Robust Response of Terrestrial Plants to Rising CO2

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.

The evolution of extant South American tropical biomes

This review explores the evolution of extant South American tropical biomes, focusing on when and why they developed. Tropical vegetation experienced a radical transformation from being dominated by non-angiosperms at the onset of the Cretaceous to full angiosperm dominance nowadays. Cretaceous tropical biomes do not have extant equivalents; lowland forests, dominated mainly by gymnosperms and ferns, lacked a closed canopy. This condition was radically transformed following the massive extinction event at the Cretaceous-Paleogene boundary. The extant lowland tropical rainforests first developed at the onset of the Cenozoic with a multistratified forest, an angiosperm-dominated closed canopy, and the dominance of the main families of the tropics including legumes. Cenozoic rainforest diversity has increased during global warming and decreased during global cooling. Tropical dry forests emerged at least by the late Eocene, whereas other Neotropical biomes including tropical savannas, montane forests, páramo/puna, and xerophytic forest are much younger, greatly expanding during the late Neogene, probably at the onset of the Quaternary, at the expense of the rainforest.

Functional phenotypic plasticity mediated by water stress and [CO2] explains differences in drought tolerance of two phylogenetically close conifers

Forests are threatened globally by increased recurrence and intensity of hot droughts. Functionally close coexisting species may exhibit differences in drought vulnerability large enough to cause niche differentiation and affect forest dynamics. The effect of rising atmospheric [CO2], which could partly alleviate the negative effects of drought, may also differ between species. We analysed functional plasticity in seedlings of two taxonomically close pine species (Pinus pinaster Ait., Pinus pinea L.) under different [CO2] and water stress levels. The multidimensional functional trait variability was more influenced by water stress (preferentially xylem traits) and [CO2] (mostly leaf traits) than by differences between species. However, we observed differences between species in the strategies followed to coordinate their hydraulic and structural traits under stress. Leaf 13C discrimination decreased with water stress and increased under elevated [CO2]. Under water stress both species increased their sapwood area to leaf area ratios, tracheid density and xylem cavitation, whereas they reduced tracheid lumen area and xylem conductivity. Pinus pinea was more anisohydric than P. pinaster. Pinus pinaster produced larger conduits under well-watered conditions than P. pinea. Pinus pinea was more tolerant to water stress and more resistant to xylem cavitation under low water potentials. The higher xylem plasticity in P. pinea, particularly in tracheid lumen area, expressed a higher capacity of acclimation to water stress than P. pinaster. In contrast, P. pinaster coped with water stress comparatively more by increasing plasticity of leaf hydraulic traits. Despite the small differences observed in the functional response to water stress and drought tolerance between species, these interspecific differences agreed with ongoing substitution of P. pinaster by P. pinea in forests where both species co-occur. Increased [CO2] had little effect on the species-specific relative performance. Thus, a competitive advantage under moderate water stress of P. pinea compared with P. pinaster is expected to continue in the future.

Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change

Trees continuously regulate leaf physiology to acquire CO₂ while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂ (iCO₂ ) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long-term records of tree-ring carbon isotope signatures with leaf physiological measurements of Quercus rubra (Quru) and Liriodendron tulipifera (Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net ), and stomatal conductance to water (g_s ) since 1940. We first show 16%-25% increases in tree iWUE since the mid-20th century, primarily driven by iCO₂ , but also document the individual and interactive effects of nitrogen (NO_x ) and sulfur (SO₂ ) air pollution overwhelming climate. We find evidence for Quru leaf gas exchange being less tightly regulated than Litu through an analysis of isotope-derived leaf internal CO₂ (C_i ), particularly in wetter, recent years. Modeled estimates of seasonally integrated A_net and g_s revealed a 43%-50% stimulation of A_net was responsible for increasing iWUE in both tree species throughout 79%-86% of the chronologies with reductions in g_s attributable to the remaining 14%-21%, building upon a growing body of literature documenting stimulated A_net overwhelming reductions in g_s as a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.

