Incident Photosynthetically Active Radiation Research Articles

A new approach for remote sensing of canopy absorbed photosynthetically active radiation. II: Proportion of canopy absorption

The amount of photosynthetically active radiation (PAR) absorbed by canopy (APAR CAN) is essential to the productivity of vegetation. Monitoring APAR CAN from space has been achieved through the retrievals of two quantities, namely, the PAR incident at the surface (PAR SFC↓) and the fraction of PAR intercepted by the canopy, FPAR. We propose a new approach that splits APAR CAN into the PAR absorbed in the surface layer below the top of the canopy (APAR SFC) and the ratio of APAR CAN / APAR SFC, RPAR. The method is introduced in two parts. Part I develops a simple parameterization that retrieves APAR SFC more readily and accurately than PAR SFC↓. Part II, presented in this paper, deals with the retrieval of RPAR. It is shown that RPAR can be derived as accurately and readily as FPAR. Hence, it is envisaged that the new approach offers an easier and more accurate means of estimating APAR CAN than the traditional one. As an investigation tool, a one-dimensional multistream and multilayer model of canopy radiative transfer is first formulated. Extensive canopy modeling is conducted with input parameters of large ranges to represent a variety of canopies and ground conditions. For vegetated land, RPAR is found to correlate well with FPAR and thus RPAR can be estimated from FPAR. RPAR is also related with the surface vegetation indices (VIs) such as NDVI, SAVI, and DVI. The relationships between RPAR and VIs are driven by the changes in leaf area index. They are not sensitive to the solar zenith angle and the fractions of direct and diffuse radiation, but to the optical properties of the canopy. The models for inferring RPAR from various VIs are given, together with the correction models to account for the dependencies of RPAR on time and cloud cover.

Shortwave radiation budget of Sahelian vegetation 2. Radiative transfer models

In semi-arid environments, vegetation canopies can often be represented by two distinct layers; the upper layer of perennial trees or shrubs, and the lower layer of annual or perennial herbaceous species. Two models of radiative transfer, one appropriate for homogeneous vegetation (the SAIL (scattering from arbitrarily inclined leaves) model), and the other for clumped plant stands (the cylinder model), were coupled to simulate photosynthetically active radiation (PAR) radiative transfer in a wide range of semi-arid vegetation canopies. The coupling was based on the assumption that the PAR incident above the herb layer was equal to the quantity of PAR transmitted by the woody layer. The fluxes predicted by the models and their coupling are validated against a data set of PAR interception acquired in small plots during the 1992 HAPEX-Sahel field campaign (Niger, West Africa). It was shown that the SAIL model alone did not adequately describe the interception of clumped vegetation communities, whereas the combined SAIL-cylinder model was able to simulate observed interception efficiencies. The coupled model was then applied to the HAPEX-Sahel West-Central grass fallow, shrub fallow, degraded shrubland and millet crop sites, to estimate daily intercepted PAR of the sites during the 1992 growing season. The results are compared with hemispherical ceptometer measurements made along transects across the sites.

Importance of Variation in Chilling Tolerance for Peanut Genotypic Adaptation to Cool, Short‐Season Environments

