Photosynthesis Model Research Articles

The impact of rising CO2 and acclimation on the response of US forests to global warming

The response of forests to climate change depends in part on whether the photosynthetic benefit from increased atmospheric CO2 (∆Ca = future minus historic CO2) compensates for increased physiological stresses from higher temperature (∆T). We predicted the outcome of these competing responses by using optimization theory and a mechanistic model of tree water transport and photosynthesis. We simulated current and future productivity, stress, and mortality in mature monospecific stands with soil, species, and climate sampled from 20 continental US locations. We modeled stands with and without acclimation to ∆Ca and ∆T, where acclimated forests adjusted leaf area, photosynthetic capacity, and stand density to maximize productivity while avoiding stress. Without acclimation, the ∆Ca-driven boost in net primary productivity (NPP) was compromised by ∆T-driven stress and mortality associated with vascular failure. With acclimation, the ∆Ca-driven boost in NPP and stand biomass (C storage) was accentuated for cooler futures but negated for warmer futures by a ∆T-driven reduction in NPP and biomass. Thus, hotter futures reduced forest biomass through either mortality or acclimation. Forest outcomes depended on whether projected climatic ∆Ca/∆T ratios were above or below physiological thresholds that neutralized the negative impacts of warming. Critically, if forests do not acclimate, the ∆Ca/∆T must be above ca 89 ppm⋅°C-1 to avoid chronic stress, a threshold met by 55% of climate projections. If forests do acclimate, the ∆Ca/∆T must rise above ca 67 ppm⋅°C-1 for NPP and biomass to increase, a lower threshold met by 71% of projections.

Ozone-induced change in the relationship between stomatal conductance and net photosynthetic rate is a factor determining cumulative stomatal ozone uptake by Fagus crenata seedlings

The change in the relationship between the net photosynthetic rate and stomatal conductance by exposure to ozone is a factor determining cumulative ozone uptake via stomata in Fagus crenata seedlings. Stomatal ozone (O3) uptake, one of the key factors determining the impact of O3 on trees, has been estimated using a coupled model of photosynthesis and stomatal conductance. Exposure to O3 increases or decreases stomatal conductance without changes in the net photosynthetic rate and maximum carboxylation velocity (Vcmax). Such changes in the coupling between photosynthesis and stomatal conductance may be important factors determining stomatal O3 uptake. We aimed to investigate impacts of O3-induced changes in stomatal conductance on the cumulative stomatal O3 uptake of Fagus crenata seedlings grown in three levels of gas treatments: charcoal-filtered air or O3 at 1.0- or 1.5-times the ambient concentration (CF, 1.0 × O3, and 1.5 × O3 treatments, respectively). In July, an O3-induced remarkable reduction in stomatal conductance compared to the net photosynthetic rate (a reduction in the intercept of the Ball–Berry–Leuning model) was observed. Moreover, the exposure to O3 significantly reduced Vcmax during the growing season. Model simulations showed that O3-induced reductions in the photosynthetic capacity, i.e., Vcmax, hardly affect stomatal conductance and cumulative O3 uptake during the growing season. Conversely, the O3-induced remarkable reduction in stomatal conductance compared to net photosynthetic rate in July limited the cumulative stomatal O3 uptake during the growing season by 6% and 10% in the 1.0 × O3 and 1.5 × O3 treatments, respectively. Therefore, we should consider O3-induced changes in the coupling between the net photosynthetic rate and stomatal conductance to calculate the cumulative O3 uptake via stomata of F. crenata using the coupled model.

Photosynthetic electron transport transients in Chlorella vulgaris under fluctuating light

Chlorella sp. is both an important model for green algal photosynthesis and is produced using industrial scale photobioreactors. In photobioreactors, cells travel through steep gradients of illumination at rates determined by photobioreactor design, mixing rates, culture density and surface irradiance levels. We used non-invasive, rapid fluorescence measures to show that Chlorella vulgaris tolerates short-term exposures to super-saturating irradiance by transiently accelerating electron transport away from Photosystem II. This capacity lasts for only 10–20s, and longer exposures to supersaturating irradiance induced down-regulation of electron transport through slowing of down-stream electron sinks, induction of non-photochemical quenching and net Photosystem II photoinactivation. Cells previously acclimated to high growth light were able to partially recover from the down regulation within 300–600s, but cells previously acclimated to low growth light suffered more sustained down-regulation after exposure to super-saturating light. These metrics can be used to guide and constrain culture density, mixing rate and irradiance regime decisions in photobioreactors.