Meeting the heavy-metal safety requirements for food crops by using biochar: An investigation using sunflower as a representative plant under different atmospheric CO2 concentrations

Global warming impacts on plant growth and food safety are emerging topics of concern, while biochar as a soil additive benefits plants. This study investigates (1) sunflower plant growth at various biochar concentrations in a soil-compost growing substrate under both ambient (420 ppm) and elevated (740 ppm) atmospheric CO₂ concentrations, and (2) concentrations of heavy metals in the growing substrates and organs of the plants. The elevated CO₂ concentration benefits the vegetative parts but harms the reproductive parts of the plants. Additionally, the elevated CO₂ concentration inhibits the beneficial effects that biochar confers on the plants at the ambient concentration. The optimum biochar concentration at both CO₂ levels was found to be 15%. At the time of harvest, most of the heavy-metal concentrations in the growing substrate increased. It was demonstrated that biochar can reduce the amount of heavy metals that accumulate in the roots and seeds whose heavy-metal concentrations complied with Singapore food safety regulations, while those for the biochar met the proposed Singapore biochar standard's thresholds. Our results show that the proposed Singapore biochar standard is practical and sound.

Feedback in tropical forests of the Anthropocene

Tropical forests are complex systems containing myriad interactions and feedbacks with their biotic and abiotic environments, but as the world changes fast, the future of these ecosystems becomes increasingly uncertain. In particular, global stressors may unbalance the feedbacks that stabilize tropical forests, allowing other feedbacks to propel undesired changes in the whole ecosystem. Here, we review the scientific literature across various fields, compiling known interactions of tropical forests with their environment, including the global climate, rainfall, aerosols, fire, soils, fauna, and human activities. We identify 170 individual interactions among 32 elements that we present as a global tropical forest network, including countless feedback loops that may emerge from different combinations of interactions. We illustrate our findings with three cases involving urgent sustainability issues: (1) wildfires in wetlands of South America; (2) forest encroachment in African savanna landscapes; and (3) synergistic threats to the peatland forests of Borneo. Our findings reveal an unexplored world of feedbacks that shape the dynamics of tropical forests. The interactions and feedbacks identified here can guide future qualitative and quantitative research on the complexities of tropical forests, allowing societies to manage the nonlinear responses of these ecosystems in the Anthropocene.

Characteristics and influencing factors of carbon fluxes in winter wheat fields under elevated CO2 concentration

Elevated carbon dioxide (ECO₂) concentration has profound impacts on ecosystem carbon fluxes, with consequent changes in carbon sequestration and its feedback to climate change. Agroecosystem plays an essential role in global carbon sequestration. However, it is not well understood how the carbon fluxes of agroecosystem respond to increasing atmospheric CO₂ concentrations. In this study, an in-situ 2-year field experiment was conducted using open-top chamber with treatments including ambient CO₂ concentration (CK) and ambient plus 200 μmol mol^-1 (T) to investigate the characteristics and main factors influencing carbon fluxes during the 2017-2019 winter wheat growing seasons. Results showed that the dynamics of CO₂ fluxes under different treatments had similar seasonal trends, with the peak flux observed at the heading-filling stage. Compared to the CK, T treatment increased the cumulative amount of CO₂ (CAC) by 17.2% and 24.0% in 2017-2018 and 2018-2019 growing seasons, respectively. In addition, the seasonal CAC was highly dependent on treatment and varied with year, while there was no interactive effect of treatment and year (p > 0.05). ECO₂ concentration increased the biomass of wheat by an average of 8.28% over two growing seasons. There was a significant positive correlation between biomass and CAC, with biomass elucidating 52% and 76% of the variations in CAC under CK and T treatments, respectively. A good correlation was found between net ecosystem exchange (NEE) and environmental variables under different treatments. During the pre-milk ripening period, the NEE mainly depended on photosynthetically active radiation (PAR) and air temperature (Ta), while NEE was mainly controlled by PAR and soil water content (SWC) during the post-milk ripening period. Overall, the findings presented here demonstrate that the carbon exchange in wheat fields under different treatments serves as carbon sequestration, while ECO₂ concentration enhances the capacity of winter wheat fields to act as carbon sinks, which may have feedback to the climate system in the future.

