Search

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8–10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the ‘leaf homeothermy hypothesis’. Warmed leaves showed significantly lower stomatal conductance (−0.05 mol m−2 s−1 or −43% across species) and net photosynthesis (−3.91 μmol m−2 s−1 or −39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming could reduce carbon assimilation via reduced photosynthesis in these forests, potentially weakening the land carbon sink in tropical and temperate forests.

There is a pressing need to better understand ecosystem resilience to droughts and heatwaves. Eco-evolutionary optimization approaches have been proposed as means to build this understanding in land surface models and improve their predictive capability, but competing approaches are yet to be tested together. Here, we coupled approaches that optimize canopy gas exchange and leaf nitrogen investment, respectively, extending both approaches to account for hydraulic impairment. We assessed model predictions using observations from a native Eucalyptus woodland that experienced repeated droughts and heatwaves between 2013 and 2020, whilst exposed to an elevated [CO2] treatment. Our combined approaches improved predictions of transpiration and enhanced the simulated magnitude of the CO2 fertilization effect on gross primary productivity. The competing approaches also worked consistently along axes of change in soil moisture, leaf area, and [CO2]. Despite predictions of a significant percentage loss of hydraulic conductivity due to embolism (PLC) in 2013, 2014, 2016, and 2017 (99th percentile PLC > 45%), simulated hydraulic legacy effects were small and short-lived (2 months). Our analysis suggests that leaf shedding and/or suppressed foliage growth formed a strategy to mitigate drought risk. Accounting for foliage responses to water availability has the potential to improve model predictions of ecosystem resilience.

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees. Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming. Tropical and subtropical species had higher temperature optima for photosynthesis (T

Cities are warmer than surrounding non-urban areas. Climate models predict that metropolitan centres will become even warmer due to the dual impacts of global warming and densification. However, the outer fringe zones of metropolitan centres will also become warmer as a consequence of urban expansion that requires replacing green and open spaces like pastures or bushland with grey infrastructure such as roads and buildings. Limiting the warming effect of urban expansion is possible. It requires dedicated heat-responsive planning and design strategies being applied systematically and at scale. But where should planners and developers start to effectively reduce urban heat? At-grade car parks are an ideal starting point. They represent the ���low-hanging fruit��� for urban cooling efforts. While unavoidable today and in the near future, at-grade car parks are predominately unshaded; made from black, heat-retaining asphalt; widespread and fairly uniform; and often large in size. Changes to current designs of at-grade car parks can therefore have a big impact. A number of strategies to effectively reduce surface heat of car parks are commercially available. Cooling car parks not only addresses their status as local heat islands, but it also leads to lower ambient air temperatures in downwind environments. This report documents: ��� Microclimates across eight car parks and reference sites covered by vegetation. Measurements of surface and air temperatures related to a range of car park surface materials. The cooling effect of shade in car parks. Current design guidelines and policies in Australia related to car parks. Alternative design solutions for cooler car parks. The empirical data and policy analysis are used to develop a set of recommendations for urban heat mitigation that can be applied to new and existing car parks. Because of the common nature of at-grade car parks around the world, the proposed cooling techniques can be applied globally, irrespective of the fact that the underlying case studies and data originated from Sydney.

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (Ca) is a major uncertainty in projecting the future trajectory of the Earth's climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated Ca (eCa), it is unclear how this stimulation translates into carbon cycle responses at the ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native eucalypt forest exposed to free-air CO2 enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19 % in response to a 38 % increase in Ca. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate the GPP and its response to eCa. We assessed the direct impact of eCa, as well as the indirect effect of photosynthetic acclimation to eCa and variability among treatment plots using different model scenarios. At the canopy scale, MAESPA estimated a GPP of 1574 g C m−2 yr−1 under ambient conditions across 4 years and a direct increase in the GPP of +11 % in response to eCa. The smaller canopy-scale response simulated by the model, as compared with the leaf-level response, could be attributed to the prevalence of RuBP regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10 %. After taking the baseline variability in the leaf area index across plots in account, we estimated a field GPP response to eCa of 6 % with a 95 % confidence interval (−2 %, 14 %). These findings highlight that the GPP response of mature forests to eCa is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the eCa responses of other components of the ecosystem carbon cycle.

