Optimal Carbon Storage During Drought.

Allocation of non-structural carbohydrates (NSC) to storage allows plants to maintain a carbon pool in anticipation of future stress. However, to do so, plants must forego use of the carbon for growth, creating a trade-off between storage and growth. It is possible that plants actively regulate the storage pool to maximise fitness in a stress-prone environment. Here, we attempt to identify the patterns of growth and storage that would result during drought stress under the hypothesis that plants actively regulate carbon storage. We use optimal control theory to calculate the optimal allocation to storage and utilisation of stored carbon over a single drought stress period. We examine two fitness objectives representing alternative life strategies: prioritisation of growth (MaxM) and prioritisation of storage (MaxS), as well as strategies in between these extremes. We find that optimal carbon storage consists of three discrete phases: 'growth', 'storage without growth', and the 'stress' phase where there is no carbon source. This trajectory can be defined by the time point when the plant switches from growth to storage. Growth-prioritising plants switch later and fully deplete their stored carbon over the stress period, while storage-prioritising plants either do not grow or switch early in the drought period. The switch time almost always occurs before soil water is depleted, meaning that growth stops before photosynthesis. We conclude that the common observation of increasing carbon storage during drought could be interpreted as an active process that optimises plant performance during stress.

Higher global gross primary productivity under future climate with more advanced representations of photosynthesis.

Gross primary productivity (GPP) is the key determinant of land carbon uptake, but its representation in terrestrial biosphere models (TBMs) does not reflect our latest physiological understanding. We implemented three empirically well supported but often omitted mechanisms into the TBM CABLE-POP: photosynthetic temperature acclimation, explicit mesophyll conductance, and photosynthetic optimization through redistribution of leaf nitrogen. We used the RCP8.5 climate scenario to conduct factorial model simulations characterizing the individual and combined effects of the three mechanisms on projections of GPP. Simulated global GPP increased more strongly (up to 20% by 2070-2099) in more comprehensive representations of photosynthesis compared to the model lacking the three mechanisms. The experiments revealed non-additive interactions among the mechanisms as combined effects were stronger than the sum of the individual effects. The modeled responses are explained by changes in the photosynthetic sensitivity to temperature and CO2 caused by the added mechanisms. Our results suggest that current TBMs underestimate GPP responses to future CO2 and climate conditions.

Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access.

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.

Mapping soil organic carbon stocks in Nepal's forests.

Comprehensive forest carbon accounting requires reliable estimation of soil organic carbon (SOC) stocks. Despite being an important carbon pool, limited information is available on SOC stocks in global forests, particularly for forests in mountainous regions, such as the Central Himalayas. The availability of consistently measured new field data enabled us to accurately estimate forest soil organic carbon (SOC) stocks in Nepal, addressing a previously existing knowledge gap. Our method involved modelling plot-based estimates of forest SOC using covariates related to climate, soil, and topographic position. Our quantile random forest model resulted in the high spatial resolution prediction of Nepal's national forest SOC stock together with prediction uncertainties. Our spatially explicit forest SOC map showed the high SOC levels in high-elevation forests and a significant underrepresentation of these stocks in global-scale assessments. Our results offer an improved baseline on the distribution of total carbon in the forests of the Central Himalayas. The benchmark maps of predicted forest SOC and associated errors, along with our estimate of 494 million tonnes (SE = 16) of total SOC in the topsoil (0-30 cm) of forested areas in Nepal, carry important implications for understanding the spatial variability of forest SOC in mountainous regions with complex terrains.

Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees.

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2 ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller gs , but greater Anet , responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs . The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.

Canopy dieback and recovery in Australian native forests following extreme drought.

