Canopy Photosynthesis Model Research Articles

Can Changes in Canopy and/or Root System Architecture Explain Historical Maize Yield Trends in the U.S. Corn Belt?

ABSTRACTContinuous increase in the yield of maize (Zea mays L.) in the U.S. Corn Belt has involved an interaction with plant density. A number of contributing traits and mechanisms have been suggested. In this study we used a modeling approach to examine whether changes in canopy and/or root system architecture might explain the observed trends. A maize crop model was generalized so that changes in canopy and root system architecture could be examined. A layered, diurnal canopy photosynthesis model was introduced to predict consequences of change in canopy architecture. A two‐dimensional root exploration model was introduced to predict consequences of change in root system architecture. Field experiments were conducted to derive model parameters for the base hybrid (Pioneer 3394). Simulation studies for various canopy and root system architectures were undertaken for a range of sites, soils, and densities. Simulated responses to density compared well with those found in field experiments. The analysis indicated that (i) change in root system architecture and water capture had a direct effect on biomass accumulation and historical yield trends; and (ii) change in canopy architecture had little direct effect but likely had important indirect effects via leaf area retention and partitioning of carbohydrate to the ear. The study provided plausible explanations and identified testable hypotheses for future research and crop improvement effort.

Carbon balance in a monospecific stand of an annual herb Chenopodium album at an elevated CO2 concentration

Elevated CO2 enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO2 effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO2 concentrations (370 and 700 μmol mol−1). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO2 exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r 2 = 0.98). The increase of canopy photosynthesis with elevated CO2 was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO2. However, their contributions were not constant, but changed with stand development.

Morphological, Anatomical, and Physiological Changes of Orchardgrass Leaves Grown under Fluctuating Light Regimes

The physiological and anatomical adaptability of pastures growing under trees in silvopastoral systems can alter the efficiency of conversion of energy to dry matter (DM). This study was conducted to determine the effects of different fluctuating light regimes (from 24 to 100% transmissivity) on leaf physiology, morphology, anatomy, and structure of orchardgrass (Dactylis glomerata L.) in a silvopastoral experiment (New Zealand). Slatted shade structures created a bimodal light regime that represented an existing silvopastoral system. Morphologically, as transmissivity decreased the length of the youngest fully expanded leaf and pseudo‐stem height increased by up to 33% and the leaf width declined up to 22%. Physiologically, leaf adaptation to different light regimes was characterized by: (i) the light‐saturated rate of net photosynthesis (Pmax) and to less extent the photosynthetic efficiency (α) in sun conditions was double; (ii) in sunny conditions plants grown under shade were photosynthetically less efficient than plants grown in full sunlight with lower Pmax and α values; (iii) when plants were exposed to severe shade, leaves adapted to severe shade condition had the highest Pmax, α, and θ, and saturated at the minimum photosynthetic photon flux density (PPFD) value. These changes in leaf photosynthesis characteristics under different light regimes were attributed to anatomical changes that caused reductions in stomatal conductance (gs), the mesophyll surface area/leaf surface area ratio (Ames/A) and maintenance respiration for shade adapted plants. These photosynthetic responses and leaf adaptations to fluctuating light regimes can be included into a canopy photosynthesis model to improve the accuracy of DM predictions in silvopastoral systems.

Functional Significance of Shade‐Induced Leaf Senescence in Dense Canopies: An Experimental Test Using Transgenic Tobacco

Canopy photosynthesis models have predicted an optimal leaf area index (LAI; leaf area per unit surface area) and leaf nitrogen distribution at which whole-plant carbon gain per unit N is maximized. In this study we experimentally tested these models, using transgenic P(SAG12)-IPT tobacco (SAG; Nicotiana tabacum L.) plants with delayed leaf senescence and therefore a greater LAI and more uniform N distribution than the wild type (WT). In a competition experiment, the increased density of surrounding WT plants caused a greater reduction in dry mass of mature SAG target plants than in that of WT target plants, indicating negative effects of delayed leaf senescence on performance at high canopy density. Vegetative SAG plants achieved a lower calculated daily carbon gain than competing WT plants because the former retained leaves with a negative carbon gain in the shaded, lower part of the canopy. Sensitivity analyses showed that the carbon gain of SAG plants would increase if these lower leaves were shed and the N reallocated from these leaves were used to form additional leaf area at the canopy top. This strategy, which is adopted by the WT, is most advantageous because it results in the shading of competing neighbors.

