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Terrestrial plants have transformed Earth's surface environments by altering water, energy, and biogeochemical cycles. Studying vegetation-climate interaction in deep time has necessarily relied on modern-plant analogs to represent paleo-ecosystems—as methods for reconstructing paleo- and, in particular, extinct-plant function were lacking. This approach is potentially compromised given that plant physiology has evolved through time, and some paleo-plants have no clear modern analog. Advancements in the quantitative reconstruction of whole-plant function provide new opportunities to replace modern-plant analogs and capture age-specific vegetation-climate interactions. Here, we review recent investigations of paleo-plant performance through the integration of fossil and geologic data with process-based ecosystem- to Earth system–scale models to explore how early vascular plants responded to and influenced climate. First, we present an argument for characterizing extinct plants in terms of ecological and evolutionary theory to provide a framework for advancing reconstructed vegetation-climate interactions in deep time. We discuss the novel mechanistic understanding provided by applying these approaches to plants of the late Paleozoic ever-wet tropics and at higher latitudes. Finally, we discuss preliminary applications to paleo-plants in a state-of-the-art Earth system model to highlight the potential implications of different plant functional strategies on our understanding of vegetation-climate interactions in deep time. ▪ For hundreds of millions of years, plants have been a keystone in maintaining the status of Earth's atmosphere, oceans, and climate. ▪ Extinct plants have functioned differently across time, limiting our understanding of how processes on Earth interact to produce climate. ▪ New methods, reviewed here, allow quantitative reconstruction of extinct-plant function based on the fossil record. ▪ Integrating extinct plants into ecosystem and climate models will expand our understanding of vegetation's role in past environmental change.

To understand Earth ́s climate variability and improve predictions of future climate change, studying past climates is an important avenue to explore. A previously published record of pCO2, across the Triassic–Jurassic boundary (TJB, ~201 Ma) of East Greenland, showed that Bennettitales (Anamozamites and Pterophyllum) responded in parallel to the empirically proven pCO2-responders Ginkgoales, reducing their stomatal densities by half across the TJB, indicating a transient doubling of pCO2. The abundance of fossil Bennettitales in Mesozoic strata and natural history museum collections worldwide offers enormous potential for further stomatal proxy pCO2 reconstructions, but a suitable nearest living equivalent (NLE) should ideally first be identified for this extinct plant group. Using specimens from herbarium collections, three species of cycads, historically considered the best NLE, were tested for pCO2 response, as well as two species of tree ferns, grown in experimental growth chambers. None responded to changes in pCO2, and were consequently rejected as NLEs. Finally, two species of ferns were selected from the literature, and produced very similar pCO2 compared to Ginkgoales. However, these understory ferns are not appropriate NLEs for Bennettitales due to differences in habitat and a distant evolutionary relationship. Future work should test additional plant groups, in particular seed plants such as basal angiosperms and Gnetales, for suitability as NLE for Bennettitales in pCO2 reconstructions, for example through biogeo- chemical fingerprinting using infrared microspectroscopy. Until an appropriate NLE is identified, Bennettitales pCO2 can be reconstructed based on cross-calibration of stomatal densities with those of co-occurring pCO2 responders, such as Ginkgoales. This work was supported by the EU Marie Curie Excellence Grant (MEXT-CT-2006-042531) to J.McE and M.S., the Swedish Research Council (VR Starting Grant NT-7 2016 04905) to M.S., and the IRCSET Embark scholarship (R10679) to C.E-K. We also gratefully acknowledge funding from the EU Marie Curie Intra-European Fellowship (PEA-IEF-2010-275626) and ERC grant (ERC-279962-OXYEVOL). We thank the following for technical assistance: Bredagh Moran, UCD; Matthew Gilroy, Conviron UK; Aidan Blake, Aaron Refrigeration Ireland.

Premise of research. The role of sulfur dioxide (SO2) in plant responses to large igneous province volcanism is difficult to determine but important in relation to understanding ecosystem responses...

Camptotheca Decaisne, an endemic genus with three species confined in central and southern China, is medically important for its anticancer effects. Fossil records indicate extinct genera Amersinia...

Variability in ryegrass responses to elevated CO2 shows that biomass stimulation strongly associates with enhancements in leaf area/tillering capacity and is largely unrelated to leaf-level photosynthetic rate., Whilst a range of strategies have been proposed for enhancing crop productivity, many recent studies have focused primarily on enhancing leaf photosynthesis under current atmospheric CO2 concentrations. Given that the atmospheric CO2 concentration is likely to increase significantly in the foreseeable future, an alternative/complementary strategy might be to exploit any variability in the enhancement of growth/yield and photosynthesis at higher CO2 concentrations. To explore this, we investigated the responses of a diverse range of wild and cultivated ryegrass genotypes, with contrasting geographical origins, to ambient and elevated CO2 concentrations and examined what genetically tractable plant trait(s) might be targeted by plant breeders for future yield enhancements. We found substantial ~7-fold intraspecific variations in biomass productivity among the different genotypes at both CO2 levels, which were related primarily to differences in tillering/leaf area, with only small differences due to leaf photosynthesis. Interestingly, the ranking of genotypes in terms of their response to both CO2 concentrations was similar. However, as expected, estimates of whole-plant photosynthesis were strongly correlated with plant productivity. Our results suggest that greater yield gains under elevated CO2 are likely through the exploitation of genetic differences in tillering and leaf area rather than focusing solely on improving leaf photosynthesis.