In 2019, south-eastern Australia experienced its driest and hottest year on record, resulting in massive canopy dieback events in eucalypt dominated forests. A subsequent period of high precipitation in 2020 provided a rare opportunity to quantify the impacts of extreme drought and consequent recovery. We quantified canopy health and hydraulic impairment (native percent loss of hydraulic conductivity, PLC) of 18 native tree species growing at 15 sites that were heavily impacted by the drought both during and 8-10 months after the drought. Most species exhibited high PLC during drought (PLC:65.1 ± 3.3%), with no clear patterns across sites or species. Heavily impaired trees (PLC > 70%) showed extensive canopy browning. In the post-drought period, most surviving trees exhibited hydraulic recovery (PLC:26.1 ± 5.1%), although PLC remained high in some trees (50-70%). Regained hydraulic function (PLC < 50%) corresponded to decreased canopy browning indicating improved tree health. Similar drought (37.1 ± 4.2%) and post-drought (35.1 ± 4.4%) percentages of basal area with dead canopy suggested that trees with severely compromised canopies immediately after drought were not able to recover. This dataset provides insights into the impacts of severe natural drought on the health of mature trees, where hydraulic failure is a major contributor in canopy dieback and tree mortality during extreme drought events.

Convergence in phosphorus constraints to photosynthesis in forests around the world.

Tropical forests take up more carbon (C) from the atmosphere per annum by photosynthesis than any other type of vegetation. Phosphorus (P) limitations to C uptake are paramount for tropical and subtropical forests around the globe. Yet the generality of photosynthesis-P relationships underlying these limitations are in question, and hence are not represented well in terrestrial biosphere models. Here we demonstrate the dependence of photosynthesis and underlying processes on both leaf N and P concentrations. The regulation of photosynthetic capacity by P was similar across four continents. Implementing P constraints in the ORCHIDEE-CNP model, gross photosynthesis was reduced by 36% across the tropics and subtropics relative to traditional N constraints and unlimiting leaf P. Our results provide a quantitative relationship for the P dependence for photosynthesis for the front-end of global terrestrial C models that is consistent with canopy leaf measurements.

Predicting resilience through the lens of competing adjustments to vegetation function.

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.

The Role of Hydraulic Failure in a Massive Mangrove Die-Off Event.

Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina, 2 years after the mortality event. We measured predawn water potential (Ψpd) and percent loss of stem hydraulic conductivity (PLC) in surviving individuals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ50) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ50 = -9.6 MPa), indicates that severe water stress (Ψpd < -10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer.

Towards species-level forecasts of drought-induced tree mortality risk.

Predicting species-level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections. We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South-Eastern Australia (mean annual precipitation range: 344-1424 mm yr-1 ). We conducted three experiments: applying CABLE to the 2017-2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO2 ). The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within-species' ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO2 in ameliorating drought stress was consistent across species. Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision-making around the resilience of restoration plantings or net-zero emission strategies.

High safety margins to drought-induced hydraulic failure found in five pasture grasses.

Determining the relationship between reductions in stomatal conductance (gs ) and leaf water transport during dehydration is key to understanding plant drought responses. While numerous studies have analysed the hydraulic function of woody species, minimal research has been conducted on grasses. Here, we sought to characterize hydraulic vulnerability in five widely-occurring pasture grasses (including both C3 and C4 grasses) and determine whether reductions in gs and leaf hydraulic conductance (Kleaf ) during dehydration could be attributed to xylem embolism. Using the optical vulnerability (OV) technique, we found that all species were highly resistant to xylem embolism when compared to other herbaceous angiosperms, with 50% xylem embolism (PX50 ) occurring at xylem pressures ranging from -4.4 to -6.1 MPa. We observed similar reductions in gs and Kleaf under mild water stress for all species, occurring well before PX50 . The onset of xylem embolism (PX12 ) occurred consistently after stomatal closure and 90% reduction of Kleaf . Our results suggest that factors other than xylem embolism are responsible for the majority of reductions in gs and Kleaf during drought and reductions in the productivity of pasture species under moderate drought may not be driven by embolism.

Pastures and Climate Extremes: Impacts of Cool Season Warming and Drought on the Productivity of Key Pasture Species in a Field Experiment.

