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Drought combined with extreme heatwaves has been increasingly identified as the important trigger of worldwide tree mortality in the context of climate change; nonetheless, our understanding of the potential hydraulic and thermal impairments of hot droughts to trees and the subsequent post-recovery process remains limited. To investigate the response of tree water and carbon relations to drought, heatwave, and combined drought-heatwave stresses, three-year-old potted seedlings of Fraxinus mandshurica Rupr., a dominant tree species in temperate forests of northeast China, were grown under well-watered and drought-stressed conditions and exposed to a rapid, acute heatwave treatment. During the heatwave treatment with a maximum temperature exceeding 40 °C for two days, the leaf temperature of drought-stressed seedlings was, on average, 5 °C higher than that of well-watered counterparts due to less effective evaporative cooling, indicating that soil water availability influenced leaf thermoregulatory capacity during hot extremes. Consistently, more pronounced crown damage, as shown by 13 % irreversible leaf scorch, was found in seedlings under the drought-heatwave treatment relative to sole heatwave treatment, alongside the more severe stem xylem embolism and leaf electrolyte leakage. While the heatwave treatment accelerated the depletion of non-structural carbohydrates in drought-stressed seedlings, the increase of branch soluble sugar concentration in response to heatwave might be related to the requirement for maintaining hydraulic functioning via osmoregulation under high dehydration risk. The coordination between leaf stomatal conductance and total non-structural carbohydrate content during the post-heatwave recovery phase implied that plant-water relations and carbon physiology were closely coupled in coping with hot droughts. This study highlights that, under scenarios of aggravating drought co-occurring with heatwaves, tree seedlings could face a high risk of crown decline in relation to the synergistically increased hydraulic and thermal impairments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (Anet) and stomatal conductance (gs). However, evidence for decoupling of gs from Anet under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum Tair > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced Anet and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well‐watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced—gs was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD. [ABSTRACT FROM AUTHOR]

How desert shrubs respond to drought and heat remains poorly understood. We investigated how drought affected the morphology and physiology of two C 4 species with different root architecture, Calligonum arborescens and Haloxylon ammodendron , in the Taklamakan Desert. We measured leaf traits, chlorophyll fluorescence and temperature responses of photosynthesis after ∼3 years of experimental drought and asked: (1) Do both species adjust similarly to drought? (2) Does transpirational cooling increase at high temperature, and if so, how is this affected by drought? Drought plants of H. ammodendron had shorter and thinner leaves than controls, and lower chlorophyll content, photochemical efficiency, and electron transport rate. In contrast, C. arborescens leaves became shorter and thicker. Drought reduced maximum photosynthesis by 63 % and 21 % in H. ammodendron and C. arborescens , respectively. The optimum temperature did not change significantly in H. ammodendron and decreased by ∼2 °C in C. arborescens. Surprisingly, above 40–45 °C, stomatal conductance (g sw) and transpiration increased—even under drought stomata partially reopened. This consistent uncoupling of photosynthesis and g sw suggests that widely-implemented stomatal optimization models may poorly reflect high-temperature behaviors in dryland ecosystems. Our study established the basis for predicting the eco-physiological responses of C4 species in hyper-arid ecosystems to climate change. • Drought affected the morphology and physiology of two C4 desert species. • C4 woody shrubs exhibit decoupling of photosynthesis and stomatal conductance. • Drought weakened, but did not fully prevent this decoupling. • Different physiological adjustment mechanisms and morphological adjustment. [ABSTRACT FROM AUTHOR]