Ecosystem process models provide unique insight into terrestrial ecosystems by employing a modern understanding of ecophysiological processes within a dynamic environmental framework. We apply this framework to deep-time ecosystems made up of extinct plants by constructing plant functional types using fossil remains and simulating—as close as possible—the in vivo response of extinct taxa to their paleoclimatic environment. To accomplish this, foliar characteristics including maximum stomatal conductance, distance from leaf vein to stomata, and cuticular carbon and nitrogen were input as model parameters derived from measurements of well-preserved Pennsylvanian-age fossil leaves. With these inputs, we modeled a terrestrial tropical forest ecosystem dominated by “iconic” plant types of the Pennsylvanian (∼323–299 Ma) including arborescent lycopsids, medullosans, cordaitaleans, and tree ferns using a modified version of the process model BIOME-BGC, which we refer to as Paleo-BGC. Modeled carbon and water—and, for the first time, nitrogen—budgets of a tropical ecosystem from Euramerica driven by daily meteorology are simulated using the Global Circulation Model GENESIS 3.0. Key findings are: lycopsids have the lowest daily leaf water potential, soil water content, surface runoff, and degree of nitrogen leaching indicating an intensive water use strategy compared to medullosans, cordaitaleans, and tree ferns that have increasingly lower simulated water use, greater surface, and nitrogen loss in this order; modeled vegetation response to aridification, which was caused by reduced precipitation and intensified through the close of the Carboniferous and into the Permian shows that lycopsids and medullosans have the lowest tolerance for precipitation decrease compared to cordaitaleans and tree ferns, consistent with the paleobotanical record of occurrence of floral turnovers through the Middle Pennsylvanian through earliest Permian; elevated atmospheric pO2, hypothesized as characteristic for the latter half of the Pennsylvanian and early Permian (∼299–272 Ma), caused higher atmospheric pressure reducing plant transpiration, higher surface water runoff rates, and increased nitrogen export for all plant types simulated, manifested most strongly in the lycopsid dominated ecosystems—with overall only a small reduction in net daily assimilation (≈1 μmol CO2 m−2 s−1). Both aridification and elevated atmospheric oxygen reduced transpiration, increased water retention in soils, with higher surface runoff. With more discharge, enhanced and higher short-term surface soil loss and silicate weathering would have been possible in broad regions of the paleotropics during the late Carboniferous and early Permian. These results are only obtainable by integrating multiple, fossil-derived measurements into the simulation framework of an ecosystem model that utilizes daily meteorology.

AimsResearch on the effects of extreme rainfall events on ecosystem function has primarily focussed on drought or flooding events, which usually include changes to mean or total rainfall, annually or over a season. However, less is known about the effects of increased rainfall variability without change to mean or total amounts. We investigated the effects of increased variation of water supply on shoot and root biomass as well as the distribution of root biomass of four grassland plant species, grown in monoculture and mixture communities.MethodsPerennial ryegrass (Lolium perenne L., shallow-rooting grass), chicory (Cichorium intybus L., deep-rooting forb), white clover (Trifolium repens L., shallow-rooting legume) and red clover (Trifolium pratense L., deep-rooting legume) were established in mesocosms. Four plants of the same species were grown in monoculture communities and one of each species grown in four-species communities. Water supply was manipulated such that; compared with a baseline level with low variation in water supply, there was a treatment with medium variation (±40%) and another with high variation (±80%). Shoot and root biomass were measured, and vertical root distribution models fitted.Important FindingsCompared with the low variation treatment, shoot biomass was significantly reduced under high variation for white clover, red clover and four-species communities. Under all conditions, four-species communities produced more shoot and root biomass than predicted by species performance in monoculture (overyielding). Under increased water variation, chicory monocultures allocated a higher proportion of root biomass to deeper soil layers while the total root biomass of white clover monocultures was significantly reduced. These results indicate that increased variability of water supply can negatively affect the shoot and root biomass production of single and multi-species grasslands. There is a need for further investigation of water variation effects on the functioning of multi-species grassland systems at field scale.

How plants have shaped Earth surface feedbacks over geologic time is a key question in botanical and geological inquiry. Recent work has suggested that biomes during the Carboniferous Period contained plants with extraordinary physiological capacity to shape their environment, contradicting the previously dominant view that plants only began to actively moderate the Earth's surface with the rise of angiosperms during the Mesozoic Era. A recently published Viewpoint disputes this recent work, thus here, we document in detail, the mechanistic underpinnings of our modeling and illustrate the extraordinary ecophysiological nature of Carboniferous plants.

Premise of research. Understanding the relationship between field-measured operating stomatal conductance (gop) and theoretical maximum stomatal conductance (gmax), calculated from stomatal density and geometry, provides an important framework that can be used to infer leaf-level gas exchange of historical, herbarium, and fossil plants. To date, however, investigation of the nature of the relationship between gop and theoretical gmax remains limited to a small number of experiments on relatively few taxa and is virtually undefined for plants in natural ecosystems.\ud \ud Methodology. We used the gop measurements of 74 species and 35 families across four biomes from a published contemporary data set of field-measured leaf-level stomatal conductance in woody angiosperms and calculated the theoretical gmax from the same leaves to investigate the relationship between gop and gmax across multiple species and biomes and determine whether such relationships are widely conserved.\ud \ud Pivotal results. We observed significant relationships between gop and gmax, with consistency in the gop ∶ gmax ratio across biomes, growth habits (shrubs and trees), and habitats (open canopy and understory subcanopy). An overall mean gop ∶ gmax ratio of 0.26 ± 0.11 (mean ± SD) was observed. The consistently observed gop ∶ gmax ratio in this study strongly agrees with previous hypotheses that an ideal gop ∶ gmax ratio exists.\ud \ud Conclusions. These results build substantially on previous studies by presenting a new reference for a consistent gop ∶ gmax ratio across many levels and offer great potential to enhance paleoclimate proxies and vegetation-climate models alike.

The Triassic-Jurassic boundary marks the third largest mass extinction event in the Phanerozoic, characterized by a rise in CO

The distribution of forest cover alters Earth surface mass and energy exchange and is controlled by physiology, which determines plant environmental limits. Ancient plant physiology, therefore, likely affected vegetation-climate feedbacks. We combine climate modeling and ecosystem-process modeling to simulate arboreal vegetation in the late Paleozoic ice age. Using GENESIS V3 global climate model simulations, varying pCO2, pO2, and ice extent for the Pennsylvanian, and fossil-derived leaf C:N, maximum stomatal conductance, and specific conductivity for several major Carboniferous plant groups, we simulated global ecosystem processes at a 2° resolution with Paleo-BGC. Based on leaf water constraints, Pangaea could have supported widespread arboreal plant growth and forest cover. However, these models do not account for the impacts of freezing on plants. According to our interpretation, freezing would have affected plants in 59% of unglaciated land during peak glacial periods and 73% during interglacials, when more high-latitude land was unglaciated. Comparing forest cover, minimum temperatures, and paleo-locations of Pennsylvanian-aged plant fossils from the Paleobiology Database supports restriction of forest extent due to freezing. Many genera were limited to unglaciated land where temperatures remained above -4 °C. Freeze-intolerance of Pennsylvanian arboreal vegetation had the potential to alter surface runoff, silicate weathering, CO2 levels, and climate forcing. As a bounding case, we assume total plant mortality at -4 °C and estimate that contracting forest cover increased net global surface runoff by up to 6.1%. Repeated freezing likely influenced freeze- and drought-tolerance evolution in lineages like the coniferophytes, which became increasingly dominant in the Permian and early Mesozoic.

