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The maximum photosynthetic carboxylation rate (V cmax ) is an influential plant trait that has multiple scaling hypotheses, which is a source of uncertainty in predictive understanding of global gross primary production (GPP). Four trait-scaling hypotheses (plant functional type, nutrient limitation, environmental filtering, and plant plasticity) with nine specific implementations were used to predict global V cmax distributions and their impact on global GPP in the Sheffield Dynamic Global Vegetation Model (SDGVM). Global GPP varied from 108.1 to 128.2 PgC yr -1 , 65% of the range of a recent model intercomparison of global GPP. The variation in GPP propagated through to a 27% coefficient of variation in net biome productivity (NBP). All hypotheses produced global GPP that was highly correlated (r = 0.85-0.91) with three proxies of global GPP. Plant functional type-based nutrient limitation, underpinned by a core SDGVM hypothesis that plant nitrogen (N) status is inversely related to increasing costs of N acquisition with increasing soil carbon, adequately reproduced global GPP distributions. Further improvement could be achieved with accurate representation of water sensitivity and agriculture in SDGVM. Mismatch between environmental filtering (the most data-driven hypothesis) and GPP suggested that greater effort is needed understand V cmax variation in the field, particularly in northern latitudes., (© 2017 UT-Battelle LLC. New Phytologist © 2017 New Phytologist Trust.)

Future climate change and increasing atmospheric CO2 are expected to cause major changes in vegetation structure and function over large fractions of the global land surface. Seven global vegetation models are used to analyze possible responses to future climate simulated by a range of general circulation models run under all four representative concentration pathway scenarios of changing concentrations of greenhouse gases. All 110 simulations predict an increase in global vegetation carbon to 2100, but with substantial variation between vegetation models. For example, at 4 °C of global land surface warming (510-758 ppm of CO2), vegetation carbon increases by 52-477 Pg C (224 Pg C mean), mainly due to CO2 fertilization of photosynthesis. Simulations agree on large regional increases across much of the boreal forest, western Amazonia, central Africa, western China, and southeast Asia, with reductions across southwestern North America, central South America, southern Mediterranean areas, southwestern Africa, and southwestern Australia. Four vegetation models display discontinuities across 4 °C of warming, indicating global thresholds in the balance of positive and negative influences on productivity and biomass. In contrast to previous global vegetation model studies, we emphasize the importance of uncertainties in projected changes in carbon residence times. We find, when all seven models are considered for one representative concentration pathway × general circulation model combination, such uncertainties explain 30% more variation in modeled vegetation carbon change than responses of net primary productivity alone, increasing to 151% for non-HYBRID4 models. A change in research priorities away from production and toward structural dynamics and demographic processes is recommended.

Terrestrial gross primary production (GPP) is the largest global CO(2) flux driving several ecosystem functions. We provide an observation-based estimate of this flux at 123 +/- 8 petagrams of carbon per year (Pg C year(-1)) using eddy covariance flux data and various diagnostic models. Tropical forests and savannahs account for 60%. GPP over 40% of the vegetated land is associated with precipitation. State-of-the-art process-oriented biosphere models used for climate predictions exhibit a large between-model variation of GPP's latitudinal patterns and show higher spatial correlations between GPP and precipitation, suggesting the existence of missing processes or feedback mechanisms which attenuate the vegetation response to climate. Our estimates of spatially distributed GPP and its covariation with climate can help improve coupled climate-carbon cycle process models.