Urban afforestation: using phytotoxicity endpoints to compare air pollution tolerance of two native Brazilian plants Aroeira (Schinus terebinthifolius) and Cuvatã (Cupania vernalis)

Urban afforestation can mitigate the effects of air pollution, but the suitability of plant species for this purpose needs to be determined according to pollution intensity and climate change. The goal of this study was to evaluate the sensitivity of different phytotoxicity endpoints using two native Brazilian plant species as models, Aroeira (Schinus terebinthifolius) and Cuvatã (Cupania vernalis). The sensitivity parameters evaluated could help in selecting the most air-pollution-tolerant plant species for use in urban afforestation programs. The two plant species were exposed, in a greenhouse, to the combustion gases of a diesel engine for 120 days, with daily intermittent gas exposure. Every 30 days, leaf injury (chlorosis and necrosis), biomass, and physiological/biochemical parameters (proteins, chlorophyll, and peroxidase enzyme activity) were evaluated for both plant species. For the two selected species, the endpoints studied can be ranked according to their sensitivity (or inversely the tolerance) to diesel oil combustion gases in the following order: peroxidase > biomass ≈ chlorophyll > protein > leaf injury. The endpoint responses of higher plants can be used to assess the suitability of particular plant species for use in urban afforestation areas with relatively intense vehicle traffic.

Antagonist Temperature Variation Affects the Photosynthetic Parameters and Secondary Metabolites of Ocimum basilicum L. and Salvia officinalis L

Climate change is one of the main challenges for actual and future generations. Global warming affects plants and animals and is responsible for considerable crop loss. This study studied the influence of antagonist successive stresses, cold-heat and heat-cold, on two medicinal plants Ocimum basilicum L. and Salvia officinalis L. The photosynthetic parameters decreased for plants under the variation of subsequent stress. Net assimilation rates and stomatal conductance to water vapor are more affected in the case of plants under cold-heat consecutive stress than heat-cold successive stress. Emissions of volatile organic compounds have been enhanced for plants under successive stress when compared with control plants. Chlorophyll concentrations for plants under successive stress decreased for basil and sage plants. The total phenolic and flavonoid contents were not affected by the successive stresses when compared with the plants under only one type of treatment.

Inter-comparisons of mean, trend and interannual variability of global terrestrial gross primary production retrieved from remote sensing approach

Many models were established to estimate gross primary production (GPP) of terrestrial ecosystems based on vegetation light use efficiency (LUE). Analysing the spatial-temporal variations of global terrestrial GPP became capable with the increasing length of satellite data. Previous studies mainly focused on evaluating the model performance or investigating the mean, the temporal trend or the interannual variability (IAV) of global terrestrial GPP based on one single or multiple models, which is difficult to identify common merits of a same cluster of GPP models. This study compared eight satellite-based LEU-type GPP models in capturing the mean, temporal trend and IAV of global GPP concurrently. Our results showed that current common-used models based on LUE methodology estimated global mean GPP ranging from 128.5 to 158.3 Pg C year^-1, and global mean IAV ranging from 0.1 to 0.35, but the trends ranging from -0.22 to 0.51 Pg C year^-1. In the context of plant functional types (PFTs) and climate classifications, no consistent feature for either of the mean, trend or IAV of GPP are identified among eight models. Future studies should integrate the latest advances on the mechanisms and associated environmental factors into models and consolidate performance of models to better understand the evolutions of terrestrial ecosystem functioning.

Does growing atmospheric CO2 explain increasing carbon sink in a boreal coniferous forest?

