Rising summer heat and more frequent and intense heatwaves impact countless metropolitan regions, including Greater Sydney, Australia. An analysis of historic air temperature measurements (1859–2020) reveals a notable increase in the number of ‘hot’ (≥35°C) days during austral summers. While in the first 120 years of records 351 hot days were identified, 478 hot days were recorded during 2000–2020 alone. Trajectories of summer heat until 2060 indicate that maximum air temperatures in Western Sydney could be ≥35°C during 160 days.A second, more granular analysis compared air temperature measurements recorded at 274 urban microsites during the summers of 2019 and 2020 with measurements of official weather stations in Central and Western Sydney. Results revealed that the number of hot (≥35°C), extreme (≥40°C), and ‘catastrophic’ (≥45°C) heat days was markedly greater than those reported by official weather stations. Underreporting of heat was greatest across the Local Government Area (LGA) of Cumberland, where data loggers recorded 32 hot and 15 extreme heat days, compared to 7 hot and 1 extreme heat day recorded by the nearest official station. Based on empirical measurements, a set of novel ‘heat risk’ maps identify suburbs and regions inside LGAs where underreporting of summer heat is high. Findings indicate that communities across Greater Sydney are exposed to more frequent and more intense heat than previously reported. Underreporting of local urban heat results in lower preparedness and thus higher risk of harm to urban populations of Greater Sydney and likely many other metropolitan regions.

1. The live fuel moisture content (LFMC) is an important precondition for wildfire activity, yet it remains challenging to predict LFMC due to the dynamic interplay between atmospheric and hydrological conditions that determine the plant's ac-cess to, and loss of water.2. We monitored LFMC and a range of plant water- use traits (predawn and mid-day leaf water potentials [Psi leaf]), leaf traits (specific leaf area [SLA]), hydrological status (soil water content [SWC] in the shallow layer and full profile) and atmos-pheric variables (air temperature, vapour pressure deficit [VPD], CO2 concentra-tions) in a mature eucalypt woodland at the Eucalyptus Free- Air CO2 Enrichment (EucFACE) facility during a drought.3. We combined plant traits, hydrological status and atmospheric variables into a biophysical model to predict LFMC dynamics, and compared these with predic-tions of LFMC based on a satellite model and established relationships between Psi leaf and LFMC from pressure- volume curves.4. Predawn Psi leaf could be well predicted from changes in SWC, but variation in midday Psi leaf and LFMC were more responsive to atmospheric than hydrological variables. The biophysical model explained up to 89% of variability in LFMC and outperformed established approaches to predict LFMC. SLA was the single most important variable to predict LFMC, followed by VPD, which explained 33% of the remaining variability in LFMC.5. Our study demonstrates that the co- variation of plant traits and atmospheric and hydrological conditions affect LFMC during drought, suggesting a new way for-ward for predicting LFMC by combining biophysical and satellite- based models of LFMC with seasonal forecasts of meteorological and hydrological variables.

To reduce increasing human morbidity and mortality due to urban overheating, urban cooling strategies need to be targeted towards intra-urban hot spots. The application of findings from numerous urban heat studies is limited because methods do not measure air temperature relevant to the human experience with a sufficient level of data granularity. In this work, we developed and tested a unit consisting of a temperature data logger and a custom-made weather shield that is substantially less expensive than published alternatives and can be deployed with high spatial flexibility to measure canopy-layer (near-surface) air temperature relevant to people living in the built environment. Accuracy of results was compared with a more expensive, commercially available scientific instrument as well as official Bureau of Meteorology weather stations in Sydney, Australia. The unit costs around 15% of the price of the tested alternative and only 25% of the least expensive published alternative. Data quality was almost identical to that provided by more expensive scientific instruments and official weather stations. Temporal and spatial coverage and the resulting granularity of air temperature data were very high. The air temperature measurement method reported here can be used in future urban heat studies to determine intra-urban hot spots. Resultant knowledge can be used to target cooling strategies that maximise benefits to the human population, reducing heat-related illnesses and death in overheating cities.

Abstract 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.

Urban Green Infrastructure (UGI) elements help lower surface and air temperatures within cities. Parks that contain UGI can be used to countermeasure local Urban Heat Island (UHI) effects. Here we investigate the Park Cool Island (PCI) effect in an increasingly dense suburb of Sydney, Australia. At Sydney Olympic Park, we monitored environmental conditions at six sites in Bicentennial Park, and six sites in an adjacent urban precinct during the summer of 2022/23. A detailed analysis of surface and tree canopy cover was done for each site and linked with site microclimate. Despite the unusually wet and cool conditions, air temperatures across the entire summer were 0.4 degrees C lower and night-time air temperatures were frequently >5 degrees C cooler in the park compared to the urban precinct. During very hot days where maximum air temperature was >35 degrees C, ambient conditions were always cooler in the park. Increasing the cover of hard surfaces led to significant warming of air during the day in the urban precinct only, and greater tree canopy cover did not result in greater cooling during day. These findings provide new insights into PCI effects that will assist in the development of heat-resilient urban planning to addresses UHI effects in warming cities.