Perturbation to aquatic photosynthesis due to high energy muons from cosmic ray bursts due to neutron star mergers

AbstractIn this work, we propose a modification of the E‐model (Cullen, Neale, and Lesser) mathematical model for aquatic photosynthesis to include the action of ionizing radiation. We evaluated, with this modified model, the perturbations that muons from neutron star fusions could cause in the biological process of aquatic photosynthesis. It is shown that neutron star fusions not far from the solar system have the potential to considerably deplete aquatic photosynthesis.

Variation in Responses of Photosynthesis and Apparent Rubisco Kinetics to Temperature in Three Soybean Cultivars.

Recent in vivo assays of the responses of Rubisco to temperature in C3 plants have revealed substantial diversity. Three cultivars of soybean (Glycine max L. Merr.), Holt, Fiskeby V, and Spencer, were grown in indoor chambers at 15, 20, and 25 °C. Leaf photosynthesis was measured over the range of 15 to 30 °C, deliberately avoiding higher temperatures which may cause deactivation of Rubisco, in order to test for differences in temperature responses of photosynthesis, and to investigate in vivo Rubisco kinetic characteristics responsible for any differences observed. The three cultivars differed in the optimum temperature for photosynthesis (from 15 to 30 °C) at 400 μmol mol−1 external CO2 concentration when grown at 15 °C, and in the shapes of the response curves when grown at 25 °C. The apparent activation energy of the maximum carboxylation rate of Rubisco differed substantially between cultivars at all growth temperatures, as well as changing with growth temperature in two of the cultivars. The activation energy ranged from 58 to 84 kJ mol−1, compared with the value of 64 kJ mol−1 used in many photosynthesis models. Much less variation in temperature responses occurred in photosynthesis measured at nearly saturating CO2 levels, suggesting more diversity in Rubisco than in electron transport thermal properties among these soybean cultivars.

Multiple-constraint inversion of SCOPE. Evaluating the potential of GPP and SIF for the retrieval of plant functional traits

The most recent efforts to provide remote sensing (RS) estimates of plant function rely on the combination of Radiative Transfer Models (RTM) and Soil-Vegetation-Atmosphere Transfer (SVAT) models, such as the Soil-Canopy Observation Photosynthesis and Energy fluxes (SCOPE) model. In this work we used ground spectroradiometric and chamber-based CO2 flux measurements in a nutrient manipulated Mediterranean grassland in order to: 1) develop a multiple-constraint inversion approach of SCOPE able to retrieve vegetation biochemical, structural as well as key functional traits, such as chlorophyll concentration (Cab), leaf area index (LAI), maximum carboxylation rate (Vcmax) and the Ball-Berry sensitivity parameter (m); and 2) compare the potential of the of gross primary production (GPP) and sun-induced fluorescence (SIF), together with up-welling Thermal Infrared (TIR) radiance and optical reflectance factors (RF), to estimate such parameters. The performance of the proposed inversion method as well as of the different sets of constraints was assessed with contemporary measurements of water and heat fluxes and leaf nitrogen content, using pattern-oriented model evaluation.The multiple-constraint inversion approach proposed together with the combination of optical RF and diel GPP and TIR data provided reliable estimates of parameters, and improved predicted water and heat fluxes. The addition of SIF to this scheme slightly improved the estimation of m. Parameter estimates were coherent with the variability imposed by the fertilization and the seasonality of the grassland. Results revealed that fertilization had an impact on Vcmax, while no significant differences were found for m. The combination of RF, SIF and diel TIR data weakly constrained functional traits. Approaches not including GPP failed to estimate LAI; however GPP overestimated Cab in the dry period. These problems might be related to the presence of high fractions of senescent leaves in the grassland. The proposed inversion approach together with pattern-oriented model evaluation open new perspectives for the retrieval of plant functional traits relevant for land surface models, and can be utilized at various research sites where hyperspectral remote sensing imagery and eddy covariance flux measurements are simultaneously taken.

