-at-site combinations (975 woody species representing 146 families, from 199 sites worldwide), and investigated how K_S varied with climatic variables, plant functional types, and biomes. Growing-season temperature and growing-season precipitation drove global variation in K_S independently. Both the mean and the variation in K_S were highest in the warm and wet tropical regions, and lower in cold and dry regions, such as tundra and desert biomes. Our results suggest that future warming and redistribution of seasonal precipitation may have a significant impact on species functional diversity, and is likely to be particularly important in regions becoming warmer or drier, such as high latitudes. This highlights an important role for K_S in predicting shifts in community composition in the face of climate change.

Water transport from stem to stomata: the coordination of hydraulic and gas exchange traits across 33 subtropical woody species

Coordination between sapwood-specific hydraulic conductivity (Ks) and stomatal conductance (gs) has been identified in previous studies; however, coordination between leaf hydraulic conductance (Kleaf) and gs, as well as between Kleaf and Ks is not always consistent. This suggests that there is a need to improve our understanding of the coordination among hydraulic and gas exchange traits. In this study, hydraulic traits (e.g., Ks and Kleaf) and gas exchange traits, including gs, transpiration (E) and net CO2 assimilation (Aarea), were measured across 33 co-occurring subtropical woody species. Kleaf was divided into two components: leaf hydraulic conductance inside the xylem (Kleaf-x) and outside the xylem (Kleaf-ox). We found that both Kleaf-x and Kleaf-ox were coordinated with gs and E, but the correlations between Kleaf-ox and gs (or E) were substantially weaker, and that Ks was coordinated with Kleaf-x, but not with Kleaf-ox. In addition, we found that Ks, Kleaf-x and Kleaf-ox together explained 63% of the variation in gs and 42% of the variation in Aarea across species, with Ks contributing the largest proportion of explanatory power, whereas Kleaf-ox contributed the least explanatory power. Our results demonstrate that the coordination between leaf water transport and gas exchange, as well as the hydraulic linkage between leaf and stem, were weakened by Kleaf-ox. This highlights the possibility that water transport efficiencies of stem and leaf xylem, rather than that of leaf tissues outside the xylem, are important determinants of stomatal conductance and photosynthetic capacity across species.

Hydraulic traits are coordinated with maximum plant height at the global scale

Water must be transported long distances in tall plants, resulting in increasing hydraulic resistance, which may place limitations on the maximum plant height (H _max) in a given habitat. However, the coordination of hydraulic traits with H _max and habitat aridity remains poorly understood. To explore whether H _max modifies the trade-off between hydraulic efficiency and safety or how water availability might influence the relationship between H _max and other hydraulic traits, we compiled a dataset including H _max and 11 hydraulic traits for 1281 woody species from 369 sites worldwide. We found that taller species from wet habitats exhibited greater xylem efficiency and lower hydraulic safety, wider conduits, lower conduit density, and lower sapwood density, which were all associated with habitat water availability. Plant height and hydraulic functioning appear to represent a single, coordinated axis of variation, aligned primarily with water availability, thus suggesting an important role for this axis in species sorting processes.

USDA-ARS Colorado maize growth and development, yield and water-use under strategic timing of irrigation, 2012-2013

This data set was collected over two years, 2012–2013, on maize under 12 irrigation treatments with varying levels of deficit during late-vegetative and grain-filling growth stages in semi-arid Northern Colorado supplied with surface drip irrigation. The data set, which can be found online at the USDA National Agricultural Library data repository (doi: 10.15482/USDA.ADC/1439968), includes hourly weather data; plant growth and canopy development over the season; final biomass, yield and harvest index; and daily water balance data including irrigation, precipitation, soil water content, and estimates of crop evapotranspiration. Soil parameters for the site, as well as data from a previous experiment on maize with different treatments can also be found online (doi: 10.15482/USDA.ADC/1254006). Here, we describe the synthesis of data collected from 2012 to 2013. These data can be used for modeling the relationship between maize yield and field-level water use under season water availability.

A blooming interest in the hydraulic traits of flowers

This article comments on: Water relations of Calycanthus flowers: Hydraulic conductance, capacitance, and embolism resistance.