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3°C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species' sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive (Festuca), or less-than-additive (Medicago), where warming reduced the magnitude of drought effects. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realized. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network.

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those 'next users' of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem's carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers.

Tropical rainforest species have larger increases in temperature optima with warming than warm-temperate rainforest trees.

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.

Mechanisms of xylem hydraulic recovery after drought in Eucalyptus saligna.

The mechanisms by which woody plants recover xylem hydraulic capacity after drought stress are not well understood, particularly with regard to the role of embolism refilling. We evaluated the recovery of xylem hydraulic capacity in young Eucalyptus saligna plants exposed to cycles of drought stress and rewatering. Plants were exposed to moderate and severe drought stress treatments, with recovery monitored at time intervals from 24 h to 6 months after rewatering. The percentage loss of xylem vessels due to embolism (PLV) was quantified at each time point using microcomputed tomography with stem water potential (Ψx ) and canopy transpiration (Ec ) measured before scans. Plants exposed to severe drought stress suffered high levels of embolism (47.38% ± 10.97% PLV) and almost complete canopy loss. No evidence of embolism refilling was observed at 24 h, 1 week, or 3 weeks after rewatering despite rapid recovery in Ψx . Recovery of hydraulic capacity was achieved over a 6-month period by growth of new xylem tissue, with canopy leaf area and Ec recovering over the same period. These findings indicate that E. saligna recovers slowly from severe drought stress, with potential for embolism to persist in the xylem for many months after rainfall events.

Patterns of post-drought recovery are strongly influenced by drought duration, frequency, post-drought wetness, and bioclimatic setting.

Understanding vegetation recovery after drought is critical for projecting vegetation dynamics in future climates. From 1997 to 2009, Australia experienced a long-lasting drought known as the Millennium Drought (MD), which led to widespread reductions in vegetation productivity. However, vegetation recovery post-drought and its determinants remain unclear. This study leverages remote sensing products from different sources-fraction of absorbed photosynthetically active radiation (FPAR), based on optical data, and canopy density, derived from microwave data-and random forest algorithms to assess drought recovery over Australian natural vegetation during a 20-year period centered on the MD. Post-drought recovery was prevalent across the continent, with 6 out of 10 drought events seeing full recovery within about 6 months. Canopy density was slower to recover than leaf area seen in FPAR. The probability of full recovery was most strongly controlled by drought return interval, post-drought hydrological condition, and drought length. Full recovery was seldom observed when drought events occurred at intervals of 3 months or less, and moderately dry (standardized water balance anomaly [SWBA] within [-1, -0.76]) post-drought conditions resulted in less complete recovery than wet (SWBA > 0.3) post-drought conditions. Press droughts, which are long term but not extreme, delayed recovery more than pulse droughts (short term but extreme) and led to a higher frequency of persistent decline. Following press droughts, the frequency of persistent decline differed little among biome types but peaked in semi-arid regions across aridity levels. Forests and savanna required the longest recovery times for press drought, while grasslands were the slowest to recover for pulse drought. This study provides quantitative thresholds that could be used to improve the modeling of ecosystem dynamics post-drought.

To what extent can rising [CO2 ] ameliorate plant drought stress?

Plant responses to elevated atmospheric carbon dioxide (eCO2 ) have been hypothesized as a key mechanism that may ameliorate the impact of future drought. Yet, despite decades of experiments, the question of whether eCO2 reduces plant water use, yielding 'water savings' that can be used to maintain plant function during periods of water stress, remains unresolved. In this Viewpoint, we identify the experimental challenges and limitations to our understanding of plant responses to drought under eCO2 . In particular, we argue that future studies need to move beyond exploring whether eCO2 played 'a role' or 'no role' in responses to drought, but instead more carefully consider the timescales and conditions that would induce an influence. We also argue that considering emergent differences in soil water content may be an insufficient means of assessing the impact of eCO2 . We identify eCO2 impact during severe drought (e.g. to the point of mortality), interactions with future changes in vapour pressure deficit and uncertainty about changes in leaf area as key gaps in our current understanding. New insights into CO2 × drought interactions are essential to better constrain model theory that governs future climate model projections of land-atmosphere interactions during periods of water stress.