The terrestrial net ecosystem productivity (NEP) has increased during the past three decades, but the mechanisms responsible are still unclear. We analyzed 17 years (2001-2017) of eddy-covariance measurements of NEP, evapotranspiration (ET) and light and water use efficiency from a boreal coniferous forest in Southern Finland for trends and inter-annual variability (IAV). The forest was a mean annual carbon sink (252 [ ± 42] gC m-2a-1 ), and NEP increased at rate +6.4-7.0 gC m-2a-1 (or ca. +2.5% a-1 ) during the period. This was attributed to the increasing gross-primary productivity GPP and occurred without detectable change in ET. The start of annual carbon uptake period was advanced by 0.7 d a-1 , and increase in GPP and NEP outside the main growing season contributed ca. one-third and one-fourth of the annual trend, respectively. Meteorological factors were responsible for the IAV of fluxes but did not explain the long-term trends. The growing season GPP trend was strongest in ample light during the peak growing season. Using a multi-layer ecosystem model, we showed that direct CO2 fertilization effect diminishes when moving from leaf to ecosystem, and only 30-40% of the observed ecosystem GPP increase could be attributed to CO2 . The increasing trend in leaf-area index (LAI), stimulated by forest thinning in 2002, was the main driver of the enhanced GPP and NEP of the mid-rotation managed forest. It also compensated for the decrease of mean leaf stomatal conductance with increasing CO2 and LAI, explaining the apparent proportionality between observed GPP and CO2 trends. The results emphasize that attributing trends to their physical and physiological drivers is challenged by strong IAV, and uncertainty of LAI and species composition changes due to the dynamic flux footprint. The results enlighten the underlying mechanisms responsible for the increasing terrestrial carbon uptake in the boreal zone.

The Influence of Elevated CO2 on Volatile Emissions, Photosynthetic Characteristics, and Pigment Content in Brassicaceae Plants Species and Varieties

Climate change will determine a sharp increase in carbon dioxide in the following years. To study the influence of elevated carbon dioxide on plants, we grew 13 different species and varieties from the Brassicaceae family at three carbon dioxide concentrations: 400, 800, and 1200 ppmv. The photosynthetic parameters (assimilation rate and stomatal conductance to water vapor) increase for all species. The emission of monoterpenes increases for plants grown at elevated carbon dioxide while the total polyphenols and flavonoids content decrease. The chlorophyll content is affected only for some species (such as Lipidium sativum), while the β-carotene concentrations in the leaves were not affected by carbon dioxide.

A constraint on historic growth in global photosynthesis due to increasing CO2

The global terrestrial carbon sink is increasing^1-3, offsetting roughly a third of anthropogenic CO₂ released into the atmosphere each decade¹, and thus serving to slow⁴ the growth of atmospheric CO₂. It has been suggested that a CO₂-induced long-term increase in global photosynthesis, a process known as CO₂ fertilization, is responsible for a large proportion of the current terrestrial carbon sink^4-7. The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO₂ concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models^7-13. Here we quantify the historic effect of CO₂ on global photosynthesis by identifying an emergent constraint^14-16 that combines terrestrial biosphere models with global carbon budget estimates. Our analysis suggests that CO₂ fertilization increased global annual photosynthesis by 11.85 ± 1.4%, or 13.98 ± 1.63 petagrams carbon (mean ± 95% confidence interval) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global photosynthesis to CO₂, and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.

Leaf and tree responses of young European aspen trees to elevated atmospheric CO2 concentration vary over the season

Elevated atmospheric CO2 concentration (eCO2) commonly stimulates net leaf assimilation, decreases stomatal conductance and has no clear effect on leaf respiration. However, effects of eCO2 on whole-tree functioning and its seasonal dynamics remain far more uncertain. To evaluate temporal and spatial variability in eCO2 effects, 1-year-old European aspen trees were grown in two treatment chambers under ambient (aCO2, 400 p.p.m.) and elevated (eCO2, 700 p.p.m.) CO2 concentrations during an early (spring 2019) and late (autumn 2018) seasonal experiment. Leaf (net carbon assimilation, stomatal conductance and leaf respiration) and whole-tree (stem growth, sap flow and stem CO2 efflux) responses to eCO2 were measured. Under eCO2, carbon assimilation was stimulated during the early (1.63-fold) and late (1.26-fold) seasonal experiments. Stimulation of carbon assimilation changed over time with largest increases observed in spring when stem volumetric growth was highest, followed by late season down-regulation, when stem volumetric growth ceased. The neutral eCO2 effect on stomatal conductance and leaf respiration measured at leaf level paralleled the unresponsive canopy conductance (derived from sap flow measurements) and stem CO2 efflux measured at tree level. Our results highlight that seasonality in carbon demand for tree growth substantially affects the magnitude of the response to eCO2 at both leaf and whole-tree level.