Modelling five years of weather-driven variation of GPP in a boreal forest

This study uses a five-year series of half-hourly eddy covariance (EC) and soil temperature measurements to estimate the time development of Gross Primary Production (GPP) of a boreal Scots pine ( Pinus sylvestris L.) forest, and compares this estimate with results from three versions of a mechanistic canopy photosynthesis model at different temporal scales. The mechanistic model is independent of the canopy-level measurements, scaling up instantaneous shoot-level photosynthesis in spatially distributed crowns. It consists of submodels for radiation interception and shoot photosynthesis. The interception model calculates the attenuation of photosynthetically active radiation (PAR) through randomly distributed individual crowns. To assess the significance of driving variables at different time scales, we used three shoot photosynthesis models with an increasing number of driving variables, starting from PAR only, adding air humidity and stomatal conductance, and a delayed temperature effect describing the temperature acclimation of photosynthetic capacity. The scaling model based on incoming PAR only explained up to 95% of the within-day variation in the EC-based estimate of GPP. The model accounting for PAR and the temperature acclimation explained about 90% of the day-to-day variation in the EC-based estimate of GPP over the five years. Models without the temperature acclimation did not capture the day-to-day variation during spring. From mid to late summer, PAR was sufficient to explain the observed day-to-day variation also. Including vapour pressure deficit (VPD) was not significant for the quantitative accuracy of the results in this boreal Scots pine stand. The mechanistic models also explained most of the year-to-year variation in the EC-based estimates, except that they all yielded larger predictions in 1999, most likely due to soil drought which was not accounted for by the models.

Leaf lifespan and lifetime carbon balance of individual leaves in a stand of an annual herb, Xanthium canadense

Leaf lifespan in response to resource availability has been documented in many studies, but it still remains uncertain what determines the timing of leaf shedding. Here, we evaluate the lifetime carbon (C) balance of a leaf in a canopy as influenced by nitrogen (N) availability. Stands of Xanthium canadense were established with high-nitrogen (HN) and low-nitrogen (LN) treatments and temporal changes of C gain of individual leaves were investigated with a canopy photosynthesis model. Daily C gain of a leaf was maximal early in its development and subsequently declined. Daily C gain at shedding was nearly zero in HN, while it was still positive in LN. Sensitivity analyses showed that the decline in the daily C gain resulted primarily from the reduction in light level in HN and by the reduction in leaf N in LN. Smaller leaf size in LN than in HN led to higher light levels in the canopy, which helped leaves of the LN stand maintain for a longer period. These results suggest that the mechanism by which leaf lifespan is determined changes depending on the availability of the resource that is most limiting to plant growth.

Validation of a canopy photosynthesis model for cocksfoot pastures grown under different light regimes