The acclimation of leaf photosynthesis of wheat and rice to seasonal temperature changes in T-FACE environments.

Crops show considerable capacity to adjust their photosynthetic characteristics to seasonal changes in temperature. However, how photosynthesis acclimates to changes in seasonal temperature under future climate conditions has not been revealed. We measured leaf photosynthesis (An ) of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) grown under four combinations of two levels of CO2 (ambient and enriched up to 500 µmol/mol) and two levels of canopy temperature (ambient and increased by 1.5-2.0°C) in temperature by free-air CO2 enrichment (T-FACE) systems. Parameters of a biochemical C3 -photosynthesis model and of a stomatal conductance (gs ) model were estimated for the four conditions and for several crop stages. Some biochemical parameters related to electron transport and most gs parameters showed acclimation to seasonal growth temperature in both crops. The acclimation response did not differ much between wheat and rice, nor among the four treatments of the T-FACE systems, when the difference in the seasonal growth temperature was accounted for. The relationships between biochemical parameters and leaf nitrogen content were consistent across leaf ranks, developmental stages, and treatment conditions. The acclimation had a strong impact on gs model parameters: when parameter values of a particular stage were used, the model failed to correctly estimate gs values of other stages. Further analysis using the coupled gs -biochemical photosynthesis model showed that ignoring the acclimation effect did not result in critical errors in estimating leaf photosynthesis under future climate, as long as parameter values were measured or derived from data obtained before flowering.

A 50-Year Journey from Phosphate to Autonomous Underwater Vehicles.

This narrative is a personal account of my evolution as a student of phytoplankton and the ocean. Initially I focused on phytoplankton nutrient physiology and uptake, later switching to photosynthetic physiology. Better models of photosynthesis naturally require a better understanding of spectral underwater light fields and absorption coefficients, which precipitated my involvement in the nascent field of bio-optical oceanography. Establishment of the now 34-year-old summer graduate course in ocean optics, which continues to attract students from around the globe, is a legacy of my jumping into optics. The importance of social interactions in advancing science cannot be underestimated; a prime example is how a TGIF gathering led to my immersion in the world of autonomous underwater vehicles for the past two decades of my career. Working with people who you like and respect is also critical; I believe collegial friendship played a major role in the great success of the 2008 North Atlantic Bloom Experiment.

Does interruption of electricity supply for supplementary lighting affect the long-term carbon dioxide exchange of greenhouse tomato crops?

Due to new electricity generation and supply schemes, optimising cost and return in greenhouse production could be achieved with interruptive schemes for supplementary lighting (SL). We analysed the effect of interrupted SL (ISL) on both the cost of electricity and crop yield of tomato cultivation during a winter in Brandenburg, Germany (LAT 52°N). We applied SL for 60 days to a tomato crop (cv. Komeett) with two nighttime regimes of nine hours using high pressure sodium lamps (HPSL; 400 W and 210 μmol m–2 s–1) resulting in equivalent daily light integral (DLI, mol m–2 day–1). CO2 assimilation of the crop stand was measured either continuously in four airtight gas exchange greenhouse compartments (28.8 m2 each) or from a sampling of total dry mass. Measurements were used to evaluate the predictions of an existing deterministic photosynthesis model. Results indicate: 1) continuously measured crop net photosynthesis (Pn) overestimated the total CO2 assimilation when measured as crop dry matter at only 1.7%, and 2) an ISL regime had only a low influence on total dry mass formation, presenting 7.6% higher total dry matter in comparison to the model results. It is concluded that 1) continuous measurements of Pn in a daylit environment can be used as a dry mass increase indicator, 2) ISL can be applied without losses in tomato yield, and 3) the existing tomato crop photosynthesis model was not suitable for ISL control. Thus, price optimised ISL can be applied with no negative effect only by looking at electricity prices.

