Thermal acclimation of leaf photosynthetic traits in an evergreen woodland, consistent with the coordination hypothesis

Ecosystem models commonly assume that key photosynthetic traits, such as carboxylation capacity measured at a standard temperature, are constant in time. The temperature responses of modelled photosynthetic or respiratory rates then depend entirely on enzyme kinetics. Optimality considerations, however, suggest this assumption may be incorrect. The coordination hypothesis (that Rubisco- and electron-transport-limited rates of photosynthesis are co-limiting under typical daytime conditions) predicts, instead, that carboxylation (Vcmax) capacity should acclimate so that it increases somewhat with growth temperature but less steeply than its instantaneous response, implying that Vcmax when normalized to a standard temperature (e.g. 25 °C) should decline with growth temperature. With additional assumptions, similar predictions can be made for electron-transport capacity (Jmax) and mitochondrial respiration in the dark (Rdark). To explore these hypotheses, photosynthetic measurements were carried out on woody species during the warm and the cool seasons in the semi-arid Great Western Woodlands, Australia, under broadly similar light environments. A consistent proportionality between Vcmax and Jmax was found across species. Vcmax, Jmax and Rdark increased with temperature in most species, but their values standardized to 25 °C declined. The ci : ca ratio increased slightly with temperature. The leaf N  :  P ratio was lower in the warm season. The slopes of the relationships between log-transformed Vcmax and Jmax and temperature were close to values predicted by the coordination hypothesis but shallower than those predicted by enzyme kinetics.This research was funded by the Terrestrial Ecosystem Research Network (TERN), Macquarie University and the Australian National University. Henrique Fürstenau Togashi was supported by an international Macquarie University International Research Scholarship (iMQRES). Iain Colin Prentice, Bradley John Evans, and Henrique Fürstenau Togashi were funded by the Ecosystem Modelling and Scaling Infrastructure (eMAST, part of TERN). TERN and eMAST have been supported by the Australian Government through the National Collaborative Research Infrastructure Strategy (NCRIS). Owen Atkin acknowledges the support of the Australian Research Council (DP130101252 and CE140100008)

Plasticity of photosynthetic heat tolerance in plants adapted to thermally contrasting biomes

In many biomes, plants are subject to heatwaves, potentially causing irreversible damage to the photosynthetic apparatus. Field surveys have documented global, temperature-dependent patterns in photosynthetic heat tolerance (PHT); however, it remains unclear if these patterns reflect acclimation in PHT or inherent differences among species adapted to contrasting habitats. To address these unknowns, we quantified seasonal variations in Tcrit (high temperature where minimal chlorophyll-a fluorescence rises rapidly, reflecting disruption to photosystem II) in 62 species native to 6 sites from 5 thermally contrasting biomes across Australia. Tcrit and leaf fatty acid (FA) composition (important for membrane stability) were quantified in three temperature-controlled glasshouses in 20 of those species. Tcrit was greatest at hot field sites and acclimated seasonally (summer > winter, increasing on average 0.34 °C per °C increase in growth temperature). The glasshouse study showed that Tcrit was inherently higher in species from warmer habitats (increasing 0.16 °C per °C increase in origin annual mean maximum temperature) and acclimated to increasing growth temperature (0.24 °C °C−1). Variations in Tcrit were positively correlated with the relative abundance of saturated FAs, with FAs accounting for 40% of Tcrit variation. These results highlight the importance of both plastic adjustments and inherent differences determining contemporary continent-wide patterns in PHT.This work was funded by grants from the Australian Research Council (Grants DP0986823, DP130101252, CE140100008) to O. K. A. We also acknowledge the support of the Australian SuperSite Network, part of the Australian Government's Terrestrial Ecosystem Research Network (www.tern.org.au)