The variable effects of global change on insect mutualisms

Insect mutualisms are essential for reproduction of many plants, protection of plants and other insects, and provisioning of nutrients for insects. Disruption of these mutualisms by global change can have important implications for ecosystem processes. Here, we assess the general effects of global change on insect mutualisms, including the possible impacts on mutualistic networks. We find that the effects of global change on mutualisms are extremely variable, making broad patterns difficult to detect. We require studies focusing on changes in cost-benefit ratios, effects of partner dependency, and degree of specialization to further understand how global change will influence insect mutualism dynamics. We propose that rapid coevolution is one avenue by which mutualists can ameliorate the effects of global change.

Precipitation alters the CO2 effect on water-use efficiency of temperate forests

Increasing water-use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO₂ (c_a ). Existing theory and empirical evidence suggest a proportional WUE increase in response to rising c_a as plants maintain a relatively constant ratio between the leaf intercellular (c_i ) and ambient (c_a ) partial CO₂ pressure (c_i /c_a ). This has been hypothesized as the main driver of the strengthening of the terrestrial carbon sink over the recent decades. However, proportionality may not characterize CO₂ effects on WUE on longer time-scales and the role of climate in modulating these effects is uncertain. Here, we evaluate long-term WUE responses to c_a and climate from 1901 to 2012 CE by reconstructing intrinsic WUE (iWUE, the ratio of photosynthesis to stomatal conductance) using carbon isotopes in tree rings across temperate forests in the northeastern USA. We show that iWUE increased steadily from 1901 to 1975 CE but remained constant thereafter despite continuously rising c_a . This finding is consistent with a passive physiological response to c_a and coincides with a shift to significantly wetter conditions across the region. Tree physiology was driven by summer moisture at multi-decadal time-scales and did not maintain a constant c_i /c_a in response to rising c_a indicating that a point was reached where rising CO₂ had a diminishing effect on tree iWUE. Our results challenge the mechanism, magnitude, and persistence of CO₂ 's effect on iWUE with significant implications for projections of terrestrial productivity under a changing climate.

Short photoperiod attenuates CO2 fertilization effect on shoot biomass in Arabidopsis thaliana

The level of carbon dioxide (CO2) in the air can affect several traits in plants. Elevated atmospheric CO2 (eCO2) can enhance photosynthesis and increase plant productivity, including biomass, although there are inconsistencies regarding the effects of eCO2 on the plant growth response. The compounding effects of ambient environmental conditions such as light intensity, photoperiod, water availability, and soil nutrient composition can affect the extent to which eCO2 enhances plant productivity. This study aimed to investigate the growth response of Arabidopsis thaliana to eCO2 (800 ppm) under short photoperiod (8/16 h, light/dark cycle). Here, we report an attenuated fertilization effect of eCO2 on the shoot biomass of Arabidopsis plants grown under short photoperiod. The biomass of two-, three-, and four-week-old Arabidopsis plants was increased by 10%, 15%, and 28%, respectively, under eCO2 compared to the ambient CO2 (aCO2, 400 ppm) i.e. control. However, the number of rosette leaves, rosette area, and shoot biomass were similar in mature plants under both CO2 conditions, despite 40% higher photosynthesis in eCO2 exposed plants. The levels of chlorophylls and carotenoids were similar in the fully expanded rosette leaves regardless of the level of CO2. In conclusion, CO2 enrichment moderately increased Arabidopsis shoot biomass at the juvenile stage, whereas the eCO2-induced increment in shoot biomass was not apparent in mature plants. A shorter day-length can limit the source-to-sink resource allocation in a plant in age-dependent manner, hence diminishing the eCO2 fertilization effect on the shoot biomass in Arabidopsis plants grown under short photoperiod.