One of the most pressing and globally recognized challenges is how to mitigate the effects of global environment change brought about by increasing emissions of greenhouse gases, especially CO(2). In this review we evaluate the potential contribution of four biological approaches to mitigating global environment change: reducing atmospheric CO(2) concentrations through soil carbon sequestration and afforestation; reducing predicted increases in global surface temperatures through increasing the albedo of crop plants; and fertilizing the oceans to increase primary productivity and CO(2) drawdown. We conclude that none of these biological approaches are 'magic bullets' capable of reversing environmental changes brought about by increasing emissions of greenhouse gases. However, it is possible that increasing crop albedo and soil carbon sequestration might contribute towards mitigation on a regional scale. In the absence of legally binding international agreements to reduce CO(2) emissions, we propose that: increased efforts are made to identify novel biological mitigatory strategies; further research is conducted to minimise the uncertainties present in all four of the biological approaches described; and pilot-level field work is conducted to examine the feasibility of the most promising strategies. Finally, it is essential to engage with the public concerning strategies for mitigating the effects of climate change because the majority of the biological approaches have effects, quite possibly of a negative nature, on ecosystem services and land usage.

We model plant species diversity globally by country to show that future plant diversity capacity has a strong dependence on changing climate and carbon dioxide concentration. CO2 increase, through its impact on net primary production and warming is predicted to increase regional diversity capacity, while warming with constant CO2 leads to decreases in diversity capacity. Increased CO2 concentrations are unlikely to counter projected extinctions of endemic species, shown in earlier studies to be more strongly dependent on changing land use patterns than climate per se. Model predictions were tested against (1) contemporary observations of tree species diversity in different biomes, (2) an independent global map of contemporary species diversity and (3) time sequences of plant naturalisation for different locations. Good agreements between model, observations and naturalisation patterns support the suggestion that future diversity capacity increases are likely to be filled from a 'cosmopolitan weed pool' for which migration appears to be an insignificant barrier.

The observation that stomatal density (number mm(-2)) on herbarium leaves had decreased over the last century represents clear evidence that plants have responded to anthropogenic increases in CO2 concentration. The mechanism of the response has proved elusive but here it is shown that density responses to both CO2 concentration and humidity are correlated with changes in whole-plant transpiration and leaf abscisic acid (ABA) concentration. The transpiration rate of a range of accessions of Arabidopsis thaliana was manipulated by changing CO2 concentration, humidity and by exogenous application of ABA. Stomatal density increased with transpiration and leaf ABA concentration. A common property of signal transduction systems is that they rapidly lose their ability to respond to the co-associated stimulus. Pathways of water movement within the plant are connected and so variations in supply and demand can be signalled throughout the plant directly, modifying stomatal aperture of mature leaves and stomatal density of developing leaves. Furthermore, the system identified here does not conform to the loss of ability to respond. A putative mechanism is proposed for the control of stomatal density by transpiration rate and leaf ABA concentration.

Light intensity and atmospheric CO2 partial pressure are two environmental signals known to regulate stomatal numbers. It has previously been shown that if a mature Arabidopsis leaf is supplied with either elevated CO2 (750 ppm instead of ambient at 370 ppm) or reduced light levels (50 micromol m-2 s-1 instead of 250 micromol m-2 s-1), the young, developing leaves that are not receiving the treatment grow with a stomatal density as if they were exposed to the treatment. But the signal(s) that it is believed is generated in the mature leaves and transmitted to developing leaves are largely unknown. Photosynthetic rates of treated, mature Arabidopsis leaves increased in elevated CO2 and decreased when shaded, as would be expected. Similarly, the levels of sugars (glucose, fructose, and sucrose) in the treated mature leaves increased in elevated CO2 and decreased with shade treatment. The levels of sugar in developing leaves were also measured and it was found that they mirrored this result even though they were not receiving the shade or elevated CO2 treatment. To investigate the effect of these treatments on global gene expression patterns, transcriptomics analysis was carried out using Affymetrix, 22K, and ATH1 arrays. Total RNA was extracted from the developing leaves after the mature leaves had received either the ambient control treatment, the elevated CO2 treatment, or the shade treatment, or both elevated CO2 and shade treatments for 2, 4, 12, 24, 48, or 96 h. The experiment was replicated four times. Two other experiments were also conducted, one to compare and contrast gene expression in response to plants grown at elevated CO2 and the other to look at the effect of these treatments on the mature leaf. The data were analysed and 915 genes from the untreated, signalled leaves were identified as having expression levels affected by the shade treatment. These genes were then compared with those whose transcript abundance was affected by the shade treatment in the mature treated leaves (1181 genes) and with 220 putative 'stomatal signalling' genes previously identified from studies of the yoda mutant. The results of these experiments and how they relate to environmental signalling are discussed, as well as possible mechanisms for systemic signalling.