The links between leaf hydraulic vulnerability to drought and key aspects of leaf venation and xylem anatomy among 26 Australian woody angiosperms from contrasting climates

Background and Aims

The structural properties of leaf venation and xylem anatomy strongly influence leaf hydraulics, including the ability of leaves to maintain hydraulic function during drought. Here we examined the strength of the links between different leaf venation traits and leaf hydraulic vulnerability to drought (expressed as P50leaf by rehydration kinetics) in a diverse group of 26 woody angiosperm species, representing a wide range of leaf vulnerabilities, from four low-nutrient sites with contrasting rainfall across eastern Australia.

Methods

For each species we measured key aspects of leaf venation design, xylem anatomy and leaf morphology. We also assessed for the first time the scaling relationships between hydraulically weighted vessel wall thickness (th) and lumen breadth (bh) across vein orders and habitats.

Key Results

Across species, variation in P50leaf was strongly correlated with the ratio of vessel wall thickness (th) to lumen breadth (bh) [(t/b)h; an index of conduit reinforcement] at each leaf vein order. Concomitantly, the scaling relationship between th and bh was similar across vein orders, with a log-log slope less than 1 indicating greater xylem reinforcement in smaller vessels. In contrast, P50leaf was not related to th and bh individually, to major vein density (Dvmajor) or to leaf size. Principal components analysis revealed two largely orthogonal trait groupings linked to variation in leaf size and drought tolerance.

Conclusions

Our results indicate that xylem conduit reinforcement occurs throughout leaf venation, and remains closely linked to leaf drought tolerance irrespective of leaf size.

Vessel scaling in evergreen angiosperm leaves conforms with Murray's law and area-filling assumptions: implications for plant size, leaf size and cold tolerance

Water transport in leaf vasculature is a fundamental process affecting plant growth, ecological interactions and ecosystem productivity, yet the architecture of leaf vascular networks is poorly understood. Although Murray's law and the West-Brown-Enquist (WBE) theories predict convergent scaling of conduit width and number, it is not known how conduit scaling is affected by habitat aridity or temperature. We measured the scaling of leaf size, conduit width and conduit number within the leaves of 36 evergreen Angiosperms spanning a large range in aridity and temperature in eastern Australia. Scaling of conduit width and number in midribs and 2° veins did not differ across species and habitats (P > 0.786), and did not differ from that predicted by Murray's law (P = 0.151). Leaf size was strongly correlated with the hydraulic radius of petiole conduits (r²  = 0.83, P < 0.001) and did not differ among habitats (P > 0.064), nor did the scaling exponent differ significantly from that predicted by hydraulic theory (P = 0.086). The maximum radius of conduits in petioles was positively correlated with the temperature of the coldest quarter (r²  = 0.67; P < 0.001), suggesting that habitat temperature restricts the occurrence of wide-conduit species in cold habitats.

Vessel diameter is related to amount and spatial arrangement of axial parenchyma in woody angiosperms

Parenchyma represents a critically important living tissue in the sapwood of the secondary xylem of woody angiosperms. Considering various interactions between parenchyma and water transporting vessels, we hypothesize a structure-function relationship between both cell types. Through a generalized additive mixed model approach based on 2,332 woody angiosperm species derived from the literature, we explored the relationship between the proportion and spatial distribution of ray and axial parenchyma and vessel size, while controlling for maximum plant height and a range of climatic factors. When factoring in maximum plant height, we found that with increasing mean annual temperatures, mean vessel diameter showed a positive correlation with axial parenchyma proportion and arrangement, but not for ray parenchyma. Species with a high axial parenchyma tissue fraction tend to have wide vessels, with most of the parenchyma packed around vessels, whereas species with small diameter vessels show a reduced amount of axial parenchyma that is not directly connected to vessels. This finding provides evidence for independent functions of axial parenchyma and ray parenchyma in large vesselled species and further supports a strong role for axial parenchyma in long-distance xylem water transport.

