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

Integrated Ca, Mg, Cu, and Zn supply upregulates leaf anatomy and metabolic adjustments in Eucalyptus seedlings

Adaptive responses to abiotic stresses such as soil acidity in Eucalyptus-the most widely planted broad-leaf forest genus globally-are poorly understood. This is particularly evident in physiological and anatomical disorders that inhibit plant development and wood quality. We aimed to explore how the supply of Ca and Mg through liming (lime), combined with Cu and Zn fertilization (CZF), influences physiological and anatomical responses during Eucalyptus grandis seedlings growth in tropical acid soil. Therefore, related parameters of leaf area and leaf anatomy, stomatal size, leaf gas exchange, antioxidant system, nutrient partitioning, and biomass allocation responses were monitored. Liming alone in Eucalyptus increased specific leaf area, stomatal density on the abaxial leaf surface, and Ca and Mg content. Also, Eucalyptus exposed only to CZF increased Cu and Zn content. Lime and CZF increased leaf blade and adaxial epidermal thickness, and improved the structural organization of the spongy mesophyll, promoting increased net CO2 assimilation, and stomatal conductance. Fertilization with Ca, Mg, Cu, and Zn positively affects plant nutrition, light utilization, photosynthetic rate, and antioxidant performance, improving growth. Our results indicate that lime and CZF induce adaptive responses in the physiological and anatomical adjustments of Eucalyptus plantation, thereby promoting biomass accumulation.

Stomatal responses of terrestrial plants to global change

Quantifying the stomatal responses of plants to global change factors is crucial for modeling terrestrial carbon and water cycles. Here we synthesize worldwide experimental data to show that stomatal conductance (g_s) decreases with elevated carbon dioxide (CO₂), warming, decreased precipitation, and tropospheric ozone pollution, but increases with increased precipitation and nitrogen (N) deposition. These responses vary with treatment magnitude, plant attributes (ambient g_s, vegetation biomes, and plant functional types), and climate. All two-factor combinations (except warming + N deposition) significantly reduce g_s, and their individual effects are commonly additive but tend to be antagonistic as the effect sizes increased. We further show that rising CO₂ and warming would dominate the future change of plant g_s across biomes. The results of our meta-analysis provide a foundation for understanding and predicting plant g_s across biomes and guiding manipulative experiment designs in a real world where global change factors do not occur in isolation.

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 (A_net ) 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 A_net and stomatal conductance (g_s ). 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 g_s , but greater A_net , 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 A_net had stronger effects than reductions in g_s . 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.

Towards a unified theory of plant photosynthesis and hydraulics

The global carbon and water cycles are governed by the coupling of CO₂ and water vapour exchanges through the leaves of terrestrial plants, controlled by plant adaptations to balance carbon gains and hydraulic risks. We introduce a trait-based optimality theory that unifies the treatment of stomatal responses and biochemical acclimation of plants to environments changing on multiple timescales. Tested with experimental data from 18 species, our model successfully predicts the simultaneous decline in carbon assimilation rate, stomatal conductance and photosynthetic capacity during progressive soil drought. It also correctly predicts the dependencies of gas exchange on atmospheric vapour pressure deficit, temperature and CO₂. Model predictions are also consistent with widely observed empirical patterns, such as the distribution of hydraulic strategies. Our unified theory opens new avenues for reliably modelling the interactive effects of drying soil and rising atmospheric CO₂ on global photosynthesis and transpiration.

Increasing water-use efficiency mediates effects of atmospheric carbon, sulfur, and nitrogen on growth variability of central European conifers

Climate controls forest biomass production through direct effects on cambial activity and indirectly through interactions with CO₂, air pollution, and nutrient availability. The atmospheric concentration of CO₂, sulfur and nitrogen deposition can also exert a significant indirect control on wood formation since these factors influence the stomatal regulation of transpiration and carbon uptake, that is, intrinsic water use efficiency (iWUE). Here we provide 120-year long tree-ring time series of iWUE, stem growth, climatic and combined sulfur and nitrogen (SN) deposition trends for two common tree species, Pinus sylvestris (PISY) and Picea abies (PCAB), at their lower and upper distribution margins in Central Europe. The main goals were to explain iWUE trends using theoretical scenarios including climatic and SN deposition data, and to assess the contribution of climate and iWUE to the observed growth trends. Our results showed that after a notable increase in iWUE between the 1950s and 1980s, this positive trend subsequently slowed down. The substantial rise of iWUE since the 1950s resulted from a combination of an accelerated increase in atmospheric CO₂ concentrations (C_a) and a stable level of leaf intercellular CO₂ (C_i). The offset of observed iWUE values above the trajectory of a constant C_i/C_a scenario was explained by trends in SN deposition (all sites) together with the variation of drought conditions (low-elevation sites only). Increasing iWUE over the 20th and 21st centuries improved tree growth at low-elevation drought-sensitive sites. In contrast, at high-elevation PCAB sites, growth was mainly stimulated by recent warming. We propose that SN pollution should be considered in order to explain the steep increase in iWUE of conifers in the 20th century throughout Central Europe and other regions with a significant SN deposition history.

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.

Process-oriented analysis of dominant sources of uncertainty in the land carbon sink

The observed global net land carbon sink is captured by current land models. All models agree that atmospheric CO₂ and nitrogen deposition driven gains in carbon stocks are partially offset by climate and land-use and land-cover change (LULCC) losses. However, there is a lack of consensus in the partitioning of the sink between vegetation and soil, where models do not even agree on the direction of change in carbon stocks over the past 60 years. This uncertainty is driven by plant productivity, allocation, and turnover response to atmospheric CO₂ (and to a smaller extent to LULCC), and the response of soil to LULCC (and to a lesser extent climate). Overall, differences in turnover explain ~70% of model spread in both vegetation and soil carbon changes. Further analysis of internal plant and soil (individual pools) cycling is needed to reduce uncertainty in the controlling processes behind the global land carbon sink.