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.

Surfaces in outdoor playgrounds get hot in the sun and can cause serious skin burns in children. In-situ measurements from 10 playgrounds in Sydney showed that the maximum and average surface temperatures of sun -exposed playground equipment and flooring surfaces were frequently above skin contact burn thresholds. Black and dark-coloured wet pour rubber and synthetic turf were the hottest floor materials, all having maximum surface temperatures (T-s_max) > 80 ?C. A blue rubber dolphin was the hottest piece of play equipment, with a T-s_max of 91.8 ?C. A systematic assessment of common synthetic flooring materials exposed to full sun showed notable differences in T(s_max )between material types and colour-tones. Synthetic turf with 40 mm long grass blades (STlng-GR) was the hottest material (T-s_max = 84.5 ?C), followed by dark blue styrene butadiene rubber (SBRD-BL, T-s_max = 81.1 ?C), dark green ethylene propylene diene polymer (EPDMD-GR-2, (Ts_max )= 77.8 ?C), dark brown thermoplastic vulcanizate (TPVD-BR, T-s_max = 71.8 ?C), and intermediate blue thermoplastic polyolefin (TPOI-BL, T-s_max = 65.0 ?C). All these materials were hot enough to cause contact burns on typical, warm summer days when children are likely to visit outdoor playgrounds. Surface temperatures were significantly reduced in the shade and never reached burn threshold temperatures. Selection of appropriate material type and colour -tone, together with the provision of shade can remove the hazard risk for contact skin burns from outdoor playgrounds. Results of this work will assist playground designers and managers to provide safer places for our children to play longer in increasingly warmer summers.

Summary 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 (ToptA) than temperate species. There was acclimation of ToptA to warmer growth temperatures. The rate of acclimation (0.35–0.78°C °C–1) was higher in tropical and subtropical than in warm‐temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of Vcmax25 and Jmax25. The temperature sensitivity of respiration (Q10) was 24% lower in tropical and subtropical compared with warm‐temperate species. Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm‐temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.

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.

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 (<10 m, 10–20 m, >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.

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.

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 (aCO(2)) and elevated (eCO(2)) 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 eCO(2) than aCO(2) across both foliage age classes. The CO2 effect was stronger in new leaves where the quantum yield was 17% higher in eCO(2) than aCO(2), but not different in old leaves between CO2 treatments. New leaves had higher transmittance of photons in eCO(2 )than aCO(2), while there was no change in old leaves. Mass-based concentrations of chlorophyll and carotenoids were reduced in eCO(2) compared to aCO(2) 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 eCO(2) 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 eCO(2) which resulted in more carbon fixation. This implies that leaves can adjust their light harvesting capacity to eCO(2), particularly in younger leaves which have higher photosynthetic activity.

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 (eCO(2)), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO(2): (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 eCO(2). 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 eCO(2) 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 eCO(2) supplying less reductant, and in situ NO-3 reductase activity was concurrently reduced (-34%) in eCO(2), 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 eCO(2) can contribute to understanding how eCO(2)-induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO(2) 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.

Elevated CO2 affects C cycling processes which in turn can influence the nitrogen (N) and phosphorus (P) concentrations of plant tissues. Given differences in how N and P are used by plants, we asked if their stoichiometry in leaves and wood was maintained or altered in a long-term elevated CO2 experiment in a mature Eucalyptus forest on a low P soil (EucFACE). We measured N and P concentrations in green leaves at different ages at the top of mature trees across 6 years including 5 years in elevated CO2. N and P concentrations in green and senesced leaves and wood were determined to evaluate both spatial and temporal variation of leaf N and P concentrations, including the N and P retranslocation in leaves and wood. Leaf P concentrations were 32% lower in old mature leaves compared to newly flushed leaves with no effect of elevated CO2 on leaf P. By contrast, elevated CO2 significantly decreased leaf N concentrations in newly flushed leaves but this effect disappeared as leaves matured. As such, newly flushed leaves had 9% lower N:P ratios in elevated CO2 and N:P ratios were not different in mature green leaves (CO2 by Age effect, P = 0.02). Over time, leaf N and P concentrations in the upper canopy slightly declined in both CO2 treatments compared to before the start of the experiment. P retranslocation in leaves was 50%, almost double that of N retranslocation (29%), indicating that this site was P-limited and that P retranslocation was an important mechanism in this ecosystem to retain P in plants. As P-limited trees tend to store relatively more N than P, we found an increased N:P ratio in sapwood in response to elevated CO2 (P < 0.01), implying N accumulation in live wood. The flexible stoichiometric ratios we observed can have important implications for how plants adjust to variable environmental conditions including climate change. Hence, variable nutrient stoichiometry should be accounted for in large-scale Earth Systems models invoking biogeochemical processes.

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.

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.

Two Acacia species adapted to contrasting habitats showed different response of photoprotective and antioxidative defence systems to imposed drought and heatwave. Predicted 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. We hypothesised that these defence systems behave differently in tree species from contrasting habitats. Acacia 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). Photosynthesis 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. Both 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.