Reimagining the relationship between Gondwanan forests and Aboriginal land management in Australia's "Wet Tropics"

The "Wet Tropics" of Australia host a unique variety of plant lineages that trace their origins to the super-continent of Gondwanaland. While these "ancient" evolutionary records are rightly emphasized in current management of the region, multidisciplinary research and lobbying by Rainforest Aboriginal Peoples have also demonstrated the significance of the cultural heritage of the "Wet Tropics." Here, we evaluate the existing archeological, paleoenvironmental, and historical evidence to demonstrate the diverse ways in which these forests are globally significant, not only for their ecological heritage but also for their preservation of traces of millennia of anthropogenic activities, including active burning and food tree manipulation. We argue that detailed paleoecological, ethnobotanical, and archeological studies, working within the framework of growing national and world heritage initiatives and active application of traditional knowledge, offer the best opportunities for sustainable management of these unique environments in the face of increasingly catastrophic climate change and bushfires.

The combined effects of elevated atmospheric CO2 and cadmium exposure on flavonoids in the leaves of Robinia pseudoacacia L. seedlings

Flavonoids participate in several plant processes such as growth and physiological protection in adverse environments. In this study, we investigated the combined effects of eCO₂ and cadmium (Cd)-contaminated soils on the total flavonoid and monomer contents in the leaves of Robinia pseudoacacia L. seedlings. Elevated CO₂, Cd, and eCO₂+ Cd increased the total flavonoids in the leaves relative to the control, and eCO₂ mostly increased (p < 0.05) the total flavonoid content under Cd exposure. Elevated CO₂ increased (p < 0.05) robinin, rutin, and acacetin contents in the leaves of 45-day seedlings and decreased (p < 0.05) the content of robinin and acacetin at 90 and 135 d under Cd exposure except for robinin at day 45 under Cd1 and acacetin on day 135 under Cd1. Quercetin content decreased (p < 0.05) under the combined conditions relative to Cd alone. Kaempferol in the leaves was only detected under eCO₂ on day 135. The responses of total chlorophyll, total soluble sugars, starch, C, N, S, and the C/N ratio in the leaves to eCO₂ significantly affected the synthesis of total flavonoids and monomers under Cd exposure. Overall, rutin was more sensitive to eCO₂+ Cd than the other flavonoids. Cadmium, CO_2, and time had significant interactive effects on the synthesis of flavonoids in the leaves of R. pseudoacacia L. seedlings. Elevated CO₂ may improve the protection and defense system of seedlings grown in Cd-contaminated soils by promoting the synthesis of total flavonoids, although robinin, rutin, quercetin, and acacetin yields may reduce with time. Additionally, increased Cd in the leaves suggested that eCO₂ could improve the phytoremediation of Cd-contaminated soils.

Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO2 and modulated by climate and plant functional types

We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (Anet) to stomatal conductance (gs), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO2 (Ca), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While Ca was a dominant environmental driver of iWUE, the effects of increasing Ca were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon-oxygen isotope approach revealed that increases in Anet dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in Anet, whereas reductions in gs occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between Ca and other environmental factors have important implications for the coupled carbon-hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in gs as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

The main effects of elevated CO2 and soil-water deficiency on 1H NMR-based metabolic fingerprints of Coffea arabica beans by factorial and mixture design