An experimentally-calibrated model of photosynthesis, assimilate partitioning and tuber yield in cassava in response to water supply to assist crop management and improvement

A model (DYNCAS 1.0) is presented of photosynthesis and partitioning of carbon-based (C) assimilate to respiration and growth of leaves, stems, tubers and roots in cassava in response to environmental atmospheric conditions and water supply. It comprises 9 state variables and 40 parameters (including 7 functional relationships), all of direct physiological significance. The important characteristic of the model is the simultaneous consideration of assimilate supply and crop water status by hourly calculations that integrate diurnal physiological responses of photosynthesis, respiration and transpiration to seasonal crop growth and water use under existing environmental conditions.Crop photosynthesis is the integral of a single leaf-response through the canopy according to intercepted radiation determined by an extinction coefficient and by temperature together with canopy conductance determined from crop water potential and vapour pressure deficit. Respiration of organs is calculated individually according to chemical composition and growth processes, including mobilization of reserves. Transpiration proceeds with photosynthesis according to canopy conductance, but in response to evaporative demand, extracting water from the root zone depending upon crop and soil water potentials and the root length that explores it. Partitioning of carbon assimilate from a single crop pool to leaves, stems, tubers and roots depends upon diurnal oscillations in supply according to organ-specific Michaelis-Menten kinetics. Phenological development, here the branching of stems, rate of node and leaf production, and maturity and senescence of leaves and of roots, is determined by temperature and crop water status.The model is shown to capture the response reported in a published study of the growth and yield of two cultivars of cassava of distinct branching habit to water supply. The principles of assimilate partition and the simulation of diurnal responses of growth processes employed here are presented as a guide for further research in cassava productivity and response to environment and management.

Divergent shifts in peak photosynthesis timing of temperate and alpine grasslands in China

The changing climate is shifting the seasonality of photosynthesis in vegetation, including the start (SOS), end (EOS), and length (LOS) of the growing season, and the peak photosynthesis timing (PPT). While the SOS, EOS, and LOS have been widely investigated, the PPT of grasslands – as a proxy for the response of seasonal plant photosynthesis to climate change – has been overlooked. In this study, we propose a hybrid generalized additive model (HGAM) method to extract PPT using the Vegetation Photosynthesis Model (VPM)-based gross primary production (GPP) product, and we examine the dynamics, drivers, and consequences of PPT changes in temperate and alpine grasslands in China over 2000–2016. We found that the PPTs in temperate and alpine grasslands have exhibited advancing (with −0.68 days yr−1, p < 0.05) and delaying (with 0.29 days yr−1, p = 0.158) trends, respectively. In addition, preseason precipitation and soil moisture were positively correlated with the PPT in temperate and alpine grasslands, respectively, while the preseason temperature consistently controlled the PPT changes in both grasslands. Furthermore, we found that an earlier PPT was associated with higher annual production in the temperate grasslands but not in the alpine grasslands in China. The divergent PPT patterns indicated the varied adaptation characteristics to climatic constraints in the temperate and alpine grasslands and also caused different consequences on carbon uptake. This study highlights the importance of PPT in understanding the spatiotemporal dynamics of vegetation photosynthesis and the carbon cycle under a changing climate.

No evidence for triose phosphate limitation of light-saturated leaf photosynthesis under current atmospheric CO2 concentration.

The triose phosphate utilization (TPU) rate has been identified as one of the processes that can limit terrestrial plant photosynthesis. However, we lack a robust quantitative assessment of TPU limitation of photosynthesis at the global scale. As a result, TPU, and its potential limitation of photosynthesis, is poorly represented in terrestrial biosphere models (TBMs). In this study, we utilized a global data set of photosynthetic CO2 response curves representing 141 species from tropical rainforests to Arctic tundra. We quantified TPU by fitting the standard biochemical model of C3 photosynthesis to measured photosynthetic CO2 response curves and characterized its instantaneous temperature response. Our results demonstrate that TPU does not limit leaf photosynthesis at the current ambient atmospheric CO2 concentration. Furthermore, our results showed that the light-saturated photosynthetic rates of plants growing in cold environments are not more often limited by TPU than those of plants growing in warmer environments. In addition, our study showed that the instantaneous temperature response of TPU is distinct from temperature response of the maximum rate of Rubisco carboxylation. The new formulations of the temperature response of TPU derived in this study may prove useful in quantifying the biochemical limits to terrestrial plant photosynthesis and improve the representation of plant photosynthesis in TBMs.