Rising atmospheric CO2 concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO2 (eCO2 ), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO2 : (a) A "dilution effect" caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO2 . This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free-Air CO2 Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate ( NO3- ) concentrations, carbohydrates and NO3- reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO2 in new leaves, while the NO3- fraction and total carbohydrate concentrations remained unchanged by CO2 treatment for either new or mature leaves. Photorespiration decreased 31% under eCO2 supplying less reductant, and in situ NO3- reductase activity was concurrently reduced (-34%) in eCO2 , especially in new leaves during summer periods. Hence, NO3- assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of NO3- reductase activity due to lower photorespiration in eCO2 can contribute to understanding how eCO2 -induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO2 on N biogeochemistry include both mechanisms, especially where NO3- is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.

Intensifying summer heat and associated Urban Heat Island Effects are a risk to public and environmental health. They contribute to higher energy consumption and associated greenhouse gas emissions in cities. Across Greater Western Sydney, home of the fastest growing urban population in Australia, increasing heat is recognised as the largest risk to local populations and economies. A range of interventions across the region aim at mitigating the negative impacts of heat. The Cool Roads Trial is one of these interventions. It addresses the contribution of unshaded road and carpark surfaces to local heat island effects. In March 2020, 14,700 m2 of road and carpark surfaces were coated with a highly reflective asphalt emulsion in the local government areas of Blacktown, Campbelltown and Parramatta to reduce surface temperatures of pavements. The trial was accompanied by an environmental monitoring program. The program used measurements of surface, air and black globe temperatures to document the effectiveness of the surface coat on cooling. Data were collected between February 2020 and March 2021 using a full-factorial design with paired impact and control sites. Results showed that surface temperatures of unshaded coated pavements were on average 6°C and at maximum 11°C cooler compared to uncoated pavements. Tree shade reduced temperatures of uncoated surfaces by 20°C and that of coated surfaces by 14°C leading to identical surface temperatures in the shade on coated and uncoated surfaces. Surface coating did not systematically reduce air temperature during the day or night. Back globe temperatures during sunny days increased by 2.7°C on coated compared to uncoated sites as a result of increased reflectivity of the surface. The higher exposure to reflected solar incident radiation resulted in lower thermal comfort in the sun on coated surfaces. The Cool Roads Trial established important information for the management of heat in Western Sydney and beyond. Increasing albedo of roads and carparks will help reduce surface Urban Heat Island Effects due to lower surface temperatures. Ambient air temperatures were not lowered as a result of coating roads and carparks, which can potentially be a matter of scale. The Cool Roads Trial worked at the microscale where air cooling benefits could be masked by continuous mixing of local with surrounding air masses. The range of thermal effects documented in this report make it clear that mitigating the impacts of urban heat will require a broad suite of solutions. A clear definition of desired thermal outcomes will be necessary on a case-by-case basis. Only once thermal outcomes are defined can resilience of urban populations, infrastructure and ecosystems against intensifying summer heat be improved effectively.