Oceanic Anoxic Event 2 (OAE2; c. 94.5–93.9 Ma) offers insight into the mechanisms of past climate change linked to organic productivity and carbon sequestration. It has been studied extensively, but the vast majority of data come from marine records, thus providing an incomplete view of past climate dynamics. Here we integrate new high-resolution data and published records on depositional environments, the carbon-isotope composition of bulk organic carbon (δ13Corg) and plant cuticles (δ13Ccut), and stomatal-index values, a proxy for pCO2, in well-preserved terrestrial through marginal-marine archives of the initial phase of OAE2. The study area is located on the western margin of the Western Interior Seaway (southwestern Utah). Age constraints are based on a new U-Pb bentonite age and correlation to an orbitally calibrated interval of the Bridge Creek Limestone. n-Alkane abundance suggests predominance of terrestrial contributions to bulk organic carbon for most samples. Despite similarities between carbon-isotope variations and transgressive-regressive shoreline movements, it is argued that δ13Corg and δ13Ccut are not strongly affected by local variables. A series of negative, ~2‰ carbon-isotope excursions is identified and attributed to changes in the size and isotopic value of the atmospheric CO2 reservoir. The temporal spacing of these anomalies (80–120 kyr) is consistent with changes in insolation modulated by orbital eccentricity. A systematic, phase-shifted relationship between the negative carbon-isotope excursions and transgressive increments further suggests a link between carbon-cycle perturbations and meter-scale sea-level change on the 100-kyr time scale. A conceptual model involving insolation-controlled aquifer charge/discharge and biomass burial/degradation in the monsoonal belt is proposed. The framework presented here is available to facilitate further research on the interplay of terrestrial and oceanic carbon reservoirs during OAE2.

A single species of fossil conifer with well-preserved cuticles from multiple layers of the Dalong Formation in two sections in Duanshan Town, Guizhou Province, Southwest China has been used to reconstruct palaeo-atmospheric pCO2. The age of these layers can be dated back to the late Changhsingian (latest Permian), corresponding to the Clarkina changxingensis and Clarkina yini conodont Zones. Two calibration approaches were employed in this study. Using the stomatal ratio method, the pCO2 curves of the two sections showed matching trends, with levels decreasing in the middle of the profiles, and similar ranges at ca. 340–510 ppm and ca. 300–440 ppm. Similar values (ca. 360–520 ppm), yet with wider error ranges, were obtained when the mechanistic model proposed by Franks et al. in 2014 was applied to one of the sections. Our values are lower than the mean pCO2 values from coeval palaeosols, but partly overlap with them when error ranges are taken into consideration, thus more independent studies are needed to evaluate the discrepancies between the two proxy methods. The low atmospheric CO2 concentrations in this study suggest a cool climate rather than extensive glaciation for the brief period of late Changhsingian, which is supported by the oxygen isotope record of conodont apatite and a major restructuring of Late Permian flora and fauna.

International audience; Palaeobotany and palynology are the main direct sources of evidence for studying vegetation diversity dynamics through geological time. However, plant fossil diversity is affected by various factors other than vegetation diversity, which need to be taken into account in such studies. The use of fossil-taxa will potentially inflate perceived plant diversities, requiring taxonomic lists to be normalised. Autochthonous floras provide the most direct evidence of vegetation diversity but these are rare; most plant beds are allochthonous with plant remains that have been subjected to varying levels of fragmentation, transportation and time averaging. Local-scale vegetation diversity is especially difficult to determine from the fossil record, even with rigorous sampling protocols and detailed sedimentological analysis. Landscape-scale and regional-scale vegetation diversities are more reliably determined but usually at the rank of family. Macrofossil and palynological data tend to reveal evidence of different aspects of plant diversity, and the best results are obtained if the two diversity signals are integrated. Despite the inherent difficulties, the plant fossil record provides clear evidence of the dynamic history of vegetation through geological times, including the effects of major processes such as climate changes and mass extinctions

The future in the past A major cause of uncertainties in climate projections is our imprecise knowledge of how much warming should occur as a result of a given increase in the amount of carbon dioxide in the atmosphere. Paleoclimate records have the potential to help us sharpen that understanding because they record such a wide variety of environmental conditions. Tierney et al. review the recent advances in data collection, statistics, and modeling that might help us better understand how rising levels of atmospheric carbon dioxide will affect future climate. Science , this issue p. eaay3701

We report the initial findings of the ‘T’ephra Bag citizen science project, a one-year pilot study that commenced in October 2019. This initiative, which is directed at primary school students and their families, aims to test the chemical weathering potential of tephra (volcanic ash) when buried in a pot of local soil and planted with a common grass species. A parallel aim is to raise awareness among participants of the potential of geological-based solutions for climate change through the reduction of carbon dioxide (CO2) concentration in the atmosphere. A second aim of the project is to investigate participants’ perceptions of climate change and carbon emission reduction technologies and to test the effectiveness of such citizen science initiatives in changing the attitudes and perceptions of school children.