Embolized Stems Recover Overnight in Zea mays: The Role of Soil Water, Root Pressure, and Nighttime Transpiration

It is not currently well-understood how much xylem conductance is lost in maize plants during the day, if conductance is recovered during the night, or what soil water conditions are required for recovery to take place. To answer these questions we designed a greenhouse experiment whereby two genetically dissimilar maize genotypes were subjected to a level of water stress commonly experienced in the field (Ψxylem ∼-2 MPa). We then measured the loss of stem-specific conductivity associated with this level of stress, as well as the overnight recovery following three re-watering treatments: Ψsoil ∼ 0 MPa, Ψsoil ∼-0.40 MPa, and Ψsoil ∼-1.70 MPa. Mid-day leaf water potentials of -1.98 MPa resulted in stem-specific conductivity (KS) values that were 31.5% of maximal (i.e., 68% loss). Returning soils to field capacity (Ψsoil ∼ 0 MPa) overnight allowed for the significant recovery of KS (76% of maximal), whereas partial watering (Ψsoil ∼-0.40 MPa) resulted KS values that were 51.7% of maximal values, whereas not watering resulted in no recovery (35.4% of maximal; Ψsoil ∼-1.7 MPa). Recovery of KS was facilitated by the generation of root pressure and low rates of nighttime transpiration.

Toward an index of desiccation time to tree mortality under drought

Research in plant hydraulics has provided important insights into plant responses to drought and species absolute drought tolerance. However, our ability to predict when plants will die from hydraulic failure under extreme drought is limited by a lack of knowledge with regards to the dynamics of plant desiccation following stomatal closure. Thus, we develop a simple hydraulics model based on branch-level traits that incorporates key aspects of allometry, rates of water loss and resistance to embolism thresholds in order to define species differences in the time it takes plants to desiccate from stomatal closure to lethal levels of drought stress.

Weak tradeoff between xylem safety and xylem-specific hydraulic efficiency across the world's woody plant species

The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety-efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r(2)  < 0.086), no species had high efficiency and high safety, supporting the idea for a safety-efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated (r(2)  < 0.02 in most cases) with higher wood density, lower leaf- to sapwood-area and shorter stature. There appears to be no persuasive explanation for the considerable number of species with both low efficiency and low safety. These species represent a real challenge for understanding the evolution of xylem.

Weak coordination among petiole, leaf, vein, and gas-exchange traits across Australian angiosperm species and its possible implications

Close coordination between leaf gas exchange and maximal hydraulic supply has been reported across diverse plant life forms. However, it has also been suggested that this relationship may become weak or break down completely within the angiosperms. We examined coordination between hydraulic, leaf vein, and gas‐exchange traits across a diverse group of 35 evergreen Australian angiosperms, spanning a large range in leaf structure and habitat. Leaf‐specific conductance was calculated from petiole vessel anatomy and was also measured directly using the rehydration technique. Leaf vein density (thought to be a determinant of gas exchange rate), maximal stomatal conductance, and net CO₂ assimilation rate were also measured for most species (n = 19–35). Vein density was not correlated with leaf‐specific conductance (either calculated or measured), stomatal conductance, nor maximal net CO₂ assimilation, with r² values ranging from 0.00 to 0.11, P values from 0.909 to 0.102, and n values from 19 to 35 in all cases. Leaf‐specific conductance calculated from petiole anatomy was weakly correlated with maximal stomatal conductance (r² = 0.16; P = 0.022; n = 32), whereas the direct measurement of leaf‐specific conductance was weakly correlated with net maximal CO₂ assimilation (r² = 0.21; P = 0.005; n = 35). Calculated leaf‐specific conductance, xylem ultrastructure, and leaf vein density do not appear to be reliable proxy traits for assessing differences in rates of gas exchange or growth across diverse sets of evergreen angiosperms.

Leaf hydraulic vulnerability to drought is linked to site water availability across a broad range of species and climates

BACKGROUND AND AIMS

Vulnerability of the leaf hydraulic pathway to water-stress-induced dysfunction is a key component of drought tolerance in plants and may be important in defining species' climatic range. However, the generality of the association between leaf hydraulic vulnerability and climate across species and sites remains to be tested.

METHODS

Leaf hydraulic vulnerability to drought (P50leaf, the water potential inducing 50 % loss in hydraulic function) was measured in a diverse group of 92 woody, mostly evergreen angiosperms from sites across a wide range of habitats. These new data together with some previously published were tested against key climate indices related to water availability. Differences in within-site variability in P50leaf between sites were also examined.

KEY RESULTS

Values of hydraulic vulnerability to drought in leaves decreased strongly (i.e. became more negative) with decreasing annual rainfall and increasing aridity across sites. The standard deviation in P50leaf values recorded within each site was positively correlated with increasing aridity. P50leaf was also a good indicator of the climatic envelope across each species' distributional range as well as their dry-end distributional limits within Australia, although this relationship was not consistently detectable within sites.