Daily net canopy photosynthesis (P n) was predicted for cocksfoot (Dactylis glomerata L.) canopies grown under different light regimes by integration of a leaf photosynthesis model developed for the light-saturated photosynthetic rate (P max), photosynthetic efficiency (α) and the degree of curvature (θ) of the leaf light–response curve. When shade was the only limiting factor, the maximum P n (P nmax) was predicted to decrease approximately linearly from 33.4 g CO2 m−2 d−1 to zero as photosynthetic photon flux density (PPFD) fell from full sunlight (1800 μmol m−2 s−1 PPFD) to 10% of this in a fluctuating light regime. It was also predicted that at 50% transmissivity P nmax was higher for a continuous light regime (10.4 g CO2 m−2 d−1) than for a fluctuating light regime with the same intensity (8.4 g CO2 m−2 d−1). The canopy photosynthesis model was then used to predict dry matter (DM) production for cocksfoot field grown pastures under a diverse range of temperature, herbage nitrogen content and water status conditions in fluctuating light regimes. This prediction required inclusion of leaf area index and leaf canopy angle from field measurements. The model explained about 85% of the variation in observed cocksfoot DM production for a range from 6 to 118 kg DM ha−1 d−1. The proposed model improves understanding of pasture growth prediction through integration of relationships between shade limitations in fluctuating light regimes and other environmental factors that affect the canopy photosynthetic rate of cocksfoot pastures in silvopastoral systems.

Considering ecological formulations for estimating deposition velocity in air quality models

A dry deposition modelling approach that includes surface feedback through photosynthesis relationships was recently developed. A canopy photosynthesis model is dynamically coupled to an atmospheric model with prognostic soil hydrology and surface energy balance. The effective surface resistance is calculated for a realistic and fully interactive estimation of gaseous deposition velocity (Vd). The model was able to correctly estimate observed ozone Vd over agricultural fields. The same model was tested for its ability to simulate ammonia Vd near an animal agricultural facility. The scheme did not reproduce the bi-directional exchange and had a much smaller range as compared to observations. The model was modified to include a simple ammonia compensation point formulation and the results were much closer to the observations. Study concludes that ecological approaches with default parameterisation and biophysical constants are convenient and effective in estimating Vd for air quality models.

Photosynthetic and structural characteristics of canopy and shrub trees in a cool-temperate deciduous broadleaved forest: Implication to the ecosystem carbon gain

To reveal the seasonal change of leaf ecophysiological and canopy characteristics and to evaluate the functional role of canopy and shrub tree species in forest CO 2 uptake, we measured forest canopy leaf area index (LAI) using a hemispherical canopy photography technique, leaf CO 2 gas exchange and shoot architecture for canopy ( Betula ermanii and Quercus crispula) and shrub ( Hydrangea paniculata and Viburnum furcatum) tree species in a deciduous broadleaved forest in a cool-temperate region in central Japan. Canopy LAI and photosynthetic capacity of canopy tree leaves increased rapidly with leaf expansion. LAI reached its maximum in early summer but photosynthetic capacity reached its maximum in late summer. Development of photosynthetic capacity was dependent on the changes of leaf mass per area and leaf chlorophyll content (evaluated by SPAD). The seasonal maximum photosynthetic capacity of the leaves at the forest canopy top ( B. ermanii and sun leaves of Q. crispula) was about more than double of the leaves in the shrub layer ( H. paniculata, shade leaves of Q. crispula and V. furcatum). Light interception and photosynthetic carbon gain at a shoot level were simulated under three air temperature conditions by a three-dimensional canopy photosynthesis model (Y-plant) involving the combined leaf photosynthesis and stomatal conductance responses and shoot architecture. Results showed that (1) calculations without considering the heterogeneous light distribution in a foliage made by geometrical feature of plants would overestimate the photosynthetic carbon gain by +40% even at the canopy surface, and (2) the steep leaf angle in B. ermanii avoided midday depression of photosynthesis while the rather horizontal leaves in Q. crispula received excess light and heat load which led larger midday depression of photosynthesis. In addition to the large capacity of photosynthetic productivity of the canopy top foliage, our model also suggests the functional role of shrub species in forest ecosystem carbon gain, due to their high photosynthetic utilization efficiency of low light incidence available in the forest understory.