Recent studies have shown genotypic variation in sensitivity of leaflet CO2 exchange rate (CER) to cool night temperatures. However, no information is currently available on the relative importance of this trait for successful peanut production in a cool, short‐season environment. The objectives of this study were to examine the physiological basis of genotype adaptation to a cool, short‐season environment and to assess the importance of genotypic differences in sensitivity to low night temperatures in that adaptation. Eight genotypes were grown in the field at Delhi, Ontario in 1991, and six of these eight were grown in 1992. Phenological development was monitored, dry matter (DM) accumulation was determined from destructive samples, and radiation use efficiency (RUE) was calculated from measurements fractional interception of incident photosynthetically active radiation (PAR). Accumulated degree‐days in 1991 (1245 °C d from sowing to frost) greatly exceeded those in 1992 (885 °C d), despite similar calendar‐day duration. Only five genotypes matured in 1991 and two in 1992. The degree‐days required to reach key reproductive stages including maturity varied considerably among genotypes and between seasons. Seasonal variation was probably due to effects of low temperatures on continuity of reproductive development and the proportion of total kernels that were mature at harvest. Total DM was reduced by 30 to 50% in 1992, compared with 1991, whereas pod and kernel yields were reduced by 40 to 75 and 38 to 83%, respectively. The lower DM in 1992, and genotypic differences in both seasons, were due primarily to the amount of incident PAR intercepted by canopies (i.e., 520–790 MJ m−2 in 1991 and 360–480 MJ m−2 in 1992). The RUE was 13 to 24% lower during 1992, compared with 1991, with relative genotype RUE generally consistent with CER sensitivity to low night temperature. Early maturity combined with an ability to rapidly expand a canopy to intercept incident PAR were the factors most important for peanut production in this environment.

Competition for light and nitrogen during a regrowth cycle in a tropical forage mixture

Abstract A dynamic analysis of light and nitrogen sharing in a grass/legume mixture during a regrowth cycle in a humid tropical climate is presented. Pure stands and mixture plots of two stoloniferous species, Arachis pintoi (a C3 legume species) and Digitaria decumbens (a C4 grass species), were established on ferralitic soil in randomised sub-plots with four replicates. Each crop was fertilised at two different nitrogen levels: 150 (N150) and 0 (N0) kg of nitrogen per hectare. Growth was analysed in terms of radiation-use efficiency (RUE) of photosynthetically active radiation (PAR) absorbed by crops. Daily absorption efficiencies of the mixture's components were estimated by a radiative-exchange model. The results demonstrated that presence of the legume reduced light interception by the grass by 13 and 28% under the N150 and N0 conditions respectively. On the other hand, competition by D. decumbens for light was very strong at both nitrogen levels since it reduced interception of incident PAR by A. pintoi by 69% (N0) and 91% (N150) in the growth cycle. Intercropping did not result in changes in the radiation-use efficiency of either species except in N150 mixed crop where A. pintoi virtually disappeared due to zero radiation interception at the end of the cycle. Uptake of nitrogen by the grass was not affected by presence of the legume. Despite different production potentials of C3 and C4 species, under N0 mixed-crop conditions, behaviour was neutral. Consequently, the mixture could be maintained at equilibrium if sward production is not intensified by nitrogen fertilisation.

Canopy Nutrient Allocation in Relation to Incident Light in the Tropical Fruit Tree Borojoa patinoi (Cuatr.)

The relationship of incident photosynthetically active radiation (PAR) and mineral nutrient allocation was evaluated in canopies of Borojoa patinoi (Cuatr.) growing in the Chocó rainforest of Colombia, South America. Allocation of P in the canopy was positively correlated with incident PAR, principally because of increased leaf frequency (number of leaves per unit volume of canopy) brought about by local branching, with a smaller contribution from increased specific leaf weight (SLW, leaf dry weight per unit leaf area). The chemical fractionation of P within leaves did not respond to incident PAR. Canopy N allocation also was positively correlated with incident PAR because of increased leaf frequency and SLW. The N partitioning to soluble protein rather than chlorophyll was positively correlated with incident PAR. The allocation of K, Ca, Mg, S, Mn, and Cu also was positively correlated with incident PAR primarily because of increased leaf frequency and secondarily because of increased SLW. The area of individual leaves and the concentration of nutrients in leaf dry weight were not important in determining nutrient allocation responses to incident PAR. Our observations suggest that leaf frequency caused by local branching, followed by changes in SLW, are the primary determinants of canopy nutrient allocation in this tropical fruit tree.