CONCLUSIONS

The findings indicate that species sorting processes have influenced distributional patterns of P50leaf across the rainfall spectrum, but alternative strategies for dealing with water deficit exist within sites. The strong link to aridity suggests leaf hydraulic vulnerability may influence plant distributions under future climates.

Whole-plant capacitance, embolism resistance and slow transpiration rates all contribute to longer desiccation times in woody angiosperms from arid and wet habitats

Low water potentials in xylem can result in damaging levels of cavitation, yet little is understood about which hydraulic traits have most influence in delaying the onset of hydraulic dysfunction during periods of drought. We examined three traits contributing to longer desiccation times in excised shoots of 11 species from two sites of contrasting aridity: (i) the amount of water released from plant tissues per decrease in xylem water potential (WΨ); (ii) the minimum xylem water potential preceding acute water stress (defined as P50L; water potential at 50% loss of leaf conductance); and (iii) the integrated transpiration rate between the points of full hydration and P50L (Wtime). The time required for species to reach P50L varied markedly, ranging from 1.3 h to nearly 3 days. WΨ, P50L and Wtime all contributed significantly to longer desiccation times, explaining 28, 22 and 50% of the variance in the time required to reach P50L. Interestingly, these three traits were nearly orthogonal to one another, suggesting that they do not represent alternative hydraulic strategies, but likely trade off with other ecological strategies not evaluated in this study. The majority of water lost during desiccation (60-91%) originated from leaves, suggesting an important role for leaf capacitance in small plants when xylem water potentials decrease below -2 MPa.

Fibre wall and lumen fractions drive wood density variation across 24 Australian angiosperms

Wood density is often considered to be a key plant functional trait. But it is not clear what actually wood density is? We rigorously quantified anatomical underpinnings of wood density variation. We found that density was mainly driven by properties of mechanical tissue such as fibre wall fraction and fibre lumen fraction. However, there was also a substantial anatomical variation independent of density. This variation suggests that different plant ecological strategies may be available to woods with the same density. Our results imply that density is a complex characteristic of wood rather than a straightforward indicator of plant ecological strategies.

Wood density is considered a key plant trait, affecting mechanical and physiological performance, yet its biological meaning is still rather unclear. Accordingly we investigated the anatomical underpinnings of wood density in trees and shrubs. We measured wood density and anatomical traits in distal stems 4–10 mm diameter under bark in 24 Australian species. Proportions of wood components that are functionally distinct were analysed, including fibre wall and lumen, vessel wall and lumen, and axial and ray parenchyma. Wood density was mainly driven by the density of wood outside vessel lumens (density_NV) rather than by vessel lumen fraction. In turn, density_NV variation was chiefly affected by fibre wall and lumen fractions. Considerable anatomical variation was observed at a given density_NV, especially among medium-density_NV species (0.60–0.85 g cm⁻³); this range of medium density_NV roughly translates to 0.50–0.75 g cm⁻³ of overall density. The anatomy of these species formed a continuum from low fibre lumen and medium parenchyma fractions to medium fibre lumen and low parenchyma fractions. Our data suggest that wood density is an emergent property influenced by a complex anatomy rather than an unambiguous functional trait, particularly in medium-density species. With much anatomical variation, they likely represent a wide range of ecological strategies.

Light requirements of Australian tropical vs. cool-temperate rainforest tree species show different relationships with seedling growth and functional traits

BACKGROUND AND AIMS

A trade-off between shade tolerance and growth in high light is thought to underlie the temporal dynamics of humid forests. On the other hand, it has been suggested that tree species sorting on temperature gradients involves a trade-off between growth rate and cold resistance. Little is known about how these two major trade-offs interact.

METHODS

Seedlings of Australian tropical and cool-temperate rainforest trees were grown in glasshouse environments to compare growth versus shade-tolerance trade-offs in these two assemblages. Biomass distribution, photosynthetic capacity and vessel diameters were measured in order to examine the functional correlates of species differences in light requirements and growth rate. Species light requirements were assessed by field estimation of the light compensation point for stem growth.

RESULTS

Light-demanding and shade-tolerant tropical species differed markedly in relative growth rates (RGR), but this trend was less evident among temperate species. This pattern was paralleled by biomass distribution data: specific leaf area (SLA) and leaf area ratio (LAR) of tropical species were significantly positively correlated with compensation points, but not those of cool-temperate species. The relatively slow growth and small SLA and LAR of Tasmanian light-demanders were associated with narrow vessels and low potential sapwood conductivity.