Variation in the degree of coupling between δ13C of phloem sap and ecosystem respiration in two mature Nothofagus forests

Day-to-day variability in the carbon isotope composition of phloem sap (delta13Chd) and ecosystem respiratory CO2 (delta13CR) were measured to assess the tightness of coupling between canopy photosynthesis (delta13Chd) and ecosystem respiration (delta13CR) in two mature Nothofagus solandri (Hook. f.) forests in New Zealand. Abundant phloem-tapping scale insects allowed repeated, nondestructive access to stem phloem sap 1-2 m above ground. delta13Chd was compared with delta13C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO2 were used to estimate delta13CR. By including a lag of 3 d, there was good agreement in the timing and direction of variation in delta13Chd and predictions by the canopy photosynthesis model, suggesting that delta13Chd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. Significant day-to-day variability in delta13CR was recorded at one of the two forests. At this site, delta13CR reflected variability in delta13Chd only on days with <2 mm rain. We conclude that the degree of coupling between canopy photosynthesis and ecosystem respiration varies between sites, and with environmental conditions at a single site.

Development of the Monsi-Saeki Theory on Canopy Structure and Function

Monsi and Saeki (1953) published the first mathematical model of canopy photosynthesis that was based on the light attenuation within a canopy and a light response of leaf photosynthesis. This paper reviews the evolution and development of their theory. Monsi and Saeki showed that under full light conditions, canopy photosynthesis is maximized at a high leaf area index (LAI, total leaf area per unit ground area) with vertically inclined leaves, while under low light conditions, it is at a low LAI with horizontal leaves. They suggested that actual plants develop a stand structure to maximize canopy photosynthesis. Combination of the Monsi-Saeki model with the cost-benefit hypothesis in resource use led to a new canopy photosynthesis model, where leaf nitrogen distribution and associated photosynthetic capacity were taken into account. The gradient of leaf nitrogen in a canopy was shown to be a direct response to the gradient of light. This response enables plants to use light and nitrogen efficiently, two resources whose supply is limited in the natural environment. The canopy photosynthesis model stimulated studies to scale-up from chloroplast biochemistry to canopy carbon gain and to analyse the resource-use strategy of species and individuals growing at different light and nitrogen availabilities. Canopy photosynthesis models are useful to analyse the size structure of populations in plant communities and to predict the structure and function of future terrestrial ecosystems.

Modelling photosynthetic efficiency ( α) for the light-response curve of cocksfoot leaves grown under temperate field conditions

Net photosynthetic rate was measured from the youngest fully expanded leaves of field grown cocksfoot ( Dactylis glomerata L.) in open pastures and under trees in the Lincoln University silvopastoral experiment (New Zealand). The photosynthetic efficiency ( α) and convexity ( θ) of the light-response curve were derived from 209 fitted non-rectangular hyperbola functions. There was no relationship between θ and any of the environmental or management variables with a stable mean value of 0.96. For α, individual functions were required for temperature (10–31 °C), nitrogen (N) concentration (1.5–5.9% N), water status (expressed as pre-dawn leaf water potential, ψ lp) (−0.01 to −1.6 MPa), regrowth duration (20–60 days), and different times (up to 180 min) under moderate (850–950 μmol m −2 s −1 photon irradiance) and severe (85–95 μmol m −2 s −1 photon irradiance) shade. The highest α of 0.036 μmol CO 2/μmol photon irradiance was found in non-limiting conditions and defined as the standardised maximum ( α s=1). Values of α s=1 were measured in optimum ranges of 10–24 °C, 4.0–5.9% N, −0.01 to −1.0 MPa and 20 days regrowth. In addition, values of α s reached a steady-state asymptote of 0.74 after 60 min of severe shade and 0.92 after 40 min of moderate shade. Individual functions of α could not be integrated into a simple multiplicative model but a ‘law of the minimum factor’ model was appropriate. Predicted results from this model were then validated with 46 independent data points collected when at least two factors were outside their optimum range. The model accounted for 88% of the variation in observed α values. This research has derived functional relationships for α that can be used to assist predictions of leaf photosynthesis and ultimately pasture growth by their inclusion in canopy photosynthesis models.