The Triassic-Jurassic Boundary marks one of the largest mass extinction events of the Phanerozoic. Across the boundary, a rise in carbon-dioxide levels and global temperatures are hypothesized to have driven significant environmental changes inducing a major floral turnover, causing vegetation structure, composition and leaf morphology to alter, and inferred wildfire activity to increase.An example of these changes can be observed at the Astartekløft site in East Greenland, where previous work identified a change in flora from broad-leaved conifer dominated to an assemblage dominated by narrow leaved conifers, coeval with a five-fold increase in charcoal abundances.Variations in carbon-dioxide concentrations have been shown to be capable of influencing leaf chemistry. It could therefore be hypothesized that carbon-dioxide-driven climate changes across the Triassic-Jurassic boundary may have been capable of not only inducing changes in leaf morphological fuel properties, but also variations in biochemical properties that are both capable of altering wildfire behaviour.In order to assess this, we selected three plant species that have ancient evolutionary origins and correspond to the dominant leaf morphotypes of litter-forming vegetation observed at the Astartekløft site across the Triassic-Jurassic boundary. We grew these species in current ambient and high carbon-dioxide (Triassic-Jurassic boundary) atmospheric conditions and analysed variations in the chemistry of the leaves, using gas chromatography mass spectrometry, and assessed aspects of their flammability using micro-calorimetry. These data were used to inform a fire behaviour model to produce estimates of variations in fire behaviour, such as surface fire spread, flame length and fireline intensity across the Triassic-Jurassic boundary at Astartekløft.Our results reveal a change in leaf chemistry that is expressed as a suppression of volatile content in the three species grown under elevated carbon-dioxide concentrations, compared to those grown under ambient conditions. By accounting for these variations in a fire behaviour model, we estimate that fire behaviour was more extreme prior to the increase in carbon-dioxide across the boundary, suggesting a switch from a period of infrequent but intense fast-moving surface fires during the Triassic, to a period of frequent but low intensity and slow spreading fires during the earliest Jurassic. Our results indicate that that increases in carbon-dioxide concentrations may have impacted leaf chemistry and thus flammability, and may therefore have played an interesting role in determining fire behaviour characteristics during this marked period of Earth’s past.

Main conclusion Our study demonstrated that the species respond non-linearly to increases in CO2 concentration when exposed to decadal changes in CO2, representing the year 1987, 2025, 2051, and 2070, respectively. Abstract There are several lines of evidence suggesting that the vast majority of C3 plants respond to elevated atmospheric CO2 by decreasing their stomatal conductance (gs). However, in the majority of CO2 enrichment studies, the response to elevated CO2 are tested between plants grown under ambient (380–420 ppm) and high (538–680 ppm) CO2 concentrations and measured usually at single time points in a diurnal cycle. We investigated gs responses to simulated decadal increments in CO2 predicted over the next 4 decades and tested how measurements of gs may differ when two alternative sampling methods are employed (infrared gas analyzer [IRGA] vs. leaf porometer). We exposed Populus tremula, Popolus tremuloides and Sambucus racemosa to four different CO2 concentrations over 126 days in experimental growth chambers at 350, 420, 490 and 560 ppm CO2; representing the years 1987, 2025, 2051, and 2070, respectively (RCP4.5 scenario). Our study demonstrated that the species respond non-linearly to increases in CO2 concentration when exposed to decadal changes in CO2. Under natural conditions, maximum operational gs is often reached in the late morning to early afternoon, with a mid-day depression around noon. However, we showed that the daily maximum gs can, in some species, shift later into the day when plants are exposed to only small increases (70 ppm) in CO2. A non-linear decreases in gs and a shifting diurnal stomatal behavior under elevated CO2, could affect the long-term daily water and carbon budget of many plants in the future, and therefore alter soil–plant–atmospheric processes.

Determining the diversity of past floras helps with interpreting both the history and predicting the future of vegetation change. For global-scale and regional-scale diversity studies especially, secondary data are often used but local-scale studies tend to be based on survey data that require rigorous sampling. The correct sampling strategies depend on the types of fossils being investigated, including their physical size, and whether the aim is to determine taxonomic richness or relative abundance. Describing and comparing diversities can use a range of different metrics, depending on whether binary presence/absence or abundance data are available. Each metric provides a different insight into the diversities and the choice of which to use depends on the research question being investigated. Various numerical approaches are available for identifying spatial and stratigraphical diversity patterns, mainly classificatory techniques (e.g., cluster and parsimony analyses) and ordination (e.g., Detrended Correspondence Analysis, Nonmetric Dimensional Scaling). The choice of technique again depends on the research question, but often it has proved useful to run both types of analysis in tandem. This article is illustrated by past biodiversity case studies from throughout the fossil record, dealing with floras ranging in age from the Devonian to the last few centuries.

The stable carbon isotopes of fossil plants are a reflection of the atmosphere and environment in which they grew. Fossil plant remains have thus stored information about the isotopic composition and concentration of atmospheric carbon dioxide ( p CO 2 ) and possibly p O 2 through time. Studies to date, utilizing extant plants, have linked changes in plant stable carbon isotopes (δ 13 C p ) or carbon isotope discrimination (Δ 13 C) to changes in p CO 2 and/or p O 2 . These studies have relied heavily on angiosperm representatives, a phylogenetic group only present in the fossil record post-Early Cretaceous (∼140 million years ago (mya)), whereas gymnosperms, monilophytes and lycophytes dominated terrestrial ecosystems prior to this time. The aim of this study was to expand our understanding of carbon isotope discrimination in all vascular plant groups of C3 plants including lycophytes, monilophytes, gymnosperms and angiosperms, under elevated CO 2 and sub-ambient O 2 to explore their utility as paleo-atmospheric proxies. To achieve this goal, plants were grown in controlled environment chambers under a range of O 2 :CO 2 ratio treatments. Results reveal a strong phylogenetic dependency on Δ 13 C, where spore-bearing (lycophytes and monilophytes) have significantly higher 13 C discrimination than seed plants (gymnosperms and angiosperms) by ∼5‰. We attribute this strong phylogenetic signal to differences in C i / C a likely mediated by fundamental differences in how spore and seed bearing plants control stomatal aperture. Decreasing O 2 :CO 2 ratio in general resulted in increased carbon isotope discrimination in all plant groups. Notably, while all plant groups respond unidirectionally to elevated atmospheric CO 2 (1900 ppm and ambient O 2 ), they do not respond equally to sub-ambient O 2 (16%). We conclude that (1) Δ 13 C has a strong phylogenetic or ‘reproductive grade’ bias, whereby Δ 13 C of spore reproducing plants is significantly different to seed reproducing taxa. (2) Δ 13 C increases with decreasing O 2 :CO 2 ratios (where significant) but is more likely a result of mechanistically uncoupled responses to elevated p CO 2 and sub-ambient O 2 ; and (3) due to this response we find δ 13 C a cannot be calculated from δ 13 C p unless environmental influences such as O 2 :CO 2 ratio can be independently constrained. Therefore, interpretations of trends in fossil plant δ 13 C p to reconstruct paleoatmospheric CO 2 concentration should include cross calibration based on a nearest living relative, appropriate nearest living equivalent, or utilizing the phylogenetic corrections produced from this study.