Climate change and water in the UK – past changes and future prospects

Climate change is expected to modify rainfall, temperature and catchment hydrological responses across the world, and adapting to these water-related changes is a pressing challenge. This paper reviews the impact of anthropogenic climate change on water in the UK and looks at projections of future change. The natural variability of the UK climate makes change hard to detect; only historical increases in air temperature can be attributed to anthropogenic climate forcing, but over the last 50 years more winter rainfall has been falling in intense events. Future changes in rainfall and evapotranspiration could lead to changed flow regimes and impacts on water quality, aquatic ecosystems and water availability. Summer flows may decrease on average, but floods may become larger and more frequent. River and lake water quality may decline as a result of higher water temperatures, lower river flows and increased algal blooms in summer, and because of higher flows in the winter. In communicating this important work, researchers should pay particular attention to explaining confidence and uncertainty clearly. Much of the relevant research is either global or highly localized: decision-makers would benefit from more studies that address water and climate change at a spatial and temporal scale appropriate for the decisions they make

Understanding and modelling wildfire regimes: An ecological perspective

© 2021 The Author(s).Recent extreme wildfire seasons in several regions have been associated with exceptionally hot, dry conditions, made more probable by climate change. Much research has focused on extreme fire weather and its drivers, but natural wildfire regimes—and their interactions with human activities—are far from being comprehensively understood. There is a lack of clarity about the 'causes' of wildfire, and about how ecosystems could be managed for the co-existence of wildfire and people. We present evidence supporting an ecosystem-centred framework for improved understanding and modelling of wildfire. Wildfire has a long geological history and is a pervasive natural process in contemporary plant communities. In some biomes, wildfire would be more frequent without human settlement; in others they would be unchanged or less frequent. A world without fire would have greater forest cover, especially in present-day savannas. Many species would be missing, because fire regimes have co-evolved with plant traits that resist, adapt to or promote wildfire. Certain plant traits are favoured by different fire frequencies, and may be missing in ecosystems that are normally fire-free. For example, post-fire resprouting is more common among woody plants in high-frequency fire regimes than where fire is infrequent. The impact of habitat fragmentation on wildfire crucially depends on whether the ecosystem is fire-adapted. In normally fire-free ecosystems, fragmentation facilitates wildfire starts and is detrimental to biodiversity. In fire-adapted ecosystems, fragmentation inhibits fires from spreading and fire suppression is detrimental to biodiversity. This interpretation explains observed, counterintuitive patterns of spatial correlation between wildfire and potential ignition sources. Lightning correlates positively with burnt area only in open ecosystems with frequent fire. Human population correlates positively with burnt area only in densely forested regions. Models for vegetation-fire interactions must be informed by insights from fire ecology to make credible future projections in a changing climate.We gratefully acknowledge support from the Leverhulme Centre for Wildfires, Environment and Society, who organized the virtual mini-workshop which initiated the writing of this paper. RKN is supported by the Leverhulme Centre. SPH and YS acknowledge support from the ERC-funded project GC2.0 (Global Change 2.0: Unlocking the past for a clearer future, Grant Number 694481). ICP, KJB and ND acknowledge support from the ERC-funded project REALM (Re-inventing Ecosystem And Land-surface Models, Grant Number 787203). JCH acknowledges funding from the ERC project SCATAPNUT (Grant Number 681885). This work is a contribution to the LEMONTREE (Land Ecosystem Models based On New Theory, obseRvations and ExperimEnts) project, funded through the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program (SPH, YS and ICP)

Thermal limits of leaf metabolism across biomes

High-temperature tolerance in plants is important in a warming world, with extreme heat waves predicted to increase in frequency and duration, potentially leading to lethal heating of leaves. Global patterns of high-temperature tolerance are documented in animals, but generally not in plants, limiting our ability to assess risks associated with climate warming. To assess whether there are global patterns in high-temperature tolerance of leaf metabolism, we quantified Tcrit (high temperature where minimal chlorophyll a fluorescence rises rapidly and thus photosystem II is disrupted) and Tmax (temperature where leaf respiration in darkness is maximal, beyond which respiratory function rapidly declines) in upper canopy leaves of 218 plant species spanning seven biomes. Mean site-based Tcrit values ranged from 41.5 °C in the Alaskan arctic to 50.8 °C in lowland tropical rainforests of Peruvian Amazon. For Tmax, the equivalent values were 51.0 and 60.6 °C in the Arctic and Amazon, respectively. Tcrit and Tmax followed similar biogeographic patterns, increasing linearly (˜8 °C) from polar to equatorial regions. Such increases in high-temperature tolerance are much less than expected based on the 20 °C span in high-temperature extremes across the globe. Moreover, with only modest high-temperature tolerance despite high summer temperature extremes, species in mid-latitude (~20-50°) regions have the narrowest thermal safety margins in upper canopy leaves; these regions are at the greatest risk of damage due to extreme heat-wave events, especially under conditions when leaf temperatures are further elevated by a lack of transpirational cooling. Using predicted heat-wave events for 2050 and accounting for possible thermal acclimation of Tcrit and Tmax, we also found that these safety margins could shrink in a warmer world, as rising temperatures are likely to exceed thermal tolerance limits. Thus, increasing numbers of species in many biomes may be at risk as heat-wave events become more severe with climate change

