Higher Gas Exchange Rates Research Articles

Seed priming attenuates the impact of salt stress and enhances lettuce yields

Salt stress is a major factor that contributes to reduced lettuce productivity. Seed priming has emerged as a promising technique to improve crop stress tolerance. In this study, Romaine lettuce seeds were primed with calcium chloride (CaCl2), distilled water (hydro), and potassium nitrate (KNO3) to test their effectiveness in improving salt stress tolerance. Lettuce seeds treated with hydro, 50 mM CaCl2, and 0.5% KNO3 were exposed to 0 or 100 mM sodium chloride (NaCl). Priming treatments significantly increased lettuce's fresh mass by 22%–61% under salt stress. Hydro-primed lettuce showed the most significant increase in root mass (109%), a 38% higher root length, a 35% increase in surface area, and a 25%–40% increase in volume, tips, forks, and crossings of roots. The hydro-primed lettuce had higher gas exchange rates than the non-primed control, followed by the KNO3- and the CaCl2-primed lettuce. Furthermore, hydro-primed seedlings exhibited the highest proline accumulation (105%) under salt stress. The accumulation of sugars (up to 50%) in hydro- and CaCl2-primed lettuce led to a 56% decrease in electrolyte leakage and membrane damage. The correlation network analysis of the traits revealed that hydro-primed seedlings exhibited a higher + ve/-ve edge ratio, which indicates their greater resilience to salt stress than other priming techniques. Priming of seeds before planting offers the potential for improving resilience to stress.

The Poisoned Chalice of Evolution in Water: Physiological Novelty Versus Morphological Simplification in Marsileaceae

The Marsileaceae is a small family of semi-aquatic ferns displaying numerous traits commonly observed in angiosperms, including heterospory, sophisticated hydraulic architecture, and high rates of atmospheric gas exchange. Despite these similar traits, Marsileaceae is comparatively ecologically limited. Most species are found in Marsilea which is sister to Regnellidium and Pilularia, together these two genera include only seven species. Here we studied the anatomy and physiology of Marsileaceae to better understand the potential constraints on ecological and species diversity in this family. We focused on epidermal anatomy and stomatal responses to changes in light and water availability, which are unique amongst ferns. We found two evolutionary strategies in Marsileaceae, one of morphological simplification, physiological inflexibility, and aquatic specialization in Pilularia; which contrasts with a strategy of maximizing photosynthetic carbon gain at the expense of high rates of water loss in Marsilea and Regnellidium. We conclude that aquatic environments provide evolutionary opportunities for physiological innovation with regard to stomatal function, as well as selective pressures that drive the canalized evolution of highly specialized aquatic forms.

Photosynthetic patterns during autumn in three different Salix cultivars grown on a brownfield site

Leaf senescence at the end of the growing season is a complex process stimulated by changes in daylength and temperature that prepares deciduous trees for winter by reducing photosynthetic rates and remobilization of nutrients. Extending the duration of photosynthetic activity could have important consequences for the translocation of heavy metals in the phytoremediation of contaminated sites using deciduous trees like willow. In the present study, three Salix cultivars (‘India,’ ‘SX67,’ and ‘Fish Creek’) that were observed to maintain green leaves late into autumn were evaluated over an 11-week period extending from mid-September to mid-November on a brownfield site in Montreal, Canada. Gas exchange rates, chlorophyll fluorescence, and leaf pigments were measured weekly. A general trend of declining stomatal conductance and transpiration were observed early in the trial, followed by reductions in photosynthetic efficiency and concentrations of chl a, chl b, and carotenoids, in agreement with other studies. In particular, the cultivar ‘Fish Creek’ had higher rates of gas exchange and pigment concentrations than either ‘SX67’ or ‘India,’ but values for these parameters also declined more rapidly over the course of the trial. Both photoperiod and soil and air temperatures were strong drivers of changes in photosynthetic activity in all three of these cultivars according to correlation analyses. Further studies should focus on their biomass production and heavy metal accumulation capacity in light of the observed variation in photosynthetic activity stimulated by seasonal changes in light and temperature.

Polar day syndrome: differences in growth, photosynthetic traits and sink-size patterns between northern and southern Finnish silver birch (Betula pendula Roth) provenances in native and non-native photoperiods.