The presence of stomata is a diagnostic trait of all living and extinct land plants with the exception of liverworts. They are preserved widely in the fossil record from anatomically pristine stomatal complexes on permineralized and charcoalified stems of the earliest land plants dating back >400

The late Palaeozoic was marked by a series of glacial–interglacial cycles. Geochemical and fossil data suggest a role for terrestrial vegetation–carbon cycle feedbacks in the climate response to orbital forcing. Earth’s last icehouse, 300 million years ago, is considered the longest-lived and most acute of the past half-billion years, characterized by expansive continental ice sheets1,2 and possibly tropical low-elevation glaciation3. This atypical climate has long been attributed to anomalous radiative forcing promoted by a 3% lower incident solar luminosity4 and sustained low atmospheric pCO2 (≤300 ppm)5. Climate models6, however, indicate a CO2 sensitivity of ice-sheet distribution and sea-level response that questions this long-standing climate paradigm by revealing major discrepancy between hypothesized ice distribution, pCO2, and geologic records of glacioeustasy2,6. Here we present a high-resolution record of atmospheric pCO2 for 16 million years of the late Palaeozoic, developed using soil carbonate-based and fossil leaf-based proxies, that resolves the climate conundrum. Palaeo-fluctuations on the 105-yr scale occur within the CO2 range predicted for anthropogenic change and co-vary with substantial change in sea level and ice volume. We further document coincidence between pCO2 changes and repeated restructuring of Euramerican tropical forests that, in conjunction with modelled vegetation shifts, indicate a more dynamic carbon sequestration history than previously considered7,8 and a major role for terrestrial vegetation–CO2 feedbacks in driving eccentricity-scale climate cycles of the late Palaeozoic icehouse.

The fossil record and models of atmospheric concentrations of O(2) and CO(2) suggest that past shifts in plant ecological dominance often coincided with dramatic changes in Earth’s atmospheric composition. This study tested the effects of past changes in atmospheric composition on the photosynthetic physiology of a limited range of early-diverging angiosperms (eight), gymnosperms (three), and ferns (two). We performed physiological measurements on all species and used the results to parameterize simulations of their photosynthetic paleophysiology using three independent modeling approaches. Unique physiological attributes were identified for the three evolutionary groups: angiosperm taxa displayed significantly higher mesophyll conductance (g(m)), yet their stomatal conductance (g(s)) was lower than that of ferns. Gymnosperm taxa displayed low g(s) and g(m), but they partially offset their significant diffusional limitations on photosynthesis through their higher maximum Rubisco carboxylation rate. Despite their high total conductance to CO(2), fern taxa lacked an optimized control of g(s), which was reflected in their low intrinsic water use efficiency. Simulations of the photosynthetic physiology of ferns, angiosperms, and gymnosperms through Earth’s history demonstrated that past fluctuations in O(2) and CO(2) concentrations may have resulted in significant shifts in the relative competitiveness of the three evolutionary groups. Although preliminary because of limited species sampling, these findings hint at a potential mechanistic basis for the observed broad temporal correlation between atmospheric change and shifts in plant evolutionary group-level richness observed in the fossil record and are presented as a framework to be tested with paleophotosynthetic proxies and through increased species sampling.

A rapid rise in CO2 has made evergreen trees more waterwise than deciduous trees, especially in cooler parts of the world., Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO2 concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO2 (~45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.

Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO2 concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO2 (~45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.

Stomatal conductance (gs) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of gs in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize gs on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level gs using porometry in 218 C3 woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 gs measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum gs (gsmax) across bioclimatic zones and whether there was similarity in the mean gsmax of C3 woody angiosperms across ecosystem types. We also tested for differential gsmax in two broadly defined habitat groups – open-canopy and understory-subcanopy – within and across bioclimatic zones. We found strong convergence in mean gsmax of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean gsmax in open-canopy habitats (266 ± 100 mmol m-2 s-1) was significantly higher than in understory-subcanopy habitats (233 ± 86 mmol m-2 s-1). There was also a central tendency in the overall dataset to operate toward a gsmax of ∼250 mmol m-2 s-1. We suggest that the observed convergence in mean gsmax of C3 woody angiosperms in the understory-subcanopy is due to a buffering of gsmax against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of gsmax to categorize vegetation according to habitat group.

Climate models predict increased frequency and severity of drought events. At an Irish and Swiss site, experimental summer droughts were applied over two successive years to grassland plots sown with one, two or four grassland species with contrasting functional traits. Mean yield and plot-to-plot variance of yield were measured across harvests during drought and after a subsequent post-drought recovery period. At both sites, there was a positive relationship between species richness and yield. Under rainfed control conditions, mean yields of four-species communities were 32% (Wexford, Ireland) and 51% (Zürich, Switzerland) higher than in monocultures. This positive relationship was also evident under drought, despite significant average yield reductions (−27% at Wexford; −21% at Zürich). Four-species communities had lower plot-to-plot variance of yield compared to monoculture or two-species communities under both rainfed and drought conditions, which demonstrates higher yield stability in four-species communities. At the Swiss but not the Irish site, a high degree of species asynchrony could be identified as a mechanism underlying increased temporal stability in four-species communities. These results indicate the high potential of multi-species grasslands as an adaptation strategy against drought events and help achieve sustainable intensification under both unperturbed and perturbed environmental conditions.