Climate change and water in the UK: past changes and future prospects

Climate change is expected to modify rainfall, temperature and catchment hydrological responses across the world, and adapting to these water-related changes is a pressing challenge. This paper reviews the impact of anthropogenic climate change on water in the UK and looks at projections of future change. The natural variability of the UK climate makes change hard to detect; only historical increases in air temperature can be attributed to anthropogenic climate forcing, but over the last 50 years more winter rainfall has been falling in intense events. Future changes in rainfall and evapotranspiration could lead to changed flow regimes and impacts on water quality, aquatic ecosystems and water availability. Summer flows may decrease on average, but floods may become larger and more frequent. River and lake water quality may decline as a result of higher water temperatures, lower river flows and increased algal blooms in summer, and because of higher flows in the winter. In communicating this important work, researchers should pay particular attention to explaining confidence and uncertainty clearly. Much of the relevant research is either global or highly localized: decision-makers would benefit from more studies that address water and climate change at a spatial and temporal scale appropriate for the decisions they mak

Knowledge gaps in our perceptual model of Great Britain’s hydrology

There is a no lack of significant open questions in the field of hydrology. How will hydrological connectivity between freshwater bodies be altered by future human alterations to the hydrological cycle? Where does water go when it rains? Or what is the future space-time variability of flood and drought events? However, the answers to these questions will vary with location due to the specific and often poorly understood local boundary conditions and system properties that control the functional behaviour of a catchment or any other hydrologic control volume. We suggest that an open, shared and evolving perceptual model of a region’s hydrology is critical to tailor our science questions, as it would be for any other study domain from the plot to the continental scale. In this opinion piece, we begin to discuss the elements of and point out some knowledge gaps in the perceptual model of the terrestrial water cycle of Great Britain. We discuss six major knowledge gaps and propose four key ways to reduce them. While the specific knowledge gaps in our perceptual model do not necessarily transfer to other places, we believe that the development of such perceptual models should be at the core of the debate for all hydrologic communities, and we encourage others to have a similar debate for their hydrologic domain

Studies in Dhāraṇī Literature II: Pragmatics of Dhāraṇīs

This article is one of a series that reassesses the dhāraṇī texts of Mahāyāna Buddhism. The article seeks to examine dhāraṇī texts by using the linguistic tools of pragmatics, especially historical pragmatics, to assist the understanding of their statements. Rather than the meaning of the term dhāraṇī as a subject term, the domain of truth-conditional semantics, this paper examines statements in texts labelled dhāraṇī. Pragmatics examines meaning in context, and the categories of speech acts developed by Searle has been especially helpful in mapping out differences within such texts and the formalization of statements across texts. The grammaticalization of specific speech elements, especially interjections, in the context of mantra-dhāraṇīs is also discussed