Continuous light (CL) is available throughout the polar day for plants in the Arctic during the growing season, whereas provenances of the same species experience a very different environment with non-CL (NCL) just a few latitudes to the south. Both provenances need to acclimate to climate warming, yet we lack comprehensive understanding of how their growth, photosynthesis and leaf traits differ. Further, the provenances presumably have morphological and physiological adaptations to their native environments and therefore differ in response to photoperiod. We tested the height growth, leaf longevity, biomass accumulation, biomass allocation and rates of gas exchange of northern (67°N) and southern (61°N) Finnish silver birch (Betula pendula Roth) origins in CL- and NCL-treatments in a 4-month chamber experiment. Irrespective of photoperiod, 67°N had higher area-based photosynthetic rate (Anet), stomatal conductance (gs) and relative height growth rate (RGR), but lower stomatal density and fewer branches and leaves than 61°N. Photoperiod affected height growth cessation, biomass and photosynthetic traits, whereas leaf longevity and many leaf functional traits remained unchanged. In CL, both provenances had lower gs, higher RGR, increased shoot:root ratio and increased sink sizes (more branching, more leaves, increased total plant dry weight) compared with NCL. In NCL, 67°N ceased height growth earlier than in CL, which altered biomass accumulation and distribution patterns. Northern conditions impose challenges for plant growth and physiology. Whether a provenance inhabits and is adapted to an area with or without CL can also affect its response to the changing climate. Northern birches may have adapted to CL and the short growing season with a 'polar day syndrome' of traits, including relatively high gas exchange rates with low leaf biomass and growth traits that are mainly limited by the environment and the earlier growth cessation (to avoid frost damage).

Context-dependent concordance between physiological divergence and phenotypic selection in sister taxa with contrasting phenology and mating systems.

The study of phenotypic divergence of, and selection on, functional traits in closely related taxa provides the opportunity to detect the role of natural selection in driving diversification. If the strength or direction of selection in field populations differs between taxa in a pattern that is consistent with the phenotypic difference between them, then natural selection reinforces the divergence. Few studies have sought evidence for such concordance for physiological traits. Herbarium specimen records were used to detect phenological differences between sister taxa independent of the effects on flowering time of long-term variation in the climate across collection sites. In the field, physiological divergence in photosynthetic rate, transpiration rate, and instantaneous water-use efficiency were recorded during vegetative growth and flowering in 13 field populations of two taxon pairs of Clarkia, each comprising a self-pollinating and a outcrossing taxon. Historically, each selfing taxon flowered earlier than its outcrossing sister taxon, independent of the effects of local long-term climatic conditions. Sister taxa differed in all focal traits, but the degree and (in one case) the direction of divergence depended on life stage. In general, self-pollinating taxa had higher gas exchange rates, consistent with their earlier maturation. In 6 of 18 comparisons, patterns of selection were concordant with the phenotypic divergence (or lack thereof) between sister taxa. Patterns of selection on physiological traits measured in heterogeneous conditions do not reliably reflect divergence between sister taxa, underscoring the need for replicated studies of the direction of selection within and among taxa.

ZEITLUPE Promotes ABA-Induced Stomatal Closure in Arabidopsis and Populus.

Plants balance water availability with gas exchange and photosynthesis by controlling stomatal aperture. This control is regulated in part by the circadian clock, but it remains unclear how signalling pathways of daily rhythms are integrated into stress responses. The serine/threonine protein kinase OPEN STOMATA 1 (OST1) contributes to the regulation of stomatal closure via activation of S-type anion channels. OST1 also mediates gene regulation in response to ABA/drought stress. We show that ZEITLUPE (ZTL), a blue light photoreceptor and clock component, also regulates ABA-induced stomatal closure in Arabidopsis thaliana, establishing a link between clock and ABA-signalling pathways. ZTL sustains expression of OST1 and ABA-signalling genes. Stomatal closure in response to ABA is reduced in ztl mutants, which maintain wider stomatal apertures and show higher rates of gas exchange and water loss than wild-type plants. Detached rosette leaf assays revealed a stronger water loss phenotype in ztl-3, ost1-3 double mutants, indicating that ZTL and OST1 contributed synergistically to the control of stomatal aperture. Experimental studies of Populus sp., revealed that ZTL regulated the circadian clock and stomata, indicating ZTL function was similar in these trees and Arabidopsis. PSEUDO-RESPONSE REGULATOR 5 (PRR5), a known target of ZTL, affects ABA-induced responses, including stomatal regulation. Like ZTL, PRR5 interacted physically with OST1 and contributed to the integration of ABA responses with circadian clock signalling. This suggests a novel mechanism whereby the PRR proteins—which are expressed from dawn to dusk—interact with OST1 to mediate ABA-dependent plant responses to reduce water loss in time of stress.