Vegetation affects feedbacks in Earth's hydrologic system, but is constrained by physiological adaptations. In extant ecosystems, the mechanisms controlling plant water used can be measured experimentally; for extinct plants in the recent geological past, water use can be inferred from nearest living relatives, assuming minimal evolutionary change. In deep time, where no close living relatives exist, fossil material provides the only information for inferring plant water use. However, mechanistic models for extinct plant water use must be built on first principles and tested on extant plants. Plants serve as a conduit for water movement from the soil to the atmosphere, constrained by tissue-level construction and gross architecture. No single feature, such as stomata or veins, encompasses enough of the complexity underpinning water-use physiology to serve as the basis of a model of functional water use in all (or perhaps any) extinct plants. Rather, a “functional whole plant” model must be used. To understand the interplay between plant and atmosphere, water use in relation to environmental conditions is investigated in an extinct plant, the seed fernMedullosa((Division Pteridospermatophyta), by reviewing methods for reconstructing physiological variables such as leaf and stem hydraulic capacity, photosynthetic rate, transpiration rate, stomatal conductance, and albedo. Medullosans had the potential for extremely high photosynthetic and assimilation rates, water transport, stomatal conductance, and transpiration—rates comparable to later angiosperms. When these high growth and gas exchange rates of medullosans are combined with the unique atmospheric gas composition of the late Paleozoic atmosphere, complex vegetation-environmental feedbacks are expected despite their basal phylogenetic position relative to post-Paleozoic seed plants.

The inverse relationship between concentrations of CO2 in the atmosphere (pCO2) and the stomatal index of vascular plant has been widely used to estimate ancient levels of atmospheric CO2. However, some atmospheric concentration of CO2 in the geological past (paleo-CO2) estimates show little congruence because they are derived using different correlative methods, or from different fossil plant species with different calibration approaches. Here we apply three methods, including (1) the empirical method of McElwain (1998) , (2) the empirical method of Barclay and Wing (2016) and (3) the mechanistic method of Franks et al., (2014) to a single fossil Ginkgo species (Ginkgoites marginatus) to track and assess their consistency of pCO2 estimates for the Early Jurassic. By using an inter-comparison of three methods, a high degree of consistency in pCO2 estimates and trends has been observed in two empirical proxy methods. In addition, the mechanistic method and both the empirical methods also show generally good consistent paleo-CO2 estimates at the bed-level. To test the congruence of paleo-CO2 estimates, we also apply all three methods to one additional Ginkgoalean fossil species (Sphenobaiera huangii). All three methods show species-dependent uncertainty in paleo-CO2 estimates when applied to different Ginkgalean fossil species collected from the same fossiliferous bed. Moreover, considering the potential effect of guard cell size to the mechanistic method, the genome size of fossil and living Ginkgo taxa was analyzed based on the significant positive relationship between genome size and guard cell size. The result demonstrates that a likely occurrence of polyploidy in Sphenobaiera huangii may result in underestimated paleo-CO2 when applying mechanistic method due to an increase in the size of the stomatal complex.

Human carbon use during the next century will lead to atmospheric carbon dioxide concentrations (pCO2) that have been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. The paleobotanical record of plants offers key insights into vegetation responses to past global change, including suitable analogs for Earth's climatic future. Past global warming events have resulted in transient poleward migration at rates that are equivalent to the lowest climate velocities required for current taxa to keep pace with climate change. Paleobiome reconstructions suggest that the current tundra biome is the biome most threatened by global warming. The common occurrence of paleoforests at high polar latitudes when pCO2 was above 500 ppm suggests that the advance of woody shrub and tree taxa into tundra environments may be inevitable. Fossil pollen studies demonstrate the resilience of wet tropical forests to global change up to 700 ppm CO2, contrary to modeled predictions of the future. The paleobotanical record also demonstrates a high capacity for functional trait evolution as an additional strategy to migration and maintenance of a species’ climate envelope in response to global change.

The Toarcian oceanic anoxic event disrupted terrestrial ecosystems as well as the marine realm, according to analyses of microfossils derived from land plants. Changes in diversity and composition were initially more rapid in terrestrial ecosystems.

During a study of macroflora from the Astartekloft locality in Jameson Land, East Greenland, endophytic insect ovipositions (egg traces) belonging to ichnogenus Paleoovoidus were recorded for the first time in ginkgoalean (Ginkgoites, Sphenobaiera, and Baiera) fossil leaves across the Triassic-Jurassic (Tr-J) transition (ca. 200 Ma). The ovipositions may have been produced by insects in the order Odonata (dragonflies and damselflies) and are relatively more abundant before than after the Tr-J transition, possibly reflecting changes in plant-insect association. Fossil clitellate annelid (leech) cocoons were also discovered in a macerated sample from a single bed within the Tr-J transition. The cocoons belong to two species: Dictyothylakos pesslerae and Pilothylakos pilosus, extending the range of the latter genus from the Early Cretaceous to the Early Jurassic. This new evidence suggests that the ecosystem and food webs were profoundly affected by the environmental degradation surrounding the end-Triassic event (ETE), which was marked by faunal mass extinctions and floral turnover. Invertebrate ichno- and body fossils may add significantly to paleoenvironmental information provided by plant fossil assemblages, and therefore a protocol for recording evidence of invertebrate activity in paleobotanical research is suggested, including analyzing a standardized number of specimens for fossil traces and bulk maceration for discovery of invertebrate body fossils. More well-designed studies on Mesozoic plant-invertebrate associations are needed and will provide deeper knowledge about the structure and evolution of complex ecosystems.

Summary Understanding the drivers of geological‐scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function.Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (g max) and leaf vein density (D v) and physiological measurements including operational stomatal conductance (g op), saturated (A sat ) and maximum (A max) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous.Our study demonstrated significant relationships between g op, g max and D v that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high g max in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high g max (> 1400 mmol m−2 s−1) would have been capable of maintaining a high A max as the atmospheric CO 2 declined through the Cretaceous, whereas gymnosperms with a low g max would experience severe photosynthetic penalty.Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high g max , D v and increased plasticity in g op , adds further functional insights into the mechanisms driving angiosperm speciation.