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (span classCombining double low line"inline-formula"iC/ia/span) is a major uncertainty in projecting the future trajectory of the Earth's climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated span classCombining double low line"inline-formula"iC/ia/span (espan classCombining double low line"inline-formula"iC/ia/span), it is unclear how this stimulation translates into carbon cycle responses at the ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native eucalypt forest exposed to free-air span classCombining double low line"inline-formula"CO2/span enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19 % in response to a 38 % increase in span classCombining double low line"inline-formula"iC/ia/span. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate the GPP and its response to espan classCombining double low line"inline-formula"iC/ia/span. We assessed the direct impact of espan classCombining double low line"inline-formula"iC/ia/span, as well as the indirect effect of photosynthetic acclimation to espan classCombining double low line"inline-formula"iC/ia/span and variability among treatment plots using different model scenarios./p At the canopy scale, MAESPA estimated a GPP of 1574 g C mspan classCombining double low line"inline-formula"-2/span yrspan classCombining double low line"inline-formula"-1/span under ambient conditions across 4 years and a direct increase in the GPP of span classCombining double low line"inline-formula"+/span11 % in response to espan classCombining double low line"inline-formula"iC/ia/span. The smaller canopy-scale response simulated by the model, as compared with the leaf-level response, could be attributed to the prevalence of RuBP regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10 %. After taking the baseline variability in the leaf area index across plots in account, we estimated a field GPP response to espan classCombining double low line"inline-formula"iC/ia/span of 6 % with a 95 % confidence interval (span classCombining double low line"inline-formula"-/span2 %, 14 %). These findings highlight that the GPP response of mature forests to espan classCombining double low line"inline-formula"iC/ia/span is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the espan classCombining double low line"inline-formula"iC/ia/span responses of other components of the ecosystem carbon cycle.

Outdoor school environments need to be safe, stimulate physical and cognitive development of children and encourage learning. These key requirements are jeopardised by increasing summer heat. Summer heat limits outdoor activities and has adverse effects on physical wellbeing of school children and teachers. Children are particularly vulnerable to heat as they regulate their core temperature through convection, which becomes less effective when it is hot. Based on empirical data collections, this report provides more than 20 practical recommendations on how to reduce the impacts of outdoor heat. Although these recommendations were devised based on work around a public school in Western Sydney, their universal character allows applying them to any school or other urban build infrastructure. Avoiding the use of artificial grass in unshaded spaces, shading black asphalt, allowing natural air flows and using shade materials with highly reflective upper surfaces should be fundamental principles in design and building guidelines for heat-smart schools.

This project identified the 100 most vulnerable schools to heat in Greater Western Sydney using a newly developed Heat Score. The Heat Score combines socio-economic information that captures exposure, sensitivity and adaptivity of local communities to heat with environmental data related to surface and air temperatures of urban space. Analyses provided in this report will assist the development of the most effective tree planting strategies for each of Sydney’s 100 hot schools.

This report documents variation of summer temperatures across the Local Government Area of Penrith in Western Sydney. Between 14 December 2019 and 31 March 2020, more than 1.4 million measurements of air temperatures were collected using 120 data loggers. The network of data loggers covered the entire Local Government Area from Emu Plains in the west to St Marys in the east and Agnes Banks in the north to Badgerys Creek in the south. Analyses of these data revealed that local populations of Penrith experienced air temperatures of more than 50°C during three individual days.Throughout the summer of 2019/20, Emu Plains was the warmest and Mulgoa the coolest suburb. Nighttime air temperatures were higher in suburbs with high cover of hard surfaces compared to those where open surfaces dominated, demonstrating clear Urban Heat Island Effects in the area. An Urban Heat Island was also detected for the town of St Marys, where the urban core remained up to 2.4°C warmer during the night compared to the surrounding residential neighbourhoods. A wide rage of temperature analyses provide a nuanced picture of how summer heat is impacting this important city in Western Sydney. A total of 12 clear recommendations developed with the help of the empirical work.

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1–5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3–5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7–10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO2 unclear4,5,7–11. Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO2 exposure. We show that, although the eCO2 treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO2 fertilization as a driver of increased carbon sinks in global forests. Carbon dioxide enrichment of a mature forest resulted in the emission of the excess carbon back into the atmosphere via enhanced ecosystem respiration, suggesting that mature forests may be limited in their capacity to mitigate climate change.