Temporal variability in evapotranspiration and energy partitioning over a seasonally flooded scrub forest of the Brazilian Pantanal

Brazilian Pantanal is the largest seasonal wetland in the world and it plays an important role in regulating the local and global water cycle and energy balance. The Pantanal landscape consists of a mosaic of temporary and permanent aquatic habitats, floodable and non-floodable pastures, and forests; however, patterns of energy exchange are poorly understood, especially for seasonally flooded forests. Thus, the main objective of this research was to assess the variability of evapotranspiration (ET) and to evaluate biophysical controls on ET for a seasonally flooded scrub-forest. We used eddy covariance to measure latent (LE) and sensible (H) heat fluxes and linked both micrometeorological and remote sensing data to assess the connection between water dynamics and phenology. Our data suggest that 72% of the net radiation (Rn) was consumed by LE + H, even during the dry season, indicating that trees can access deep water when surface water is limiting. The decoupling factor (0.63 ± 0.08) suggests that the canopy is decoupled from the atmosphere and ET is essentially sensitive to Rn. The patterns of ET and vapor pressure deficit suggest that ET was more dependent on the Rn received and less dependent on stomatal control, but still exhibited a strong relationship with biotic factors such as enhanced vegetation index (EVI), which is sensitive to seasonal variations in leaf area index and canopy cover. Stomatal limitations of this seasonally flooded forest appear to be minimal, even in the dry season, presumably due to the relatively high-water table, the ability of trees to acquire subsurface water, and the predominance of secondary species with high rates of leaf gas exchange. These data indicate that seasonally flooded tropical forests of the Pantanal are consistent sources of water vapor to the atmosphere even during seasonal drought.

Spatio-temporal differences in leaf physiology are associated with fire, not drought, in a clonally integrated shrub.

In highly disturbed environments, clonality facilitates plant survival via resprouting after disturbance, resource sharing among interconnected stems and vegetative reproduction. These traits likely contribute to the encroachment of deep-rooted clonal shrubs in tallgrass prairie. Clonal shrubs have access to deep soil water and are typically thought of as relatively insensitive to environmental variability. However, how leaf physiological traits differ among stems within individual clonal shrubs (hereafter ‘intra-clonal’) in response to extreme environmental variation (i.e. drought or fire) is unclear. Accounting for intra-clonal differences among stems in response to disturbance is needed to more accurately parameterize models that predict the effects of shrub encroachment on ecosystem processes. We assessed intra-clonal leaf-level physiology of the most dominant encroaching shrub in Kansas tallgrass prairie, Cornus drummondii, in response to precipitation and fire. We compared leaf gas exchange rates from the periphery to centre within shrub clones during a wet (2015) and extremely dry (2018) year. We also compared leaf physiology between recently burned shrubs (resprouts) with unburned shrubs in 2018. Resprouts had higher gas exchange rates and leaf nitrogen content than unburned shrubs, suggesting increased rates of carbon gain can contribute to recovery after fire. In areas recently burned, resprouts had higher gas exchange rates in the centre of the shrub than the periphery. In unburned areas, leaf physiology remained constant across the growing season within clonal shrubs (2015 and 2018). Results suggest single measurements within a shrub are likely sufficient to parameterize models to understand the effects of shrub encroachment on ecosystem carbon and water cycles, but model parameterization may require additional complexity in the context of fire.