The global net land sink is relatively well constrained. However, the responsible drivers and above/below-ground partitioning are highly uncertain. Model issues regarding turnover of individual plant and soil components are responsible.

Nocturnal warming accelerates drought-induced seedling mortality of two evergreen tree species

Extreme drought is one of the key climatic drivers of tree mortality on a global scale. However, it remains unclear whether the drought-induced tree mortality will increase under nocturnal climate warming. Here we exposed seedlings of two wide-ranging subtropical tree species, Castanopsis sclerophylla and Schima superba, with contrasting stomatal regulation strategies to prolonged drought under ambient and elevated night-time temperature by 2 °C. We quantified the seedling survival time since drought treatment by measuring multiple leaf traits such as leaf gas exchange, predawn leaf water potential and water-use efficiency. The results showed that all seedlings in the ambient temperature died within 180 days and 167 days of drought for C. sclerophylla and S. superba, respectively. Night warming significantly shortened the survival time of C. sclerophylla, by 31 days, and S. superba by 28 days, under the drought treatment. A survival analysis further showed that seedlings under night warming suffered a 1.6 times greater mortality risk than those under ambient temperature. Further analyses revealed that night warming suppressed net leaf carbon gain in both species by increasing the nocturnal respiratory rate of S. superba across the first 120 days of drought and decreasing the photosynthetic rate of both species generally after 46 days of drought. These effects on net carbon gain were more pronounced in S. superba than C. sclerophylla. After 60 days of drought, night warming decreased the predawn leaf water potential and leaf water-use efficiency of C. sclerophylla but not S. superba. These contrasting responses are partially due to variations in stomatal control between the two species. These findings suggest that stomatal traits can regulate the response of leaf gas exchange and plant water-use to nocturnal warming during drought. This study indicates that nocturnal warming can accelerate tree mortality during drought. Night warming accelerates the mortality of two subtropical seedlings under drought.Night warming differently affects the drought response of leaf gas exchange and plant water-use between the two species due to species-specific stomatal morphological traits.Carbon metabolism changes and hydraulic damage play differential roles in driving night-warming impacts on the drought-induced mortality between the two species.

Climate Adaptation, Drought Susceptibility, and Genomic-Informed Predictions of Future Climate Refugia for the Australian Forest Tree Eucalyptus globulus

Understanding the capacity of forest tree species to adapt to climate change is of increasing importance for managing forest genetic resources. Through a genomics approach, we modelled spatial variation in climate adaptation within the Australian temperate forest tree Eucalyptus globulus, identified putative climate drivers of this genomic variation, and predicted locations of future climate refugia and populations at-risk of future maladaptation. Using 812,158 SNPs across 130 individuals from 30 populations (i.e., localities) spanning the species’ natural range, a gradientForest algorithm found 1177 SNPs associated with locality variation in home-site climate (climate-SNPs), putatively linking them to climate adaptation. Very few climate-SNPs were associated with population-level variation in drought susceptibility, signalling the multi-faceted nature and complexity of climate adaptation. Redundancy analysis (RDA) showed 24% of the climate-SNP variation could be explained by annual precipitation, isothermality, and maximum temperature of the warmest month. Spatial predictions of the RDA climate vectors associated with climate-SNPs allowed mapping of genomically informed climate selective surfaces across the species’ range under contemporary and projected future climates. These surfaces suggest over 50% of the current distribution of E. globulus will be outside the modelled adaptive range by 2070 and at risk of climate maladaptation. Such surfaces present a new integrated approach for natural resource managers to capture adaptive genetic variation and plan translocations in the face of climate change.

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

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.

Greening drylands despite warming consistent with carbon dioxide fertilization effect

The rising atmospheric CO₂ concentration leads to a CO₂ fertilization effect on plants-that is, increased photosynthetic uptake of CO₂ by leaves and enhanced water-use efficiency (WUE). Yet, the resulting net impact of CO₂ fertilization on plant growth and soil moisture (SM) savings at large scale is poorly understood. Drylands provide a natural experimental setting to detect the CO₂ fertilization effect on plant growth since foliage amount, plant water-use and photosynthesis are all tightly coupled in water-limited ecosystems. A long-term change in the response of leaf area index (LAI, a measure of foliage amount) to changes in SM is likely to stem from changing water demand of primary productivity in water-limited ecosystems and is a proxy for changes in WUE. Using 34-year satellite observations of LAI and SM over tropical and subtropical drylands, we identify that a 1% increment in SM leads to 0.15% (±0.008, 95% confidence interval) and 0.51% (±0.01, 95% confidence interval) increments in LAI during 1982-1998 and 1999-2015, respectively. The increasing response of LAI to SM has contributed 7.2% (±3.0%, 95% confidence interval) to total dryland greening during 1999-2015 compared to 1982-1998. The increasing response of LAI to SM is consistent with the CO₂ fertilization effect on WUE in water-limited ecosystems, indicating that a given amount of SM has sustained greater amounts of photosynthetic foliage over time. The LAI responses to changes in SM from seven dynamic global vegetation models are not always consistent with observations, highlighting the need for improved process knowledge of terrestrial ecosystem responses to rising atmospheric CO₂ concentration.