Effects of chronic N additions on tissue chemistry, photosynthetic capacity, and carbon sequestration potential of a red pine ( Pinus resinosa Ait.) stand in the NE United States

Temperate forests are predicted to play a key role as important sinks for atmospheric carbon dioxide, which could be enhanced by nitrogen (N) deposition. However, experimental evidence suggests that the impact of N deposition on temperate forest productivity may not be as great as originally assumed. We investigated how chronic N addition affects needle morphology, nitrogen metabolism (or partitioning) photosynthetic capacity and foliage productivity. The investigation is based at the Harvard Forest (MA) as part of a now 15-year long N amendment study. Foliar N content in red pine ( Pinus resinosa) of the high N treatment has significantly increased, but this increase was accompanied by a de-coupling of the photosynthesis–N relationship. In addition needle longevity in the high N trees was significantly lower compared to the control trees. Conifers of the high N treatment did not use the surplus of N to optimize the amount of photosynthetically active metabolites. Instead N accumulated as soluble protein (other than Rubisco), amino acids and chlorophyll. Photosynthetic capacity in the control trees was about 50% higher than in the fertilized trees. These results indicate that the increase in leaf N is not accompanied by a greater capacity for carbon assimilation in the high N treatment. Using a simple model (PnET-Day) of canopy photosynthesis and carbon allocation, we assessed the long-term effect of these physiological changes on ecosystem carbon balance. The model results emphasize and reinforce the large difference between rates of carbon accumulation predicted to occur if net photosynthesis remained linearly related to foliar N concentration, and rates measured in the field.

A canopy photosynthesis model to predict the dry matter production of cocksfoot pastures under varying temperature, nitrogen and water regimes

AbstractDaily net canopy photosynthesis (Pn) of cocksfoot (Dactylis glomerata L.) was predicted for combinations of temperature, herbage nitrogen (N) concentration and water status from the integration of models of leaf photosynthesis of the light‐saturated photosynthetic rate (Pmax), photosynthetic efficiency (α) and the degree of curvature (θ) of leaf light‐response curves. The effect on Pn, maximum Pn (Pn max) and the optimum leaf area index (LAI at Pn max) was examined when any one of these factors was limiting. The ranges that gave the optimum values of Pn (Pn max = 30·8–33·5 g CO2m−2 d−1) for temperature (19–22°C) and N concentration (40–50 g N kg−1 DM) were smaller than those for net leaf photosynthesis. Also, Pn fell to 0 at a lower level of water stress (pre‐dawn leaf water potential, ψlp = −12·5 bar) than for Pmax. The canopy photosynthesis model was then used to compare predicted and measured dry matter (DM) production for cocksfoot pastures grown under a diverse range of environmental conditions with field data from New Zealand and Argentina. To predict DM production leaf area index and leaf canopy angle were included from field measurements. The model explained about 0·85 of the variation in cocksfoot DM production for the range of 6·5–134 kg DM ha−1 d−1. The canopy model overestimated the DM production by 0·10 which indicates that a further Pmax function for leaves of different ages and a partitioning sub‐model may be needed to improve predictions of DM production.

Ecosystem UV-B experiments in terrestrial communities: a review of recent findings and methodologies