Daily entropy of dissolved oxygen reveals different energetic regimes and drivers among high‐mountain stream types

AbstractHigh‐resolution time series of dissolved oxygen (DO) have revealed different ecosystem energetics regimes across various stream types. Ecosystem energetic regimes are relevant to better understand the transformation and retention of nutrients and carbon in stream ecosystems. However, the patterns and controls of stream energetics in high‐mountain landscapes remain largely unknown. Here we monitored percent DO saturation (every 10 min) over 2 years in a glacier‐fed, krenal (groundwater‐fed) and a nival (snowmelt‐fed) stream as they are typical for the high mountains. We used daily Shannon entropy to explore the temporal dynamics of stream water DO and to infer information on the ecosystem energetics and on the potential drivers. We found that discharge modulated the drivers of DO variations at daily and seasonal scales. Elevated bed movement along with high turbidity and very high gas exchange rates drove the daily DO patterns in the glacier‐fed stream during snow and ice melt, whereas light seemed to drive DO dynamics in the krenal and nival streams. We found a window of favorable conditions for potential gross primary production (GPP) during the onset of the snowmelt in the glacier‐fed stream, whereas potential GPP seemed to extend over longer periods in the krenal and nival streams. Our findings suggest how the energetic regimes of these high‐mountain streams may change in the future as their biological and physical drivers change owing to climate warming.

Stomatal density and mechanics are critical for high productivity: insights from amphibious ferns.

Angiosperm dominance in terrestrial landscapes is partially attributable to high photosynthetic capacities. Angiosperms benefit from diverse anatomical and physiological adaptations, making it difficult to determine which factors may have been prerequisites for the evolution of enhanced photosynthetic rates in this group. We employed a novel approach to this problem: comparisons between angiosperms and Marsileaceae, a family of semi-aquatic ferns that are among the only land plants to match angiosperm photosynthetic rates. We found that Marsileaceae have very high stomatal densities and, like angiosperms but unlike all other ferns previously studied, exhibit wrong-way stomatal responses to excision. These results suggest that stomatal density and a little-studied angiosperm trait, the capacity for lateral displacement of guard cells into neighboring epidermal cells, are crucial for facilitating high rates of gas exchange. Our analysis also associates these adaptations in Marsileaceae with an increased risk of excessive water loss during drought. Our findings indicate that evolution in stomatal physiology was a prerequisite for high photosynthetic capacities in vascular plants and a key driver of the abrupt Cretaceous rise of the angiosperms.

Atavistic Stomatal Responses to Blue Light in Marsileaceae.

Stomata respond to changes in light environment through multiple mechanisms that jointly regulate the tradeoff between carbon assimilation and water loss. The stomatal response to blue light is highly sensitive, rapid, and not driven by photosynthesis. It is present in most vascular plant groups but is believed to have been lost in the ancestor of leptosporangiate ferns. Schizaeales and Salviniales are the only leptosporangiate orders that have not been tested for stomatal responses to a low fluence of blue light. We report that these stomatal responses are absent in Lygodium japonicum (Schizaeales). In contrast, we observed stomatal responses to a low fluence of blue light in Regnellidium diphyllum and Marsilea minuta (Marsileaceae, Salviniales). In R. diphyllum, blue light triggered stomatal oscillations. The oscillations were more sensitive to atmospheric carbon dioxide concentration than to humidity, suggesting that the blue light responses of Marsileaceae stomata differ from those of angiosperms. Our findings suggest that Marsileaceae have physiologically diverged from other leptosporangiate ferns, achieving unusually high photosynthetic capacities through amphibious lifestyles and numerous anatomical convergences with angiosperms. Blue light stomatal responses may have contributed to this divergence by enabling high rates of leaf gas exchange in Marsileaceae.

Steering and in situ monitoring of drying phenomena during film fabrication

During film fabrication, the phenomena of crack formation and delamination are often observed, dramatically hindering the discovery and characterization of new materials for energy applications. In this work, we report on a novel approach to fully steer the drying parameters or “knobs” that are commonly used during electrode manufacture. It allows us to precisely in situ control and monitor the solvent-specific evaporation rates that affect the development of suspension composition during drying. We managed to control the capillary stress inside the layer by precisely controlling the selectivity of solvent evaporation. Large cracks result when the surface tension increases over time and layer delamination occurs. When using an n-propanol/water system, critical crack formation is achieved when water is enriched by decreasing the gas exchange during drying or preloading the gas phase with water vapor. High gas exchange rates inhibit the water’s enrichment, and therefore, only small surface cracks develop. The experiments also surprisingly indicate that the drying temperature has no significant effect on crack formation. These results are of fundamental meaning for the future development of electrodes as the drying step has a high impact on the products specification and now can be ultimately controlled. The future development of electrodes will surely benefit from this achievement in the controlled fabrication of films for a variety of applications.