The metabolic response of Coffea arabica trees in the face of the rising atmospheric concentration of carbon dioxide (CO₂) combined with the reduction in soil-water availability is complex due to the various (bio)chemical feedbacks. Modern analytical tools and the experimental advance of agronomic science tend to advance in the understanding of the metabolic complexity of plants. In this work, Coffea arabica trees were grown in a Free-Air Carbon Dioxide Enrichment dispositive under factorial design (2²) conditions considering two CO₂ levels and two soil-water availabilities. The ¹H NMR mixture design-fingerprinting effects of CO₂ and soil-water levels on beans were strategically investigated using the principal component analysis (PCA), analysis of variance (ANOVA) - simultaneous component analysis (ASCA) and partial least squares-discriminant analysis (PLS-DA). From the ASCA, the CO₂ factor had a significant effect on changing the ¹H NMR profile of fingerprints. The soil-water factor and interaction (CO₂ × soil-water) were not significant. ¹H NMR fingerprints with PCA, ASCA and PLS-DA analysis determined spectral profiles for fatty acids, caffeine, trigonelline and glucose increases in beans from current CO₂, while quinic acid/chlorogenic acids, malic acid and kahweol/cafestol increased in coffee beans from elevated CO₂. PLS-DA results revealed a good classification performance between the significant effect of the atmospheric CO₂ levels on the fingerprints, regardless of the soil-water availabilities. Finally, the PLS-DA model showed good prediction ability, successfully classifying validation data-set of coffee beans collected over the vertical profile of the plants and included several fingerprints of different extracting solvents. The results of this investigation suggest that the association of experimental design, mixture design, PCA, ASCA and PLS-DA can provide accurate information on a series of metabolic changes provoked by climate changes in products of commercial importance, in addition to minimizing the extra work necessary in classic analytical approaches, encouraging the development of similar strategies.

Hysteresis of tropical forests in the 21st century

Tropical forests modify the conditions they depend on through feedbacks at different spatial scales. These feedbacks shape the hysteresis (history-dependence) of tropical forests, thus controlling their resilience to deforestation and response to climate change. Here, we determine the emergent hysteresis from local-scale tipping points and regional-scale forest-rainfall feedbacks across the tropics under the recent climate and a severe climate-change scenario. By integrating remote sensing, a global hydrological model, and detailed atmospheric moisture tracking simulations, we find that forest-rainfall feedback expands the geographic range of possible forest distributions, especially in the Amazon. The Amazon forest could partially recover from complete deforestation, but may lose that resilience later this century. The Congo forest currently lacks resilience, but is predicted to gain it under climate change, whereas forests in Australasia are resilient under both current and future climates. Our results show how tropical forests shape their own distributions and create the climatic conditions that enable them.

Elevated CO2-induced changes in photosynthesis, antioxidant enzymes and signal transduction enzyme of soybean under drought stress

Rising atmospheric [CO2] influences plant growth, development, productivity and stress responses. Soybean is a major oil crop. At present, it is unclear how elevated [CO2] affects the physiological and biochemical pathways of soybean under drought stress. In this study, changes in the photosynthetic capacity, photosynthetic pigment and antioxidant level were evaluated in soybean at flowering stages under different [CO2] (400 μmol mol-1 and 600 μmol mol-1) and water level (the relative water content of the soil was 75-85% soil capacity, and the relative water content of the soil was 35-45% soil capacity under drought stress). Changes in levels of osmolytes, hormones and signal transduction enzymes were also determined. The results showed that under drought stress, increasing [CO2] significantly reduced leaf transpiration rate (E), net photosynthetic rate (PN) and chlorophyll b content. Elevated [CO2] significantly decreased the content of malondialdehyde (MDA) and proline (PRO), while significantly increased superoxide dismutase (SOD) and abscisic acid (ABA) under drought stress. Elevated [CO2] significantly increased the transcript and protein levels of calcium-dependent protein kinase (CDPK), and Glutathione S- transferase (GST). The content of HSP-70 and the corresponding gene expression level were significantly reduced by elevated [CO2], irrespective of water treatments. Taken together, these results suggest that elevated [CO2] does not alleviate the negative impacts of drought stress on photosynthesis. ABA, CDPK and GST may play an important role in elevated CO2-induced drought stress responses.