Ecosystem-level UV-B experiments have revealed a variety of responses in terrestrial communities. These include decreases in the growth of some plant species (usually small changes but occasionally >25%), both decreases and increases in herbivory, occasional altered decomposition patterns, changes in populations of fungi and invertebrates, and morphological changes in the growth patterns of mosses. These field experiments have been pursued both with solar UV-B exclusion using selective filters and with supplementation of UV-B using lamps. For UV-B-exclusion studies it is critical that the UV-B-excluding and UV-B-transmitting filters pass the same amount of photosynthetically active radiation (PAR, 400–700 nm) and also infrared. Canopy photosynthesis models indicate that even small differences in PAR transmittance can, under many circumstances, have significant effects on plant carbon gain. It is also important that filters in UV-B-exclusion studies allow precipitation to pass uniformly to the plots unless there is some form of irrigation underneath the filters. For UV-B-supplementation studies with lamps, many factors are involved in the realism of the ozone depletion simulation. We believe the major lamp-system error is excessive supplementation of UV-B by timer-controlled lamps when weather conditions (primarily clouds) decrease ambient solar UV-B irradiance. The choice of biological spectral weighting functions (BSWF) used in adjusting lamp flux is also critical in determining the level of ozone depletion simulated. In addition, shading by the lamp arrays influences plant growth and becomes particularly important when BSWF with substantial weighting in the UV-A are employed.

Modelling net photosynthetic rate of field‐grown cocksfoot leaves to account for regrowth duration

Maximum light‐saturated photosynthetic rate (Pmax ) of field‐grown cocksfoot (Dactylis glomerata L.) leaves was measured in a temperate, sub‐humid environment (Canterbury, New Zealand). The aim was to derive an individual function for Pmax of newly expanded leaves against regrowth duration when other environmental factors were non‐limiting. The decrease in Pmax with regrowth duration was described by a quadratic function. From 20 to 25 days regrowth, Pmax per unit of leaf was constant and maximal (27.4 μmol CO2 m –2 s−1). It then decreased by 0.42 μmol CO2 m –2 s −1 per day of regrowth. The decline in Pmax was attributed to (i) differences in chronological age of the youngest expanded leaf as shown by changes in tiller morphology over time, and (ii) shading within the canopy during leaf expansion. These factors affected Pmax by decreasing the leaf nitrogen and chlorophyll content, and stomatal conductance. The function for regrowth duration was an additional factor included in a multiplicative model to predict Pmax with different levels of temperature, nitrogen, and water status, expressed as pre‐dawn leaf water potential (?lp). The only interaction detected was when water stress increased (?lp < ‐1.2 bar) and leaves had grown for 40–60 days. In this limited situation, stomatal closure at 40–60 days was greater than expected from the non‐limiting condition. The inclusion of this function into a simple multiplicative model enabled 80% of the variation in Pmax for individual cocksfoot leaves to be explained by their temperature, nitrogen, water, and regrowth status. These functions could then be used to develop a canopy photosynthesis model for the prediction of cocksfoot pasture production.

An integrated model for predicting maximum net photosynthetic rate of cocksfoot (Dactylis glomerata) leaves in silvopastoral systems

Net light-saturated photosynthetic rate (Amax) of field grown cocksfoot (Dactylis glomerata L.) leaves in a radiata pine (Pinus radiata D. Don) silvopastoral system (Canterbury, New Zealand) was measured at different times under severe shade (85–95 µmol m–2 s–1 photosynthetic photon flux density, PPFD) and in full sunlight (1900 µmol m–2 s–1 PPFD). The aim was to integrate individual functions for Amax against air temperature (2 to 37 oC), water status, expressed as pre-dawn leaf water potential (ψlp) (-0.01 to −1.6 MPa), herbage nitrogen (N) (1.5 to 5.9%), regrowth duration (20 to 60 days) and time under shade (1 to 180 min) into a multiplicative model. The highest Amax value obtained was 27.4 µmol CO2 m–2 s–1 in non-limiting conditions with full sunlight. This value was defined as standardised dimensionless Amaxs = 1 for comparison of factor effects. The canopy temperature of the cocksfoot sward was up to 7.4 oC cooler than air temperature for plants under shade. Therefore, canopy temperature was used to predict Amax. The only interaction was between time under severe shade (5% of the open PPFD) and water stress (ψlp = −0.4 to −1.3 MPa) and this was included in the model. Validation of this model indicated 78% of the variation in Amax could be accounted for using these five factors by the addition of the interaction function. This model could be used to assist the prediction of pasture growth in silvopastoral systems through incorporation into a canopy photosynthesis model.