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (span classCombining double low line"inline-formula"iC/ia/span) is a major uncertainty in projecting the future trajectory of the Earth's climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated span classCombining double low line"inline-formula"iC/ia/span (espan classCombining double low line"inline-formula"iC/ia/span), it is unclear how this stimulation translates into carbon cycle responses at the ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native eucalypt forest exposed to free-air span classCombining double low line"inline-formula"CO2/span enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19andthinsp;% in response to a 38andthinsp;% increase in span classCombining double low line"inline-formula"iC/ia/span. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate the GPP and its response to espan classCombining double low line"inline-formula"iC/ia/span. We assessed the direct impact of espan classCombining double low line"inline-formula"iC/ia/span, as well as the indirect effect of photosynthetic acclimation to espan classCombining double low line"inline-formula"iC/ia/span and variability among treatment plots using different model scenarios./p At the canopy scale, MAESPA estimated a GPP of 1574andthinsp;gandthinsp;Candthinsp;mspan classCombining double low line"inline-formula"-2/spanandthinsp;yrspan classCombining double low line"inline-formula"-1/span under ambient conditions across 4 years and a direct increase in the GPP of span classCombining double low line"inline-formula"+/span11andthinsp;% in response to espan classCombining double low line"inline-formula"iC/ia/span. The smaller canopy-scale response simulated by the model, as compared with the leaf-level response, could be attributed to the prevalence of RuBP regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10andthinsp;%. After taking the baseline variability in the leaf area index across plots in account, we estimated a field GPP response to espan classCombining double low line"inline-formula"iC/ia/span of 6andthinsp;% with a 95andthinsp;% confidence interval (span classCombining double low line"inline-formula"-/span2andthinsp;%, 14andthinsp;%). These findings highlight that the GPP response of mature forests to espan classCombining double low line"inline-formula"iC/ia/span is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the espan classCombining double low line"inline-formula"iC/ia/span responses of other components of the ecosystem carbon cycle. © Author(s) 2020. Martin G. De Kauwe was supported by the NSW Research Attraction and Acceleration Program (RAAP). Euc-FACE was built as an initiative of the Australian Government as part of the Nation Building Economic Stimulus Plan and is supported by the Australian Commonwealth in collaboration with Western Sydney University.

The response of mature forest ecosystems to rising atmospheric carbon dioxide concentration (Ca) is a major uncertainty in projecting the future trajectory of the Earth’s climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated Ca (eCa), it is unclear how this stimulation translates into carbon cycle responses at whole-ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native Eucalypt forest exposed to Free Air CO2 Enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19 % in response to a 38 % increase in Ca. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate Gross Primary Production (GPP) and its response to eCa. We assessed the direct impact of eCa, as well as the indirect effect of photosynthetic acclimation to eCa and variability among treatment plots via different model scenarios. At the canopy scale, MAESPA estimated a GPP of 1574 g C m−2 yr−1 under ambient conditions across four years and a direct increase in GPP of +11 % in response to eCa. The smaller canopy-scale response simulated by the model, as compared to the leaf-level response, could be attributed to the prevalence of RuBP-regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10 %. Considering variability in leaf area index across plots, we estimated a mean GPP response to eCa of 6 % with a 95 % CI of (−2 %, 14 %). These findings highlight that the GPP response of mature forests to eCa is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting eCa responses of other components of the ecosystem carbon cycle.

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1,2,3,4,5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2concentration6. While evidence gathered from young aggrading forests has generally indicated a strong CO2fertilization effect on biomass growth3,4,5, it is unclear whether mature forests respond to eCO2in a similar way. In mature trees and forest stands7,8,9,10, photosynthetic uptake has been found to increase under eCO2without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO24, 5, 7,8,9,10,11. Here, using data from the first ecosystem-scale Free-Air CO2Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responds to four years of eCO2exposure. We show that, although the eCO2treatment of ambient +150 ppm (+38%) induced a 12% (+247 gCm-2yr-1) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone contributing ∼50% of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on CO2fertilization as a driver of increased carbon sinks in standing forests and afforestation projects.