Plant water-use strategy mediates stomatal effects on the light induction of photosynthesis.

More efficient gas exchange strategies under dynamic light environments have been hypothesised to contribute to the dominance of angiosperms in the vascular plant flora. However, we still lack a clear understanding of how stomatal dynamics affect photosynthetic dynamics and whether differences exist between lineages. Stomatal and photosynthetic dynamics following changes in irradiance were studied in 15 species, encompassing ferns, gymnosperms and angiosperms. We determined the effect of stomatal speed on dynamic photosynthesis and water loss. Moreover, we assessed whether dynamic behaviour followed evolutionary lineage divisions, or whether ecological adaptation to maximise light fleck use could describe dynamic behaviour. We found that species with fast stomatal opening, such as ferns, forgo less photosynthesis during photosynthetic induction. By contrast, there was no relationship between stomatal closure speed and the water wasted by transiently more-open stomata, because species with higher rates of gas exchange also showed faster stomatal closure. Shade-adapted species possessed fast-opening but slow-closing stomata, consistent with ecological adaptation to maximise light fleck use. Our results suggest dynamic behaviour follows adaptive ecological trends more strongly than evolutionary ones, but angiosperms may benefit from relatively faster photosynthetic induction by adopting a less conservative water-use strategy.

Measurement of gas-exchange rate in streams by the oxygen–carbon method

The gas-exchange rate between streams and the atmosphere is needed to measure in-stream ecological processes and C processing in rivers and streams. Current methods include empirical relationships to hydraulics, direct injection of a tracer gas, and modeling based on O2 or C diel curves. All existing methods have strengths and drawbacks and most are limited to point measurements or are unable to measure diel variation in exchange rate. Researchers continue to search for better techniques, particularly for steep streams with high rates of gas exchange and low primary productivity. We present the O2−C (OC) method for calculating gas-exchange rates via simultaneous measurement of O2 and dissolved inorganic C (DIC). Gas-exchange rates are calculated by solving the combined stream transport equation for O2 and DIC. The output is a time-series of aeration rates at the same sampling frequency as the input O2 and C data. Field tests in a 4th-order montane stream in Oregon, USA, indicate that the method is suitable for stream reaches with high downstream gas-concentration gradients and saturation deficits. The mean modeled aeration rate adjusted to 17°C (3.25/h) agreed well with the value of 3.22/h from direct gas injection. Net ecosystem production calculated with the modeled aeration rate (−1.69 g O2 m−2 d−1) was consistent with the result obtained with direct gas injection (−1.60 g O2 m−2 d−1). An assumption of the model is a constant respiration quotient, but results indicated that the respiration quotient may be time variable. Sensitivity analysis indicated that application of the OC method is limited to reaches with a suitable change in combined O2 and CO2 concentration ≥ ∼4 μmol/L and combined O2 and CO2 saturation deficits ≈ 4 μmol/L, characteristic of smaller gaining streams. Preliminary application of the OC method indicates it could be useful to practitioners interested in continuous measurement of gas-exchange rates.

Variation of stomatal traits from cold temperate to tropical forests and association with water use efficiency

Abstract Stomata control carbon and water vapour exchange between leaves and the atmosphere, thus it can influence water use efficiency (WUE) and reflect plant adaptation to climate. However, the spatial patterns of leaf stomatal traits and relationships between stomatal trait and WUE across natural communities remain unclear. We measured stomatal density, stomatal size and stomatal area fraction for 737 plant species from nine forests ranging from tropical to cold temperate forests. Stomatal density, stomatal size and stomatal area fraction were all log‐normally distributed, and different across species, plant functional groups (trees, shrubs, and herbs), and communities. At the regional scale, variation in stomatal traits was primarily related to species, followed by climate and soil types. The community‐weighted mean of stomatal size increased linearly with latitude, whereas those of stomatal density and stomatal area fraction showed humpbacked relationship. The community‐weighted mean of stomatal area fraction was correlated with climatic aridity, consistent with the adaptation strategies of plant species to achieve high maximum rates of gas exchange in arid regions when water is available. Further, community‐weighted mean of stomatal area fraction was positively correlated with WUE in natural forest communities, indicating that plants have lower stomatal conductance in order to adapt greater aridity conditions. These findings highlight the strong associations of stomatal traits with plant functional group and climate at a regional scale, representing the adaptation strategies of stomatal traits across natural forest communities to climate. A plain language summary is available for this article.