Contrasting Performance of the Remotely-Derived GPP Products over Different Climate Zones across China

Precise quantification of terrestrial gross primary production (GPP) has been recognized as one of the most important components in understanding the carbon balance between the biosphere and the atmosphere. In recent years, although many large-scale GPP estimates from satellite data and ecosystem models have been generated, few attempts have been made to compare the different GPP products at national scales, particularly for various climate zones. In this study, two of the most widely-used GPP datasets were systematically compared over the eight climate zones across China’s terrestrial ecosystems from 2001 to 2015, which included the moderate resolution imaging spectroradiometer (MODIS) GPP and the breathing Earth system simulator (BESS) GPP products. Additionally, the coarse (0.05o) GPP estimates from the vegetation photosynthesis model (VPM) at the same time scale were used for auxiliary analysis with the two products. Both MODIS and BESS products exhibited a decreasing trend from the southeast region to the northwest inland. The largest GPP was found in the tropical humid region with 5.49 g C m−2 d−1 and 5.07 g C m−2 d−1 for MODIS and BESS, respectively, while the lowest GPP was distributed in the warm temperate arid region, midtemperate semiarid region and plateau zone. Meanwhile, the work confirmed that all these GPP products showed apparent seasonality with the peaks in the summertime. However, large differences were found in the interannual variations across the three GPP products over different climate regions. Generally, the BESS GPP agreed better than the MODIS GPP when compared to the seasonal and interannual variations of VPM GPP. Furthermore, the spatial correlation analysis between terrestrial GPP and the climatic factors, including temperature and precipitation, indicated that natural rainfall dominated the variability in GPP of Northern China, such as the midtemperate semiarid region, while temperature was a key controlling factor in the Southern China and the Tibet Plateau area.

Trends and controls of terrestrial gross primary productivity of China during 2000–2016

Terrestrial gross primary productivity (GPP) is an important flux that drives the global carbon cycle. However, quantifying the trend and the control factor of GPP from the pixel level to the regional level is still a challenge. We generated monthly GPP dataset using the vegetation photosynthesis model and calculated the interannual linear trend for China during 2000–2016. The Breaks For Additive Seasonal and Trend method was applied to detect the timing of breakpoint and trends shift of monthly GPP, while boosted regression tree analysis was used to identify the most important factor and its relative influence on GPP based on gridded leaf area index (LAI), aerosol optical thickness, and NCEP-DOE Reanalysis II meteorological data. The results show that annual mean GPP was significantly (P < 0.001, R2 = 0.78) increased, especially in the Loess Plateau and South China, from 2000 to 2016. The change rate of annual mean GPP declined from 18.82 g C m−2 yr−1 in 2000–2008 to 3.48 g C m−2 yr−1 in 2008–2016. About 55.4% of the breakpoints occur between 2009 and 2011 and was mainly distributed in Qinghai-Tibet Plateau, Central China, Southwestern China, and South China, and negative oriented GPP trends variation type still accounts for about 28.76%. LAI and temperature related factors generally had the highest relative influence on GPP in the north part and south part of China, respectively. Our study indicates that the ecological restoration projects and rapid urbanization have respectively induced the most obvious increase and decrease trends of GPP in China. Land cover change and climate change are the main reasons for GPP dynamics in the north and south part of China, respectively.

Protein Cross-Interactions for Efficient Photosynthesis in the Cassava Cultivar SC205 Relative to Its Wild Species.

The underlying mechanisms of the higher photosynthetic efficiency of cultivated cassava relative to its wild species are poorly understood. In the present study, proteins in leaves and chloroplasts were analyzed to compare the differences among the cultivar SC205, its wild ancestor W14, and the related species Glaziovii. The functions of differential proteins are associated with 10 ontology groups including photosynthesis, carbohydrate and energy metabolism, as well as potential signal pathway. The protein-protein networks among 41 differential proteins showed that PGK1 is a hub protein and protein cross-interactions affected the differentiation of photosynthetic rate. Anatomy patterns and PEPC detection suggested that SC205 has more C4 photosynthesis characteristics than Glaziovii and W14. Finally, a mechanism model of the efficient photosynthesis was proposed based on the remarkable variations in photosynthetic parameters and protein functions in the domestic cultivars.