Summary A long‐standing research focus in phytology has been to understand how plants allocate leaf epidermal space to stomata in order to achieve an economic balance between the plant's carbon needs and water use. Here, we present a quantitative theoretical framework to predict allometric relationships between morphological stomatal traits in relation to leaf gas exchange and the required allocation of epidermal area to stomata.Our theoretical framework was derived from first principles of diffusion and geometry based on the hypothesis that selection for higher anatomical maximum stomatal conductance (g smax) involves a trade‐off to minimize the fraction of the epidermis that is allocated to stomata. Predicted allometric relationships between stomatal traits were tested with a comprehensive compilation of published and unpublished data on 1057 species from all major clades.In support of our theoretical framework, stomatal traits of this phylogenetically diverse sample reflect spatially optimal allometry that minimizes investment in the allocation of epidermal area when plants evolve towards higher g smax.Our results specifically highlight that the stomatal morphology of angiosperms evolved along spatially optimal allometric relationships. We propose that the resulting wide range of viable stomatal trait combinations equips angiosperms with developmental and evolutionary flexibility in leaf gas exchange unrivalled by gymnosperms and pteridophytes., See also the Commentary on this article by Lawson & McElwain, 210: 1149–1151.

Background and Aims: Studies have indicated that plant stomatal conductance (gs) decreases in response to elevated atmospheric CO2, a phenomenon of significance for the global hydrological cycle. However, gs increases across certain CO2 ranges have been predicted by optimisation models. The aim of this work was to demonstrate that under certain environmental condition, gs can increase in response to elevated CO2. Methods: When using (i) an extensive, up-to-date, synthesis of gs responses in FACE experiments, (ii) in situ measurements across four biomes showing dynamic gs responses to a CO2 rise of ~50ppm (characterising the change in this greenhouse gas over the past three decades) and (iii) a photosynthesis-stomatal conductance model, it is demonstrated that gs can in some cases increase in response to increasing atmospheric CO2. Key Results: Field observations are corroborated by an extensive synthesis of gs responses in FACE experiments showing that 11.8% of gs responses under experimentally elevated CO2 are positive. They are further supported by a strong data-model fit (r2=0.607) using a stomatal optimization model applied to the field gs dataset. A parameter space identified in the Farquhar-Ball-Berry photosynthesis-stomatal conductance model confirms field observations of increasing gs under elevated CO2 in hot dry conditions. It was shown that contrary to the general assumption, positive gs responses to elevated CO2, although relatively rare, are a feature of woody taxa adapted to warm, low-humidity conditions, and that this response is also demonstrated in global simulations using the Community Land Model (CLM4). Conclusions: The results contradict the over-simplistic notion that global vegetation always responds with decreasing gs to elevated CO2, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level. Irish Research Council Science Foundation Ireland

In the Supplementary Information file originally published, the caption for Supplementary Figure 3d-f was missing. This has now been corrected.

The evolutionary relationships between extinct species are almost exclusively based on the shape and structure of their fossil specimens. Now, a spectroscopic technique that records a ‘chemical fingerprint’ of fossil plant cuticles is being used to re-interpret the histories of thousands of specimens languishing in museum collections.

Climate change is likely to have altered the ecological functioning of past ecosystems, and is likely to alter functioning in the future; however, the magnitude and direction of such changes are difficult to predict. Here we use a deep-time case study to evaluate the impact of a well-constrained CO2-induced global warming event on the ecological functioning of dominant plant communities. We use leaf mass per area (LMA), a widely used trait in modern plant ecology, to infer the palaeoecological strategy of fossil plant taxa. We show that palaeo-LMA can be inferred from fossil leaf cuticles based on a tight relationship between LMA and cuticle thickness observed among extant gymnosperms. Application of this new palaeo-LMA proxy to fossil gymnosperms from East Greenland reveals significant shifts in the dominant ecological strategies of vegetation found across the Triassic–Jurassic transition. Late Triassic forests, dominated by low-LMA taxa with inferred high transpiration rates and short leaf lifespans, were replaced in the Early Jurassic by forests dominated by high-LMA taxa that were likely to have slower metabolic rates. We suggest that extreme CO2-induced global warming selected for taxa with high LMA associated with a stress-tolerant strategy and that adaptive plasticity in leaf functional traits such as LMA contributed to post-warming ecological success.

Contents 'Summary' 1333 I. 'Introduction' 1334 II. 'Plants of the Pennsylvanian Tropical Realm' 1335 III. 'Conceptual insights into paleoecophysiology' 1339 IV. 'High-productivity Carboniferous plants' 1344 V. 'Lessons learned' 1347 VI. 'The big picture: an active role for early forests In Late Paleozoic climate' 1347 'Acknowledgements' 1348 'Author contributions' 1348 References 1348 Summary The Carboniferous, the time of Earth's penultimate icehouse and widespread coal formation, was dominated by extinct lineages of early-diverging vascular plants. Studies of nearest living relatives of key Carboniferous plants suggest that their physiologies and growth forms differed substantially from most types of modern vegetation, particularly forests. It remains a matter of debate precisely how differently and to what degree these long-extinct plants influenced the environment. Integrating biophysical analysis of stomatal and vascular conductivity with geochemical analysis of fossilized tissues and process-based ecosystem-scale modeling yields a dynamic and unique perspective on these paleoforests. This integrated approach indicates that key Carboniferous plants were capable of growth and transpiration rates that approach values found in extant crown-group angiosperms, differing greatly from comparatively modest rates found in their closest living relatives. Ecosystem modeling suggests that divergent stomatal conductance, leaf sizes and stem life span between dominant clades would have shifted the balance of soil–atmosphere water fluxes, and thus surface runoff flux, during repeated, climate-driven, vegetation turnovers. This synthesis highlights the importance of ‘whole plant’ physiological reconstruction of extinct plants and the potential of vascular plants to have influenced the Earth system hundreds of millions of years ago through vegetation–climate feedbacks.