Effects of wave-induced lightflecks in the intertidal zone on photosynthesis in the macroalgae Postelsia palmaeformis and Hedophyllum sessile (Phaeophyceae)

Intertidal algae are exposed to a highly variable photic regime because of crashing waves. We measured photosynthetic rate of whole fronds of the seaweeds Postelsia palmaeformis (Ruprecht) (at Bodega Marine Laboratory, 1991) and Hedophyllum sessile (Setchell) (Phaeophyceae) (at Friday Harbor Laboratories, 1990) in flashing and steady-state photosynthetically active radiation (PAR) of equal irradiance, using a recirculating metabolism chamber designed to minimize the thickness of diffusional boundary layers on the surfaces of algal tissues. The dimensionless ratio of photosynthetic rate under flecking PAR (Pfleck) and the sum of photosynthetic rate under steady state PAR (Ps) of high and low irradiance was computed for lightfleck periods from 0.2 to 100 s. Pfleck:Ps is a measure of the light-flash utilization-efficiency in flashing light, and was greater than unity at periods between lighflecks of 2 to 30 s, with a peak at 10 s. We used a novel optical fiber irradiance meter to measure PAR incident on fronds of P. palmaeformis as they were washed about by waves breaking in the intertidal zone, and compared the light records to that obtained by a stationary sensor under the canopy. PAR flashing period was closely correlated with the period of breaking waves in stands of P. palmaeformis. We estimated the seasonal spectrum of period between light flashes in stands of this species by examining the spectral density of ocean waves at Bodega Marine Laboratory, Bodega Bay, California. The wave spectrum peaks at a period (10 s) where light-flashes may theoretically enhance the light-flash utilization-efficiency of seaweeds. We calculate that the enhanced light-flash utilization-efficiency wrought by wave-induced light-flashes may contribute to significant gains in primary productivity of these macroalgae under some conditions.

Light transmission into apple fruit and leaves

Light transmission into tissue of fruit or leaves of apple ( Malus pumila Mill.) cultivars ‘Golden Delicious’, ‘Gloster’ and ‘Cox's Orange Pippin’ was studied both quantitatively and qualitatively. For comparative purposes, fruit or leaf was exposed to a constant photosynthetically active radiation (PAR) of 1750 μmol m −2 s −1. Under these conditions, transmission of PAR into the inner tissues of apple fruit ranged between 0.04 and 8.4 μmol m −2 s −1, dependent on development stage, cultivar and position of the apple tissue relative to the light source. Parenchyma of apple fruit transmitted 1.5–2% of the incident PAR 1.5 mm −1 fruit tissue. Peel of apple fruit transmitted 1–3% of incident PAR at 400 nm, increasing to 10–12% at 850 nm. Whole apple leaves transmitted up to 2%, whereas an upper epidermis and mesophyll layer transmitted up to 7% of incident PAR. With green fruit, transmission and action spectra were dominated by chlorophyll absorption at 678 nm, whereas apple fruit with red or yellow peel additionally absorbed at 480 nm.

Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions

ABSTRACTThe C3–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ∼100% incident PAR), ‘semi‐exposed’ (∼60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH+, the dawn‐dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time‐courses of photosynthetic light responses (as O2 evolution) showed marked variations in apparent quantum yield (AQY) and light‐saturated rates for both exposed and semi‐exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that Fv/Fm declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m−2), indicating non‐photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, Fv/Fm recovered during the afternoon. The decrease in Fv/Fm was reduced from 0·72 to 0·64 under 24 mol photon m−2 d−1. The long–term recovery of photo‐synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17‐d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g−1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and Fv/Fm had recovered after 2d shading. The differences between timing of recovery for Fv/Fm during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long‐term photoinhibitory damage under natural conditions during the dry season.