Maize radiation use-efficiency response to optimally distributed foliar-nitrogen-content depends on canopy leaf-area index

Canopy foliar nitrogen content is an important determinant of crop radiation use efficiency (RUE). Previous studies suggested that nitrogen-deficient maize (Zea mays L.) crops may face a trade-off between light interception and RUE. Quantifying the relevance of this trade-off, however, demands for analytical approaches on the response of canopy photosynthesis to foliar nitrogen content. In this study we set up a theoretical framework based on canopy photosynthesis modelling to determine for maize (i) whether RUE response to total canopy foliar nitrogen (TCFN) is affected by canopy leaf area index (LAI), within a range of conventional TCFN and LAI values, and (ii) the relative advantage, in terms of crop RUE, of optimal vs. uniform foliar nitrogen distribution within the canopy. Vertical profiles of optimally-distributed foliar nitrogen became steeper with increases in LAI and reductions in TCFN. In addition, the higher the LAI, the more pronounced the reductions in RUE in response to reductions in TCFN. Accordingly, the relative advantage of optimal vs. uniform foliar nitrogen distribution increased with canopy LAI and increased with the deficiency of foliar nitrogen. With a small effect of LAI, the optimal profiles of critical-TCFN (i.e. the minimal TCFN to achieve the 95% of the maximal RUE) appeared to be appropriately described by a single curve as a function of the light distribution profile within the canopy. Results of this study highlight the importance of coupling the canopy LAI to the nitrogen availability (or vice-versa) among the crop management decisions oriented to maximize crop production while reducing the negative environmental impacts of nitrogen inputs. We also discuss the potential utility of these results as reference values in diagnosis or monitoring the nitrogen status of maize.

Simulating solar-induced chlorophyll fluorescence in a boreal forest stand reconstructed from terrestrial laser scanning measurements

Solar-induced chlorophyll fluorescence (SIF) has been shown to be a suitable remote sensing proxy of photosynthesis at multiple scales. However, the relationship between fluorescence and photosynthesis observed at the leaf level cannot be directly applied to the interpretation of retrieved SIF due to the impact of canopy structure. We carried out a SIF modelling study for a heterogeneous forest canopy considering the effect of canopy structure in the Discrete Anisotropic Radiative Transfer (DART) model. A 3D forest simulation scene consisting of realistic trees and understory, including multi-scale clumping at branch and canopy level, was constructed from terrestrial laser scanning data using the combined model TreeQSM and FaNNI for woody structure and leaf insertion, respectively. Next, using empirical data and a realistic range of leaf-level biochemical and physiological parameters, we conducted a local sensitivity analysis to demonstrate the potential of the approach for assessing the impact of structural, biochemical and physiological factors on top of canopy (TOC) SIF. The analysis gave insight into the factors that drive the intensity and spectral properties of TOC SIF in heterogeneous boreal forest canopies. DART simulated red TOC fluorescence was found to be less affected by biochemical factors such as chlorophyll and dry matter contents or the senescent factor than far-red fluorescence. In contrast, canopy structural factors such as overstory leaf area index (LAI), leaf angle distribution and fractional cover had a substantial and comparable impact across all SIF wavelengths, with the exception of understory LAI that affected predominantly far-red fluorescence. Finally, variations in the fluorescence quantum efficiency (Fqe) of photosystem II affected all TOC SIF wavelengths. Our results also revealed that not only canopy structural factors but also understory fluorescence should be considered in the interpretation of tower, airborne and satellite SIF datasets, especially when acquired in the (near-) nadir viewing direction and for forests with open canopies. We suggest that the modelling strategy introduced in this study, coupled with the increasing availability of TLS and other 3D data sources, can be applied to resolve the interplay between physiological, biochemical and structural factors affecting SIF across ecosystems and independently of canopy complexity, paving the way for future SIF-based 3D photosynthesis models.