Stomatal numbers are tightly controlled by environmental signals including light intensity and atmospheric CO(2) partial pressure. This requires control of epidermal cell development during the early phase of leaf growth and involves changes in both the density of cells on the leaf surface and the proportion of cells that adopt a stomatal fate. This paper reviews the current understanding of how stomata develop and describes recent advances that have given insights into the regulatory mechanisms involved using mutant Arabidopsis plants that implicates a role for long-chain fatty acids in cell-to-cell communication. Evidence is presented which indicates that long-distance signalling from mature to newly developing leaves forms part of the mechanism by which stomatal development responds to environmental cues. Analysis of mutant plants suggests that the plant hormones abscisic acid, ethylene and jasmonates are implicated in the long-distance signalling pathway and that the action may be mediated by reactive oxygen species.

Stomatal pores on the leaf surface control both the uptake of CO2 for photosynthesis and the loss of water during transpiration. Since the industrial revolution, decreases in stomatal numbers in parallel with increases in atmospheric CO2 concentration have provided evidence of plant responses to changes in CO2 levels caused by human activity. This inverse correlation between stomatal density and CO2 concentration also holds for fossil material from the past 400 million years and has provided clues to the causes of global extinction events. Here we report the identification of the Arabidopsis gene HIC (for high carbon dioxide), which encodes a negative regulator of stomatal development that responds to CO2 concentration. This gene encodes a putative 3-keto acyl coenzyme A synthase--an enzyme involved in the synthesis of very-long-chain fatty acids. Mutant hic plants exhibit up to a 42% increase in stomatal density in response to a doubling of CO2. Our results identify a gene involved in the signal transduction pathway responsible for controlling stomatal numbers at elevated CO2.

The maximum photosynthetic carboxylation rate (Vcmax ) is an influential plant trait that has multiple scaling hypotheses, which is a source of uncertainty in predictive understanding of global gross primary production (GPP). Four trait-scaling hypotheses (plant functional type, nutrient limitation, environmental filtering, and plant plasticity) with nine specific implementations were used to predict global Vcmax distributions and their impact on global GPP in the Sheffield Dynamic Global Vegetation Model (SDGVM). Global GPP varied from 108.1 to 128.2 PgC yr-1 , 65% of the range of a recent model intercomparison of global GPP. The variation in GPP propagated through to a 27% coefficient of variation in net biome productivity (NBP). All hypotheses produced global GPP that was highly correlated (r = 0.85-0.91) with three proxies of global GPP. Plant functional type-based nutrient limitation, underpinned by a core SDGVM hypothesis that plant nitrogen (N) status is inversely related to increasing costs of N acquisition with increasing soil carbon, adequately reproduced global GPP distributions. Further improvement could be achieved with accurate representation of water sensitivity and agriculture in SDGVM. Mismatch between environmental filtering (the most data-driven hypothesis) and GPP suggested that greater effort is needed understand Vcmax variation in the field, particularly in northern latitudes.

The physiological effects of high CO2 concentrations, i.e., [CO2], on plant stomatal responses may be of major importance in understanding the consequences of climate change, by causing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic--Jurassic boundary transition (Tr--J) succession of East Greenland. This interval marks a major high CO2-driven environmental upheaval, with faunal mass extinctions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm²) decreased significantly during the Tr--J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%--60% drop in stomatal and canopy transpiration at the Tr--J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of stomatal responses to elevated [CO2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editor discusses an article on the impact of natural drought in Amazon, the quality of various global circulation models (GCMs) climate situations which are employed to drive dynamic global vegetation models (DGVMs), and the mechanisms involve in DGVM processes.