The fifth leaf and spike organs of barley (Hordeum vulgare L.) display different physiological and metabolic responses to drought stress.

BackgroundPhotosynthetic organs of the cereal spike (ear) provide assimilate for grain filling, but their response to drought is poorly understood. In this study, we characterized the drought response of individual organs of the barley spike (awn, lemma, and palea) and compared them with a vegetative organ (fifth leaf). Understanding differences in physiological and metabolic responses between the leaf and spike organs during drought can help us develop high yielding cultivars for environments where terminal drought is prevalent.ResultsWe exposed barley plants to drought by withholding water for 4 days at the grain filling stage and compared changes in: (1) relative water content (RWC), (2) osmotic potential (Ψs), (3) osmotic adjustment (OA), (4) gas exchange, and (5) metabolite content between organs. Drought reduced RWC and Ψs in all four organs, but the decrease in RWC was greater and there was a smaller change in Ψs in the fifth leaf than the spike organs. We detected evidence of OA in the awn, lemma, and palea, but not in the fifth leaf. Rates of gas exchange declined more rapidly in the fifth leaf than awn during drought. We identified 18 metabolites but, only ten metabolites accumulated significantly during drought in one or more organs. Among these, proline accumulated in all organs during drought while accumulation of the other metabolites varied between organs. This may suggest that each organ in the same plant uses a different set of osmolytes for drought resistance.ConclusionsOur results suggest that photosynthetic organs of the barley spike maintain higher water content, greater osmotic adjustment, and higher rates of gas exchange than the leaf during drought.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0922-1) contains supplementary material, which is available to authorized users.

Reconstructing Extinct Plant Water Use for Understanding Vegetation–Climate Feedbacks: Methods, Synthesis, and a Case Study Using the Paleozoic-Era Medullosan Seed Ferns

Vegetation affects feedbacks in Earth's hydrologic system, but is constrained by physiological adaptations. In extant ecosystems, the mechanisms controlling plant water used can be measured experimentally; for extinct plants in the recent geological past, water use can be inferred from nearest living relatives, assuming minimal evolutionary change. In deep time, where no close living relatives exist, fossil material provides the only information for inferring plant water use. However, mechanistic models for extinct plant water use must be built on first principles and tested on extant plants. Plants serve as a conduit for water movement from the soil to the atmosphere, constrained by tissue-level construction and gross architecture. No single feature, such as stomata or veins, encompasses enough of the complexity underpinning water-use physiology to serve as the basis of a model of functional water use in all (or perhaps any) extinct plants. Rather, a “functional whole plant” model must be used. To understand the interplay between plant and atmosphere, water use in relation to environmental conditions is investigated in an extinct plant, the seed fernMedullosa((Division Pteridospermatophyta), by reviewing methods for reconstructing physiological variables such as leaf and stem hydraulic capacity, photosynthetic rate, transpiration rate, stomatal conductance, and albedo. Medullosans had the potential for extremely high photosynthetic and assimilation rates, water transport, stomatal conductance, and transpiration—rates comparable to later angiosperms. When these high growth and gas exchange rates of medullosans are combined with the unique atmospheric gas composition of the late Paleozoic atmosphere, complex vegetation-environmental feedbacks are expected despite their basal phylogenetic position relative to post-Paleozoic seed plants.

The Evolution of Unidirectional Pulmonary Airflow.