Climate change can negatively affect sensitive tree species, affecting their acclimation and adaptation strategies. A common garden experiment provides an opportunity to test whether responses of trees from different provenances are genetically driven and if this response is related to factors at the site of origin. We hypothesized that antioxidative defence systems and leaf mass area ofAcacia melanoxylonR. Br. samples collected from different provenances will vary depending on local rainfall. Thirteen provenances ofA. melanoxylonoriginating from different rainfall habitats (500-2000 mm) were grown for 5 years in a common garden. For 2 years, phyllode samples were collected during winter and summer, for measurements of leaf mass area and concentrations of glutathione and ascorbic acid. Leaf mass area varied between seasons, years and provenances ofA. melanoxylon, and an increase was associated with decreasing rainfall at the site of origin. Ascorbic acid and glutathione concentrations varied between seasons, years (i.e., environmental factors) and among provenances ofA. melanoxylon In general, glutathione and ascorbic acid concentrations were higher in winter compared with summer. Ascorbic acid and glutathione were different among provenances, but this was not associated with rainfall at the site of origin.

Summer air temperatures will continue to rise in metropolitan regions due to climate change and urbanization, intensifying daytime and nighttime air temperatures and result in greater thermal discomfort for city dwellers. Urban heat may be reduced by trees which provide shade, decreasing air and surface temperatures underneath their canopies. We asked whether tree height and canopy density can help to identify species that provide greater microclimate benefits during day and night. We also asked if increased canopy cover of street trees provides similar microclimate benefits. We used continuous measurements of near-surface air temperatures under 36 park trees and from two urban streets to assess these questions. In the park, trees were grouped according to their height (20 m) and canopy density (low, high), while the effect of canopy cover was tested using streets with high (31%) and low (11%) cover. Daytime near-surface air temperature declined with increasing height and canopy density providing significant cooling benefits. However, this trend was reversed at night when tall trees with dense canopies restricted longwave radiative cooling and trapped warm air beneath their crowns. High canopy cover of street trees reduced daytime air temperatures more, resulting in a lower number of days with hot (>35 °C) and extreme (>40 °C) air temperatures compared to the street that had low canopy cover. These findings suggest that tree species and streetscapes with dense canopy cover improve local thermal conditions during the day but do not seem ideal to allow for nighttime cooling, creating potential discomfort for residents during hot summer nights. Our results indicate that classifying trees using a simple metric can assist in selecting tree species that can alleviate the local negative effect of urban heat during the day, but at the same time, their effect in preventing optimal longwave radiative cooling during the night must be factored into planting strategies.

Only a few previous studies have examined how photosynthetically active radiation absorptance, pigments and electron flow change in mature trees exposed to long-term increase in CO2 concentration. We investigated pigment concentrations, leaf optical properties and quantum yield of old and new leaves exposed to ambient (aCO2) and elevated (eCO2) CO2 treatments. Leaf absorptance was around 90% in E. tereticornis trees across both foliage age classes and CO2 treatments. New leaves had 15% higher quantum yield with increased absorptance within the blue spectrum than old leaves; while they reflected and transmitted more photons. In addition, young foliage had increased mass-based concentrations of chlorophyll and carotenoids; however, pigment concentrations were reduced when expressed on area-basis. Quantum yield was 9% higher in eCO2 than aCO2 across both foliage age classes. The CO2 effect was stronger in new leaves where the quantum yield was 17% higher in eCO2 than aCO2, but not different in old leaves between CO2 treatments. New leaves had higher transmittance of photons in eCO2 than aCO2, while there was no change in old leaves. Mass-based concentrations of chlorophyll and carotenoids were reduced in eCO2 compared to aCO2 while concentrations of anthocyanins were higher in response to CO2 treatment. There was a significant effect of Age x CO2 interaction on ratio a/b with larger eCO2-related reductions in old leaves (–5%) but no change in new leaves. Generally, new leaves were more efficient in utilizing the absorbed photons than old leaves, especially under eCO2 which resulted in more carbon fixation. This implies that leaves can adjust their light harvesting capacity to eCO2, particularly in younger leaves which have higher photosynthetic activity.