CONCLUSIONS

The conservative xylem traits, small LAR and modest RGR of Tasmanian light-demanders are consistent with selection for resistance to freeze-thaw embolism, at the expense of growth rate. Whereas competition for light favours rapid growth in light-demanding trees native to environments with warm, frost-free growing seasons, frost resistance may be an equally important determinant of the fitness of light-demanders in cool-temperate rainforest, as seedlings establishing in large openings are exposed to sub-zero temperatures that can occur throughout most of the year.

Global convergence in the vulnerability of forests to drought

Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many individual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow (<1 megapascal) hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk.

Safety and streamlining of woody shoots in wind: an empirical study across 39 species in tropical Australia

• Wind is a key mechanical stress for woody plants, so how do shoot traits affect performance in wind? • We used a vehicle mounted apparatus to measure drag, streamlining and mechanical safety in 127 vertical lead-shoots, 1.2 m long, across 39 species in tropical Australia. • Shoot dimensions and stem tissue properties were closely coupled so that shoots with low stem specific gravity or larger projected area had thicker stems. Thicker stems provide larger second moment of area (I), which increased shoot safety and bending stiffness but impeded shoot reconfiguration in strong winds, including frontal area reduction. Nonetheless, increasing I also improved streamlining. Streamlining was unrelated to traits except I. Stem tissue material properties only had small effects. Higher modulus of rupture increased shoot safety and higher Young's modulus impeded shoot reconfiguration. • We found no conflict between bending stiffness and streamlining for woody shoots. Stiffness might help streamlining by increasing damping and stability, thereby reducing flagging in wind. Tissue-level traits did influence shoot-level mechanical safety and behaviour, but shoot geometry was much more important. Variable shoot and stem traits, which all influenced shoot biomechanics, were integrated in shoots to yield a relatively narrow range of outcomes in wind.

Assessing and mitigating the effects of windblown soil on rare and common vegetation

Acting under the auspices of the US Endangered Species Act, we quantified wind erosion and its effects on rare and common plant species on a semi-arid military installation in Hawaii. Our goal was to develop management strategies, based on local data, to aid the conservation of rare and common indigenous plants and their habitats. We collected windblown soil coming off of roads and other disturbed soils to assess likely impacts to plants occurring at certain heights and distances from disturbed surfaces. We then subjected plants in a glasshouse to windblown dust treatments, designed from our field data to simulate erosion events, and evaluated the effect of these treatments on photosynthesis and survival. We also designed several field experiments to examine the in-situ effects of windblown soil and soil substrate on germination, growth rate, and survival of indigenous and nonindigenous plants. We conclude from these experiments that most direct effects of windblown soil to plants can be effectively mitigated by locating roads and training areas at least 40 m from sensitive plant habitats and through vegetation management to maintain at least 11% aerial cover on disturbed surfaces. Effects of soil type on germination, growth, and survival was species-specific, emphasizing the importance of species trials prior to, or during, rehabilitation efforts.

Photosynthesis, carbohydrate storage and survival of a native and an introduced tree species in relation to light and defoliation

Fraxinus uhdei (Wenz.) Lingelsh (tropical ash), a species introduced to Hawaii from Mexico, invades forests of the endemic tree Acacia koa A.Gray (koa). We examined physiological and morphological characteristics of koa and tropical ash to explore possible mechanisms that may facilitate invasion of koa forests by tropical ash. Seedlings of both species were grown in a greenhouse in three light treatments: 100% photosynthetic photon flux (PPF); 18% PPF; and 2% PPF inside the greenhouse. Light compensation point, maximum CO2 assimilation rate and dark respiration rate of seedlings differed significantly among light treatments, but were similar between species. A defoliation experiment indicated that tropical ash was better able to survive defoliation than koa, especially under high-light conditions. Tropical ash seedlings allocated more carbon (C) and nitrogen (N) to storage per unit PPF than koa seedlings. Total nonstructural carbohydrates were positively correlated with plant survival in both species. The patterns of C and N allocation associated with tropical ash seedlings favor their survival in high light, under intense herbivory and on sites where N availability is seasonal or highly variable. Variation in carbohydrate storage between koa and tropical ash greatly exceeded variation in photosynthetic performance at the leaf level.