Ricciisporites tuberculatus Lundblad is a prominently sculptured palynomorph that is dispersed at maturity as tetrads. It has a wide geographic distribution and reaches a stratigraphically important acme in the Norian and Rhaetian of Europe. As it has not yet been found in situ in a fossilized reproductive structure, its botanical affinity is poorly understood. Recent work on the morphology and ultrastructure of R. tuberculatus favours an affinity with gymnosperms rather than spore plants. In the present contribution, we re-evaluate Triassic–Jurassic (Tr-J) palynological records and discuss the consequences of a gymnosperm affinity for R. tuberculatus for Late Triassic vegetation reconstruction, as well as inferred palaeoecological interpretations. At a Tr-J boundary locality in East Greenland, the relative abundance of R. tuberculatus through time is similar to the Bennettitales Anomozamites and Pterophyllum. We suggest that R. tuberculatus may have been produced by a gymnosperm characterised by an herbaceous ruderal life habit. The poor preservation potential of the vegetative structures of such a plant may explain why R. tuberculatus has not yet been found in situ in a fossil reproductive structure. Moreover, principal component analysis of Late Triassic palynological records show that the vectors of R. tuberculatus and the common pollen type Classopollis plot in opposite directions. We suggest that the parent plants of R. tuberculatus and Classopollis (the Cheirolepidiaceae) may have had different ecological and/or climatological preferences.

The Triassic-Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and global warming as the Central Atlantic Magmatic Province developed. This resulted in both marine and terrestrial extinctions, with terrestrial plants thought to experience thermal stress as global temperatures and atmospheric CO2 levels rose. As plant compression fossils typically only preserve external morphological features, it has not been possible to reconstruct plant paleophysiology in order to elucidate the mechanisms underlying plant stress and extinction. Here we present a new approach allowing us to infer the photosynthetic performance and stress physiology of fossil plants, applied to fossil Ginkgoales across the TJB. We use correlations between the adaxial epidermal cell density of extant Ginkgo biloba and photosynthetic and protective stress physiology to infer the paleophysiological condition of Late Triassic-Early Jurassic-aged plants from Astartekloft, East Greenland. The density of fossil leaf adaxial epidermal cells indicates that photosynthetic performance of Ginkgoales became increasingly impaired towards the latter stages of the Triassic, before improving into the Early Jurassic. This is consistent with d13C isotope values, paleo-[CO2] levels, and global mean temperatures, suggesting that photosynthetic performance was influenced by the prevailing environmental conditions during the TJB event. Dissipation of absorbed energy as heat would also have risen towards the boundary as plant stress increased, in order to protect the photosynthetic physiology. The increase in dissipation of energy as heat, associated with a reduction in convective heat loss due to reduced transpiration rates, would have exacerbated plant thermal stress at the TJB, thus contributing to sudden biodiversity loss and ecological change.

Atmospheric composition, particularly levels of CO2 and O2, impacts all aspects of life but its role in relation to plant preservation in the fossil record is largely unconsidered. Plants, angiosperms in particular, have been widely shown to increase leaf mass per area (LMA) under high CO2 conditions and decrease LMA in low CO2 conditions. Leaf thickness has long been known to be a contributory factor in preservation potential in the plant fossil record, with thicker leaves considered to have a greater recalcitrance than thinner ones. Therefore, any change in leaf density/thickness, through changes to LMA, could lead to an increased or decreased preservation potential of fossil leaves at times of elevated or decreased CO2, respectively. . Additionally, the impact of changes to atmospheric O2 and to the atmospheric CO2:O2 ratio on LMA has not been previously considered in detail. This investigation examines the effect of simulated Mesozoic atmospheres, times of high CO2 and low O2, on LMA in a suite of gymnosperms that act as nearest living equivalents for common elements of Mesozoic floras. Exposure to high CO2 (~ 1,500 ppm) led to a statistically significant (p < 0.001) increase in LMA in four out of 6 species, and exposure to combined high CO2 and low O2 (~ 13%) induced a statistically significant (p < 0.001) increase in LMA in all six species. The investigation also examined the effects of atmospheric composition on %N, a key plant trait known to co-vary with LMA under modern atmospheric compositions that provides information on plant function and relates to photosynthetic efficiency. Most species showed decreased %N in treatments with increased LMA in agreement with modern ecological studies and supporting the co-varying nature of LMA and %N regardless of CO2:O2 ratio. These findings suggest that atmospheric composition has a pronounced impact on LMA. Based on these results, we propose the hypothesis that atmospheric composition is an important taphonomic filter of the fossil leaf record. Further research is now required to test the significance of atmospheric composition versus other well-known taphonomic filters.

Background and aims Fluctuations in [CO 2 ] have been widely studied as a potential driver of plant evolution; however, the role of a fluctuating [O 2 ]:[CO 2 ] ratio is often overlooked. The present study aimed to investigate the inherent physiological plasticity of early diverging, extant species following acclimation to an atmosphere similar to that across the Triassic-Jurassic mass extinction interval (TJB, approx. 200 Mya), a time of major ecological change. Methods Mature plants from two angiosperm ( Drimys winteri and Chloranthus oldhamii ), two monilophyte ( Osmunda claytoniana and Cyathea australis ) and one gymnosperm ( Ginkgo biloba ) species were grown for 2 months in replicated walk-in Conviron BDW40 chambers running at TJB treatment conditions of 16 % [O 2 ]-1900 ppm [CO 2 ] and ambient conditions of 21 % [O 2 ]-400 ppm [CO 2 ], and their physiological plasticity was assessed using gas exchange and chlorophyll fluorescence methods. Key results TJB acclimation caused significant reductions in the maximum rate of carboxylation ( V Cmax ) and the maximum electron flow supporting ribulose-1,5-bisphosphate regeneration ( J max ) in all species, yet this downregulation had little effect on their light-saturated photosynthetic rate ( A sat ). Ginkgo was found to photorespire heavily under ambient conditions, while growth in low [O 2 ]:[CO 2 ] resulted in increased heat dissipation per reaction centre ( DI o / RC ), severe photodamage, as revealed by the species' decreased maximum efficiency of primary photochemistry ( F v / F m ) and decreased in situ photosynthetic electron flow ( Jsitu ). Conclusions It is argued that the observed photodamage reflects the inability of Ginkgo to divert excess photosynthetic electron flow to sinks other than the downregulated C 3 and the diminished C 2 cycles under low [O 2 ]:[CO 2 ]. This finding, coupled with the remarkable physiological plasticity of the ferns, provides insights into the underlying mechanism of Ginkgoales' near extinction and ferns' proliferation as atmospheric [CO 2 ] increased to maximum levels across the TJB.