Thermal limits of leaf metabolism across biomes

High-temperature tolerance in plants is important in a warming world, with extreme heat waves predicted to increase in frequency and duration, potentially leading to lethal heating of leaves. Global patterns of high-temperature tolerance are documented in animals, but generally not in plants, limiting our ability to assess risks associated with climate warming. To assess whether there are global patterns in high-temperature tolerance of leaf metabolism, we quantified Tcrit (high temperature where minimal chlorophyll a fluorescence rises rapidly and thus photosystem II is disrupted) and Tmax (temperature where leaf respiration in darkness is maximal, beyond which respiratory function rapidly declines) in upper canopy leaves of 218 plant species spanning seven biomes. Mean site-based Tcrit values ranged from 41.5 °C in the Alaskan arctic to 50.8 °C in lowland tropical rainforests of Peruvian Amazon. For Tmax, the equivalent values were 51.0 and 60.6 °C in the Arctic and Amazon, respectively. Tcrit and Tmax followed similar biogeographic patterns, increasing linearly (˜8 °C) from polar to equatorial regions. Such increases in high-temperature tolerance are much less than expected based on the 20 °C span in high-temperature extremes across the globe. Moreover, with only modest high-temperature tolerance despite high summer temperature extremes, species in mid-latitude (~20–50°) regions have the narrowest thermal safety margins in upper canopy leaves; these regions are at the greatest risk of damage due to extreme heat-wave events, especially under conditions when leaf temperatures are further elevated by a lack of transpirational cooling. Using predicted heat-wave events for 2050 and accounting for possible thermal acclimation of Tcrit and Tmax, we also found that these safety margins could shrink in a warmer world, as rising temperatures are likely to exceed thermal tolerance limits. Thus, increasing numbers of species in many biomes may be at risk as heat-wave events become more severe with climate change.Access to the two Peruvian sites was also facilitated by a Moore Foundation grant (Oliver Phillips, Yadvinder Mahli, and Jon Lloyd; www.rainfor.org). This work was funded by grants/fellowships from the Australian Research Council (DP0986823, DP130101252, CE140100008, FT0991448) to O.K.A., DP140103415 to M.G.T., FT110100457 to P.M., Natural Environment Research Council (UK) to P.M. (NERC NE/F002149/1), USA National Science Foundation to K.L.G. (DEB-1234162), U.S. Department of Energy to P.B.R. (DE-FG02-7ER64456), and U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (BER) through the Southeastern Regional Center of the National Institute for Climatic Change Research at Duke University to M.G.T and Texas AgriLife Research to M.G.T

Randomized placebo-controlled trial on azithromycin to reduce the morbidity of bronchiolitis in Indigenous Australian infants: rationale and protocol

Background: Acute lower respiratory infections are the commonest cause of morbidity and potentially preventable mortality in Indigenous infants. Infancy is also a critical time for post-natal lung growth and development. Severe or repeated lower airway injury in very young children likely increases the likelihood of chronic pulmonary disorders later in life. Globally, bronchiolitis is the most common form of acute lower respiratory infections during infancy. Compared with non-Indigenous Australian infants, Indigenous infants have greater bacterial density in their upper airways and more severe bronchiolitis episodes. Our study tests the hypothesis that the anti-microbial and anti-inflammatory properties of azithromycin, improve the clinical outcomes of Indigenous Australian infants hospitalised with bronchiolitis.Methods: We are conducting a dual centre, randomised, double-blind, placebo-controlled, parallel group trial in northern Australia. Indigenous infants (aged ≤ 24-months, expected number = 200) admitted to one of two regional hospitals (Darwin, Northern Territory and Townsville, Queensland) with a clinical diagnosis of bronchiolitis and fulfilling inclusion criteria are randomised (allocation concealed) to either azithromycin (30 mg/kg/dose) or placebo administered once weekly for three doses. Clinical data are recorded twice daily and nasopharyngeal swab are collected at enrolment and at the time of discharge from hospital. Primary outcomes are 'length of oxygen requirement' and 'duration of stay,' the latter based upon being judged as 'ready for respiratory discharge'. The main secondary outcome is readmission for a respiratory illness within 6-months of leaving hospital. Descriptive virological and bacteriological (including development of antibiotic resistance) data from nasopharyngeal samples will also be reported.Discussion: Two published studies, both involving different patient populations and settings, as well as different macrolide antibiotics and treatment duration, have produced conflicting results. Our randomised, placebo-controlled trial of azithromycin in Indigenous infants hospitalised with bronchiolitis is designed to determine whether it can reduce short-term (and potentially long-term) morbidity from respiratory illness in Australian Indigenous infants who are at high risk of developing chronic respiratory illness. If azithromycin is efficacious in reducing the morbidly of Indigenous infants hospitalised with bronchiolitis, the intervention would lead to improved short term (and possibly long term) health benefits. Trial registration: Australia and New Zealand Clinical Trials Register (ANZCTR): ACTRN12610000326099