Increasing aridity will not offset CO2 fertilization in fast-growing eucalypts with access to deep soil water.

Rising atmospheric [CO2 ] (Ca ) generally enhances tree growth if nutrients are not limiting. However, reduced water availability and elevated evaporative demand may offset such fertilization. Trees with access to deep soil water may be able to mitigate such stresses and respond more positively to Ca . Here, we sought to evaluate how increased vapor pressure deficit and reduced precipitation are likely to modify the impact of elevated Ca (eCa ) on tree productivity in an Australian Eucalyptus saligna Sm. plantation with access to deep soil water. We parameterized a forest growth simulation model (GOTILWA+) using data from two field experiments on E. saligna: a 2-year whole-tree chamber experiment with factorial Ca (ambient =380, elevated =620 µmol mol-1 ) and watering treatments, and a 10-year stand-scale irrigation experiment. Model evaluation showed that GOTILWA+ can capture the responses of canopy C uptake to (1) rising vapor pressure deficit (D) under both Ca treatments; (2) alterations in tree water uptake from shallow and deep soil layers during soil dry-down; and (3) the impact of irrigation on tree growth. Simulations suggest that increasing Ca up to 700 µmol mol-1 alone would result in a 33% increase in annual gross primary production (GPP) and a 62% increase in biomass over 10 years. However, a combined 48% increase in D and a 20% reduction in precipitation would halve these values. Our simulations identify high D conditions as a key limiting factor for GPP. They also suggest that rising Ca will compensate for increasing aridity limitations in E. saligna trees with access to deep soil water under non-nutrient limiting conditions, thereby reducing the negative impacts of global warming upon this eucalypt species. Simulation models not accounting for water sources available to deep-rooting trees are likely to overestimate aridity impacts on forest productivity and C stocks.

Hydraulic failure and tree size linked with canopy die-back in eucalypt forest during extreme drought.

Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die-back in eucalypt forests. The role of hydraulic failure and tree size on canopy die-back in three eucalypt tree species during this drought was examined. We measured pre-dawn and midday leaf water potential (Ψleaf ), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought-induced xylem embolism. Tree size and tree health was also surveyed. Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78-100%). This is in contrast to trees with partial canopy die-back (30-70% canopy die-back: 72-78% native embolism), or relatively healthy trees (little evidence of canopy die-back: 25-31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die-back (-2.7 to -6.3 MPa), compared with relatively healthy trees (-2.1 to -4.5 MPa). In two of the species the majority of individuals showing complete canopy die-back were in the small size classes. Our results indicate that hydraulic failure is strongly associated with canopy die-back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

Drought by CO2 interactions in trees: a test of the water savings mechanism.

Elevated atmospheric CO2 (eCa ) may benefit plants during drought by reducing stomatal conductance (gs ) but any 'water savings effect' could be neutralized by concurrent stimulation of leaf area. We investigated whether eCa enhanced water savings, thereby ameliorating the impact of drought on carbon and water relations in trees. We report leaf-level gas exchange and whole-plant and soil water relations during a short-term dry-down in two Eucalyptus species with contrasting drought tolerance. Plants had previously been established for 9 to 11 months in steady-state conditions of ambient atmospheric CO2 (aCa ) and eCa , with half of each treatment group exposed to sustained drought for 5 to 7 months. The lower stomatal conductance under eCa did not lead to soil moisture savings during the dry-down due to the counteractive effect of increased whole-plant leaf area. Nonetheless, eCa -grown plants maintained higher photosynthetic rates and leaf water potentials, making them less stressed during the dry-down, despite being larger. These effects were more pronounced in the xeric species than the mesic species, and in previously water-stressed plants. Our findings indicate that eCa may enhance plant performance during drought despite a lack of soil water savings, especially in species with more conservative growth and water-use strategies.