Conventional wisdom holds that the avian respiratory system is unique because air flows in the same direction through most of the gas-exchange tubules during both phases of ventilation. However, recent studies showing that unidirectional airflow also exists in crocodilians and lizards raise questions about the true phylogenetic distribution of unidirectional airflow, the selective drivers of the trait, the date of origin, and the functional consequences of this phenomenon. These discoveries suggest unidirectional flow was present in the common diapsid ancestor and are inconsistent with the traditional paradigm that unidirectional flow is an adaptation for supporting high rates of gas exchange. Instead, these discoveries suggest it may serve functions such as decreasing the work of breathing, decreasing evaporative respiratory water loss, reducing rates of heat loss, and facilitating crypsis. The divergence in the design of the respiratory system between unidirectionally ventilated lungs and tidally ventilated lungs, such as those found in mammals, is very old, with a minimum date for the divergence in the Permian Period. From this foundation, the avian and mammalian lineages evolved very different respiratory systems. I suggest the difference in design is due to the same selective pressure, expanded aerobic capacity, acting under different environmental conditions. High levels of atmospheric oxygen of the Permian Period relaxed selection for a thin blood-gas barrier and may have resulted in the homogeneous, broncho-alveolar design, whereas the reduced oxygen of the Mesozoic selected for a heterogeneous lung with an extremely thin blood-gas barrier. These differences in lung design may explain the puzzling pattern of ecomorphological diversification of Mesozoic mammals: all were small animals that did not occupy niches requiring a great aerobic capacity. The broncho-alveolar lung and the hypoxia of the Mesozoic may have restricted these mammals from exploiting niches of large body size, where cursorial locomotion can be advantageous, as well as other niches requiring great aerobic capacities, such as those using flapping flight. Furthermore, hypoxia may have exerted positive selection for a parasagittal posture, the diaphragm, and reduced erythrocyte size, innovations that enabled increased rates of ventilation and more rapid rates of diffusion in the lung.

Unidirectional flow in lizard lungs: a paradigm shift in our understanding of lung evolution in Diapsida

Conventional wisdom has held that unidirectional pulmonary airflow is unique to birds and is an adaption enabling high rates of gas exchange, essential for sustaining flight as well as an endothermic metabolism. Recent visualizations and measurements of flow in the lungs of monitor and iguanid lizards show a bird-like pattern of unidirectional flow in these lineages. These findings call for a paradigm shift in our understanding of lung evolution in diapsids. This pattern of flow is not unique to birds. It is much older than previously believed, and it may be advantageous to the low-energy lifestyles typical of ectothermic animals.

Wood harvest by pollarding exerts long-term effects on Populus euphratica stands in riparian forests at the Tarim River, NW China

Populus euphratica (Euphrates poplar) is the dominant tree species of the riparian (“tugai”) forests of Central Asia, which provide important ecosystem services to a rapidly growing population. However, overuse of the forests by wood harvest (pollarding) contributes markedly to their destabilisation. At the upper reaches of the Tarim River (Xinjiang, NW China), we investigated the effects of past pollarding (in the 1970s and 1980s) on the stand structure, tree morphology, stem diameter increment and intrinsic water use efficiency (iWUE) of poplars with different pollarding intensities in the past (‘No Use’, ‘Moderate’, ‘Intense’) growing on three adjacent plots with the same distance to the groundwater level. Compared to the non-used trees, the pollarded poplars (in particular, the intensely used trees) exhibited smaller figures of the following morphological variables: ratio of tree height to diameter at breast height, vertical crown extension, crown projection area, crown volume, and tree-ring width as well as basal area increment (BAI) during the past 24years; but a higher number of secondary shoots, a higher percentage of hollow stems and a higher degree of hollowness of the stems. The pollarded trees were capable of regenerating to a certain extent, which was obvious from the formation of secondary shoots and, in the intensely pollarded trees, from a lower iWUE (inferred by more negative δ13C isotopic ratios of the tree rings; most probably due to higher rates of gas exchange) as well as from their capability of re-establishing a crown efficiency similar to non-pollarded trees. However, the BAI of the main trunk and the secondary branches of the pollarded trees decreased continuously during the last 24years of investigation. Whilst moderate intensities of pollarding seem to be sustainable for the riparian P. euphratica forests, intense pollarding reduces the growth increment of the trees and, even more importantly, results in a distinct increase in the percentage of hollow stems, which can render the trees less stable and more susceptible to secondary damaging factors. The significant correlations between BAI and morphological variables of crown projection area and crown volume are promising for developing approaches to assess the productivity of P. euphratica stands on a landscape level using methods of remote sensing.