• The observation that stomatal density (number mm−2) on herbarium leaves had decreased over the last century represents clear evidence that plants have responded to anthropogenic increases in CO2 concentration. The mechanism of the response has proved elusive but here it is shown that density responses to both CO2 concentration and humidity are correlated with changes in whole-plant transpiration and leaf abscisic acid (ABA) concentration. • The transpiration rate of a range of accessions of Arabidopsis thaliana was manipulated by changing CO2 concentration, humidity and by exogenous application of ABA. Stomatal density increased with transpiration and leaf ABA concentration. • A common property of signal transduction systems is that they rapidly lose their ability to respond to the co-associated stimulus. Pathways of water movement within the plant are connected and so variations in supply and demand can be signalled throughout the plant directly, modifying stomatal aperture of mature leaves and stomatal density of developing leaves. Furthermore, the system identified here does not conform to the loss of ability to respond. • A putative mechanism is proposed for the control of stomatal density by transpiration rate and leaf ABA concentration. [ABSTRACT FROM AUTHOR]

Summary • Stomatal density responses by 48 accessions of Arabidopsis , to CO2 enrichment, broadly parallel interspecific observations. • Accessions differing in the degree of stomatal response to both CO2 and drought differed in flower production. Under well watered conditions flowering benefits from a small reduction in stomatal density with CO2 enrichment, but benefits from a large reduction under drought. • Stomatal density increases with altitude in Vaccinium myrtillus but is also strongly influenced by exposure. Exposed plants had higher stomatal densities than plants at the same altitude but in a community of individuals. This difference might be explained by systemic signalling within the plant as mature leaves detect both irradiance and [CO2 ], subsequently controlling the response of stomatal development in developing leaves. • Plants with the highest stomatal densities also had the highest stomatal conductances and photosynthetic rates. This suggests that signalling from mature to developing leaves predetermines the potential of the developing leaf to maximize its photosynthetic potential, including associated features such as nitrogen allocation, during early stages of development in the enclosed bud. [ABSTRACT FROM AUTHOR]

A survey of 100 species and 122 observations has shown an average reduction in stomatal density of 143% (SE ± 2.2%) with CO2 enrichment, with 74% of the cases exhibiting a reduction in stomatal density. A sign test demonstrated that stomata! density decreases, in response to CO2, significantly more often than expected by chance. Repeated observations on the same species indicated a significant repeatability in the direction of the stomatal response. Analyses which removed the potential effect of taxonomy on this data set showed no significant patterns in the dependency of the degree of stomatal change on growth form (woodiness vs. non-woodiness; trees vs. shrubs), habitat (cool VS. warm) or stomatal distribution on the leaf (amphi- vs. hypostomatous). Forty-three of the observations had been made in controlled environments and under a typical range in CO2 enrichment of 350-700 μmol mol-1. For these cases the average stomata! density declined by 9% (SE ± 3.3%) and 60% of the cases showed reductions in stomatal density. When analyses were restricted to these 43 observations, amphistomatous samples more frequently had greater changes in stomatal density than did hypostomatous samples. The degree of reduction in stomatal density with increasing CO2 increases with initial stomatal density, after the influence of taxonomy is removed using analyses of independent contrasts. When the data were examined for patterns that might be due explicitly to the effects of relatedness, the subclasses of the Hamamelidae and the Rosidae showed highly significant reductions in stomatal density with CO2 (87% of the specks studied in the Hamamelidae and 80% of the species in the Rosidae showed reduction with CO2 enrichment) and correlations between initial stomatal density and degree of reduction in stomatal density. [ABSTRACT FROM AUTHOR]

