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.

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 (T_optA) than temperate species. There was acclimation of T_optA 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 V_cmax25 and J_max25. 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.

Extreme heat can push plants beyond their thermal safety margin (TSM) if maximum leaf temperature (T_{leaf_max}) exceeds leaf critical temperature (T_crit). The TSM is potentially useful for assessing heat vulnerability across species but needs further validation, so we exposed 50 tree/shrub species in controlled glasshouses to a 6‐day heatwave (peak air temperature = 41°C). Many species increased their mean T_crit during the heatwave (42%), with ΔT_crit ranging from +1°C to 4°C, but other species did not acclimate or were impaired by heat stress (58%). Species T_{leaf_max} explained ~55% of the variation in species T_crit and was a key correlate of the plasticity of T_crit among species. Species with high ΔT_crit also had higher ΔT_{leaf_max}, with leaves being 7°‒12°C hotter during the heatwave than under baseline conditions. Both T_{leaf_max} and TSMs were correlated with heatwave damage across diverse species from contrasting climate zones. Species differences in TSMs were stable across measurement temperatures, correctly identified the most vulnerable species, and were strongly associated with T_{leaf_max}. Our results suggest that (1) T_{leaf_max} alone is more informative than T_crit for ranking species heat tolerance, and (2) species vulnerability to heatwaves is most reliably assessed by using TSMs that integrate T_{leaf_max} with T_crit across species.

Urban parks are critical cooling assets for cities in a warming world. In Sydney, Australia, summers are becoming longer and hotter, and visitors to local parks experience varying levels of heat depending on each park's cooling potential. The suburb of Sydney Olympic Park includes a large regional park, Bicentennial Park, and five small urban parks. We measured canopy-layer air temperatures in the regional park and five small parks over 2 years to quantify differences in cooling performance across park types. Air temperatures were significantly lower and more variable in the regional park than among small parks, with a mean difference of –2.4 vs –0.8°C. The average temperature differential between park types increased on warm days and was greater at night than during the day (–1.7 vs –0.3°C). Reducing impervious surfaces in and around the small parks had the greatest cooling effect. Daytime air temperatures during hot days were lowered by 1 °C when the area covered by impervious materials was reduced from 90% to 60%. The area covered by pervious surfaces and tree canopy was positively associated with cooling in the regional park. These results provide a nuanced understanding of how park design and size influence cooling potential. As cities densify, expand and warm, our results highlight the importance of large urban parks for cooling and climate adaptation. Small parks (<1 ha) that lack tree canopy and are surrounded by hardscapes and buildings are less important for urban cooling, yet they can play an important role as community assets.