Impact of atmospheric aerosol light scattering and absorption on terrestrial net primary productivity

Scattering and absorption of sunlight by anthropogenic aerosols reduce the photosynthetically active radiation (PAR) incident upon the Earth's surface, but increase the fraction of the PAR that is diffuse. These alterations to irradiance may elicit conflicting responses in terrestrial plants: photosynthesis and net primary productivity (NPP) are slowed by reductions in total PAR, but enhanced by increases in diffuse PAR. In this paper, we use two canopy photosynthesis models to estimate the net effect of aerosols on carbon assimilation by green plants during summertime at midlatitudes. The model calculations indicate that the net effect of PAR scattering and absorption by atmospheric aerosols on NPP can be positive, neutral, or negative. Two parameters that strongly influence the net effect are the aerosol optical depth (integral of light extinction with height) and the cloud cover. On cloudless days NPP peaks under moderately thick aerosol loadings. On overcast days, aerosols slow NPP. The implications of these results for various regions of the globe and possible directions for future studies on the effect of aerosols on plant growth are discussed.

Net photosynthetic rate of cocksfoot leaves under continuous and fluctuating shade conditions in the field

AbstractMaximum light‐saturated photosynthetic rate (Pmax) and stomatal conductance (gs) of field‐grown cocksfoot (Dactylis glomerataL.) leaves in a silvopastoral system were measured at different times under moderate (850–950 µmol m−2s−1photosynthetic photon flux density, PPFD) and severe shade (85–95 µmol m−2s−1PPFD). Also Pmax and gs were measured after 30, 60 and 180 min of severe shade to determine the lag in the rise of photosynthesis rate from low to high irradiance levels (induction state). The highest Pmax and gs values obtained were 26·5 µmol CO2 m−2s−1and 0·41 mol H2O m−2s−1in non‐limiting conditions with full sunlight (1900 µmol m−2s−1PPFD). These values were defined as standardized dimensionless Pmaxs=1 and gss=1 for comparison of treatment effects. The Pmaxsunder severe shade decreased by 0·004 units per minute from 1 to 180 min and reached a steady‐state of 0·37 units after 140 min. Under moderate shade, Pmaxsdecreased by 0·002 units per minute from 1 to 120 min and reached a steady‐state of 0·76 units. The time required to reach full induction on return to full sun (Pmaxs=1) was 15 min after 30 min of severe shade and 37 min after 180 min of shade. Mathematical equations were derived to describe the changes in Pmaxsand gssunder severe and moderate shade and during induction. The rate of change of gsswas slower than for Pmaxson entering shade and also slower during the subsequent induction process. This indicated other factors in addition to gs were operating in the reduction and increment of Pmax and a two‐step model to explain this is proposed. The defined photosynthetic responses of cocksfoot leaves to fluctuating light regimes could be used to develop quantitative predictions of Pmax for inclusion in a canopy photosynthesis model of silvopastoral systems.

Eco-physiological modelling of canopy photosynthesis and growth of a Chinese fir plantation

A eco-physiological semi-process-based model for canopy photosynthesis and growth, PHOTOS, was developed, by which canopy photosynthesis, net primary production, and growth for an average tree in a Chinese fir plantation on a sloped site was calculated based on modelling of radiation, needleleaf density distribution and photosynthetic parameters. The 2D needleleaf area density (NAD) distribution was introduced and both within-crown shading and between-crown shading were taken into account in the radiation modelling. A tree crown, assumed to be cone-shaped, was divided into six vertical layers and 1–3 horizontal sections in eight directions, totalling 96 small cells. Crown net photosynthesis and respiration was obtained using the summation of all cells, and their daily and seasonal values were integrated in 1 h steps. The calculation was tested with an 18-year-old Chinese fir plantation and good agreement between measurements and calculation was obtained. Uncertainties are also discussed.