Effects of electric power supply for artificial illumination of greenhouse crops at volatile current supply

In Germany, the increasing generation of electricity based on renewable energy is accompanied by a high volatility in the current availability. This requires not only flexible capacities for current production, but also for current use and results in a high variability of the electricity tariff. Greenhouses could be such flexible users of volatile current supply. They can use it for heating the greenhouse and illuminating the crops. The present study considers the use of volatile current supply for the artificial illumination of greenhouse crops. In a model study, the effect of illuminating a tomato crop on photosynthesis and the potential yield at different times of the day and of the year is evaluated. In addition, the lamps release heat into the greenhouse and thus decrease the heat energy consumption. The photosynthesis model was evaluated for the application of artificial light using measurements in a greenhouse on complete crops. Simulated scenarios show the effects of variable artificial lighting. Obviously, the effect is higher at night and during winter time compared to summer. However, even during bright summer days, a photosynthesis driven model generates a slight increase in yield. The aspect of deriving the crop yield directly from the crop photosynthesis is discussed.

Spatiotemporal differentiation of the terrestrial gross primary production response to climate constraints in a dryland mountain ecosystem of northwestern China

Monitoring seasonal and interannual variability in gross primary production (GPP) and attributing these changes to climate change across various ecosystems helps to predict the future climate-carbon cycle feedback. However, such studies are scarce in dryland mountain ecosystems, possibly because of high spatial heterogeneity in landscapes and terrain. To better understand how carbon fluxes of the dryland mountain ecosystem respond to meteorology, we identified the trend and driving mechanism related to GPP in the Qilian Mountains (QLMs) of northwestern China from 2000 to 2016 by adopting the vegetation photosynthesis model that incorporates satellite and meteorological data. Our results revealed contrasting GPP trends in the growing season (May–September) between 2000–2010 and 2010–2016. In the later period, widespread GPP reductions were found across almost the whole area, especially at the middle and end of the growing season. In the central part of the QLMs, GPP reductions were induced by warming hiatus in contrast to drought in the western and eastern parts. Responses of GPP to temperature, precipitation and solar radiation differed in seasons and biomes. The positive effect of rising temperature that increased GPP was dominant during the growing season. The interannual variability in GPP was positively related to precipitation in June and July, but was negatively related to precipitation in other months. A positive correlation between forest GPP and solar radiation occurred in all months but July. Desert GPP responded negatively to solar radiation in all months but September. Temperature and solar radiation accounted for most of the interannual variability in forest GPP. Temperature was the major climate constraints on the interannual variability in grassland GPP. Precipitation and solar radiation primarily controlled the interannual variability in desert GPP from July to September, while temperature became more limited than precipitation and solar radiation for desert GPP in May and June.

Nitrogen supply and other controls of carbon uptake of understory vegetation in a boreal Picea abies forest

In boreal forests, carbon (C) uptake by understory may be too large to be ignored and too variable in space to be assumed a constant fraction of the ecosystem gross primary production. To improve estimates of understory production in these ecosystems, we need to better account for its main controls. In this study, we estimated C uptake of field-layer vegetation, dominated by Vaccinium myrtillus, V. vitis-idaea, and Deschampsia flexuosa, in a boreal Picea abies stand in northern Sweden. Nitrogen (N) availability in the stand has been manipulated through annual N additions since 1996 at the rates of 0, 12.5, and 50 kg N ha−1 yr−1. To assess the relative importance of N supply, and interannual fluctuations in leaf biomass and weather, in controlling field-layer photosynthetic production, we calculated C uptake over eight growing seasons using a canopy photosynthesis model. Without N additions, tree leaf area index (L) was already high (8.5) and field-layer C uptake was small, 27 g C m−2 (or ∼3% of stand C uptake). An increase in tree L with N additions further reduced light availability for the understory, yet the concurrent increase in the relative abundance of the more physiologically active D. flexuosa sustained the contribution of the field-layer to stand photosynthetic production. Based on a literature survey, in which site quality or stand age generated a wide range in L, understory contribution to ecosystem C uptake increases linearly with the fraction of available light reaching the forest floor across high latitude forests. Understory contributes only ∼5% to ecosystem C uptake where trees intercept ∼80% of incoming light, increasing to 100% after clearcut tree harvest. While the availability of solar energy, both spatially and temporally, is the primary driver of understory production, our analyses suggest that the predicted increases in drought severity and frequency at high latitudes may affect understory communities more than trees. Future empirical and modeling studies should focus on functional and ecological responses to drought of not only trees but also understory species, which contribute to biodiversity and convert their photosynthates to important non-timber products.