Efficiency of Maize Dry Matter Production During Periods of Complete Leaf Area Expansion

AbstractCrop dry matter production, in the absence of other limiting factors, is determined by the absorption of photosynthetically active radiation (PAR) by a crop canopy and the efficiency with which absorbed PAR is converted into dry matter. This study was conducted to investigate the effects of plant density and phase of development on PAR absorption and conversion efficiency of two maize (Zea mays L.) hybrids. The maize hybrids Pioneer 3851 and Pioneer 3925 were grown in 1984 and 1985 at plant densities of 3.9, 6, and 10 plants m−2 in irrigated field experiments near Elora, Ontario, on a London loam (Aquic Hapludalf) soil that had been systematically tile drained. Conversion efficiency was estimated from rates of aboveground dry matter accumulation and PAR absorption from approximately 2 wk before to 6 wk after silking. PAR absorption was estimated from continuous measurements of incident PAR, PAR reflectance and PAR transmittance. Crop growth rates declined during the phase of development under study due to a decline in incident PAR and a decline in conversion efficiency from 5.8 to 3.5%; PAR absorptance was approximately 89% and remained fairly stable. Leaf area index among density‐hybrid treatments ranged from 2.0 to 4.8, and PAR absorption of the latter was 9% higher than that of the former. Although crop growth rates of the plant densities were not different during the grain‐filling period, differences in PAR absorption were too small to show a significant effect of plant density on conversion efficiency. Photosynthetic conversion efficiency appears to be influenced by phase of development, but the effects of plant density on efficiency appear to be small.

Modelling planting configuration and canopy architecture effects on diurnal light absorption changes in cotton

Abstract A simplified model that accounts for diurnal solar zenith and azimuth angle illumination changes and plant geometry effects on photosynthetically active radiation (PAR) was developed, h was tested using diurnal absorbed PAR (APAR) measurements acquired for cotton (Gossvpium hirsuium L.) grown at Weslaco, Texas, during the 1984 and 1986 growing seasons. Diurnal canopy reflectance and transmittance measurements were automatically collected at 3-minute intervals from about 9.00 to 14.50 local standard time (LST) using radiometers mounted 3·7m above the soil surface and PAR light bar sensors located beneath the crop canopy perpendicular to the crop rows. Plant growth measurements estimated by the model were significantly correlated with observed measurements during crop development. Model-estimated heliotropic leaf elevation trends agreed with published results for cotton canopies. APAR, relative to incident PAR, was generally lower for east-west rows than for north-south rows during crop development...

Estimation of radiation interception by winter wheat from measurements of leaf area

SummaryThe percentage (P) of incident photosynthetically active radiation (PAR) reaching the ground was measured in winter wheat crops throughout the season, and related to projected area index (L) of leaves, plus sheaths and stems when present. Between unfolding of the first leaf and appearance of the flag leaf the mean value for the exponential extinction coefficient (K) was 0·46±0·007. The value did not vary with the growth stage of the wheat and was scarcely affected by experimental treatments, apart from effects resulting from the presence of senescent tissues that intercepted light but whose area was not included in L. After anthesis, in crops having between 340 and 540 ears/m2, between 68 and 89% of incident PAR penetrated to below the ears. Relation between the further penetration to the base of the crop canopy and the area of green plus senescent leaf and stem tissue gave K = 0·50. The relation did not change significantly between anthesis and complete senescence.The relation between P and L was similar whether P was obtained from instantaneous measurements made near midday, 24 h integrals of PAR or percentage ground cover measured photographically.

Wheat Seedling Growth and Developmental Response to Incident Photosynthetically Active Radiation1