KEY MESSAGE : Two species from warmer climates, but not the corresponding congeneric species from relatively cooler ones, decreased stomatal conductance upon heatwaves, and one of them showed significant decrease in the efficiency of open reaction centres of PSII in the light. In contrast, responses of major antioxidants ascorbic acid and glutathione to heatwaves were more similar between congeneric species than between species from similar climates. According to climate change predictions, heatwaves will increase in frequency and intensity. This can threaten survival of sensitive tree species. Heatwaves affect photosynthetic capacity and cause an imbalance between light driven electron transport and carbon fixation. This can increase the concentration of reactive oxygen species, and can cause photo-oxidative stress. Heat dissipation and antioxidants are crucial in plant defence against photo-oxidative stress. We hypothesised that stomatal regulations, heat dissipation as measured by chlorophyll fluorescence and responses of major antioxidants ascorbic acid and glutathione to heatwaves will vary according to the climate at the site of origin of tree species. Tree seedlings from warmer (Eucalyptus grandis, Acacia aneura) and cooler climates (Eucalyptus tricarpa, Acacia melanoxylon) were exposed to air temperatures about 5 °C above control levels for 5 days. The two species from warmer climates responded to heatwaves with stomatal closure restricting transpiration and carbon fixation, and E. grandis also significantly increased heat dissipation (as judged by decreased efficiency of open reaction centres of PSII in the light). Species from cooler climates kept stomata open allowing continuing carbon assimilation and transpirational cooling. Heatwaves did not reduce maximum quantum efficiency of PSII in Acacia species, with insignificant decreases in Eucalyptus species. Glutathione concentrations increased significantly upon heatwave in E. tricarpa (from cooler climates), showed a time-dependent response to heatwave in E. grandis (from warmer climates) with an increase at later stages, and showed a tendency to decrease upon heatwave in both Acacia species (non-significant only for A. melanoxylon). Ascorbic acid concentrations increased significantly in E. tricarpa (cooler climates), and did not change significantly in other species. The redox state of ascorbic acid or glutathione only changed significantly and transiently in response to heatwave in A. aneura (warmer climates; GSSG % and ASC %) and A. melanoxylon (cooler climates; GSSG %), but not Eucalyptus species. Therefore, differences in stomatal regulations upon heatwaves aligned with the origin of species in warmer or cooler climates, whereas responses of ascorbic acid and glutathione to heatwaves were more aligned with the two genera.

International audience; AbstractKey messageTwoAcaciaspecies adapted to contrasting habitats showed different response of photoprotective and antioxidative defence systems to imposed drought and heatwave.ContextPredicted increases in drought frequency and intense heatwaves are expected to lead to dieback of sensitive tree species. Stomatal closure restricts CO2 input into the leaf, resulting in imbalances between light energy-driven electron transport rate and electron consumption in the Calvin cycle. Reactive oxygen species formed under these circumstances have to be kept under control by photoprotective and antioxidative defence systems.AimsWe hypothesised that these defence systems behave differently in tree species from contrasting habitats.MethodsAcacia aneura (adapted to arid habitats) and Acacia melanoxylon (adapted to humid habitats) were exposed to two water treatments for 50 days including two short heatwave periods. Responses were assessed by gas exchange, chlorophyll fluorescence and concentrations of antioxidants (phyllodes, roots).ResultsPhotosynthesis and quantum yield of photochemistry decreased significantly in both Acacia species, especially after water was withheld in combination with the second heatwave episode. In phyllodes, the concentration of antioxidants remained unchanged until exposure to severe drought and heatwave conditions (except for A. melanoxylon where changes in glutathione concentration were observed prior to exposure to severe stress), but after water was withheld and the second heatwave occurred, oxidised forms of glutathione increased. After exposure to the second heatwave, well-watered seedlings of A. melanoxylon but not A. aneura increased ascorbic acid concentration in phyllodes. Under well-watered conditions, Acacia species also showed increased concentration of antioxidants in roots following heatwaves.ConclusionsBoth Acacia species showed photodamage to photosystem II (PSII) after water was withheld and the second heatwave imposed, but with more gradual response in A. aneura. Total concentration of investigated antioxidants increased in response to the first (A. melanoxylon) and second (A. aneura) heatwaves rather than drought stress alone.