Rising levels of atmospheric CO2, climate change, and fertilizer pollution provide the ecological imperative for investigating the interaction between plant responses to atmospheric CO2 concentration, temperature and nutrient supply. In this study spring wheat (Triticum aestivum L, cv. Wemhley) was grown at 40, 50, 60 and 70 Pa atmospheric CO2 pressure and three experiments were conducted to investigate interactions between growth responses to the CO2 treatment and: (i) temperature (24/16 °C vs. 18/10 °C day/night), (ii) nutrient solution nitrate concentration (2.5, 5, 10 and 15 mM Ca(NO3,)2. 4H2O). and (iii) phosphate concentration (0.025 and 0.5 mM KH2PO4. Dry mass and root/shoot ratio increased with CO2, level at the higher temperature. These responses were reversed at the lower temperature. The increase in yield with CO2 enhancement was limited by low rates of nutrient supply in both absolute and relative terms. In the elevated CO2 treatments, the shoot nitrogen concentration was reduced, as was the proportional allocation to the uppermost leaves. These results are discussed with respect to possible physiological mechanisms and physiological for improved crop performance in a future, elevated CO2 World. [ABSTRACT FROM AUTHOR]

1. Populations of an annual plant, Abutilon theophrasti, were grown at four densities (100. 500. 1500 and 4000m[Sup-2]) and two CO[Sub2] concentrations (350 and 700 μ11[Sup-1] to examine the influence of CO[Sub2] environment on density-dependent patterns of demography and reproduction. Variables measured included survivorship, proportion of plants flowering and fruiting, number of fruiting individuals, number of seeds per individual, total seed production per population, mean seed mass, and germination of seeds produced in each environment. 2. All variables, except the number of fruiting individuals, declined with increasing density, and at the highest density no individuals set seed. The number of fruiting individuals was highest at a density of 500m[Sup-2]. In the elevated CO[Sub2] environment, survivorship was significantly reduced but the proportion of plants flowering and fruiting and the number of fruiting individuals in each population all increased. Total population seed production was higher in the elevated CO[Sub2] environment at all densities. although the differences were not significant. Significant effects of CO[Sub2] concentration were observed only for population-level variables, but not for mean individual fecundity or seed size. Seed germination declined with increasing maternal density. and no germination was recorded For seeds produced at 1500 m[Sup-2]. 3. Simple models of population dynamics, utilizing difference equations, were constructed to examine potential population-level consequences of these density and CO[Sub2] effects. In the absence of a persistent Seed pool, the simulated populations exhibited damped or stable oscillations under low germination values, but displayed non-cyclic ('chaotic') oscillations or went extinct for higher germination due to the complete failure of seed-set at high density. [ABSTRACT FROM AUTHOR]

It is sometimes assumed that because increases in atmospheric CO2 concentration usually enhance water use efficiency per unit leaf area, there will be a tendency for plants to show greater drought tolerance as well as increased biomass in the future. A critical examination of the responses to elevated CO2 in three temperate tree species shows that this assumption might be incorrect in the case of two of them. Both beech (Fagus sylvatica L.) and birch (Betula pubescens Ehrh.) display minimal stomatal closing responses to elevated CO2, and in the case of F. sylvatica the stomatal control of transpiration per unit leaf area appears to be unable to compensate for the greater development of leaf area. By contrast, the stomata of oak (Quercus robur L.) close appreciably in elevated CO2, to an extent which might be sufficient to compensate for an increase in total leaf area. A simple model for the controls on water supply and consumption for the whole tree suggests that in F. sylvatica the potential height attainment for a given sapwood area might decrease as the atmospheric CO2 concentration rises. The conclusions drawn from experimental data and from modelling are supported by field observations made in the UK in 1995, when the three species responded very differently to severe drought. We suggest that the progressive increase in the concentration of atmospheric CO2 over the past 200 yr might have accentuated differences in drought sensitivity between these species. [ABSTRACT FROM AUTHOR]