AbstractShade from residues of previous crops in high residue seedbeds may limit development of winter cereal seedlings. A growth chamber experiment with photosynthetic photon flux densities of 140, 275, or 500 µmol−2s−1 for 12 h each day was designed to reveal the growth and development patterns of winter wheat (Triticum aestivum L. em Thell.) under light limiting conditions. Daily totals of light received during winter months at 46°N Lat are within this range. Leaf and tiller growth and development were observed daily on 6 to 12 separately potted plants in each of seven treatments for 1 month after emergence. At 140 µmol m−2s−1, a higher fraction of specific tiller buds failed to elongate than at either higher irradiance. Failure to elongate one tiller bud did not influence the point of development of subsequent tillers. Plants developed at 0.0097,0.0117, and 0.0138 leaves per degree day (0°C base) under 140, 275, and 500 µmol m−2s−1 treatments, respectively. An average of only four tillers per plant were present after 27 days under 140 µmol m−2s−1 while 12 tillers per plant were produced under 500 µmol m−2s−1. The basic relationship between main stem leaf and individual tiller development was not changed by different amounts of light. Specific tillers appeared at essentially the same point in plant development regardless of irradiance. At 140 µmol m−2s−1, the slower leaf development and reduced tillering both contributed to reduce plant weights to only one‐fifth those from the 500 µmol m−2s−1 treatment. The irradiance required for maximum development rate of winter wheat seedlings exceeds that expected in shaded areas of a seedbed from November through February at 46° N Lat. Based on this experiment, irradiance will not change the point of development of specific tillers relative to main stem leaf development. Irradiance will influence the fraction of plants with specific tillers and the rate (leaves per degree day) of leaf appearance. Leaf development rate (leaves per degree day) appears to be buffered by short term (a few days) energy reserves. Winter wheat seedlings that are shaded over winter will skip a higher fraction of the early tillers (T0, T1) than unshaded seedlings, reducing the population of potentially large heads.

Vertical Diffusivity in the Stratified Layers of the Mixolimnion of Green Lake, Jamesville, NY

ABSTRACT The distribution of the turbulent component of vertical diffusivity for heat (Khz′), and the relative contributions of vertical mixing and solar radiation to heating, in the stratified layers of the mixolimnion of bio-genitally meromictic Green Lake, Jamesville, NY, were evaluated for a six month period of 1978. Field measurements included temperature and PAR (photo-synthetically active radiation) extinction profiles, collected biweekly on 13 occasions, and continuously monitored incident PAR. Khz′ varied temporally and vertically. The values were particularly low in the metalimnion, where the major mechanism of vertical heat transfer was absorption of solar radiation coupled with molecular diffusivity. Most of the heating in the hypolimnion was accomplished through turbulent processes. The vertical distribution of Khz′ was well described by Khz′ = b(N2)m, where b and m are ecosystem specific constants, and N is the Brunt-Vaisala frequency, supporting a previous theoretical prediction and the res...

Spine influences on PAR interception, stem temperature, and nocturnal acid accumulation by cacti

Abstract. The influence of spines on nocturnal acid accumulation was examined for two Crassulacean acid metabolism plants, the barrel cactus Ferocactus acanthodes (Lemaire) Britton & Rose var. lecontei (Engelm.) Lindsay and the cylindropuntia Opuntia bigelovii Engelm. var. bigelovii. The ambient photo‐synthetically active radiation (PAR) required for 90% saturation of nocturnal acid accumulation by O. bigelovii was 23 mol m−2 d−1 for stems that were 32% shaded by spines and 16 mol m−2 d−1 when the spines were removed. For F. acanthodes, 90% saturation occurred for an ambient PAR of 45 mol m−2 d−1 where the stem was 78% shaded by spines and 21 mol m−2 d−1 when the spines were removed. For comparison, the ambient PAR averaged 21 mol m−2 d−1 at various times of the year and stem positions for O. bigelovii fully exposed to solar irradiation. Although spines can reflect some PAR toward the stem surface, their presence decreases the PAR incident on the stem surface, thereby decreasing the nocturnal acid accumulation. For O. bigelovii, periodically removing the spines led to a 60% greater increase in stem volume over 21/2 years. The effect of variation in spine‐shading on stem surface temperature was determined for F. acanthodes using computer simulations and an energy budget model. A substantial reduction in daytime stem temperatures and a smaller increase in night‐time temperatures as the spine‐shading increased had very little influence on nocturnal acid accumulation for both winter and summer days. Thus, the main effect of spines on the metabolism of cacti is apparently to reduce the PAR incident on the stem surface and thereby to reduce productivity.

Assessing the interception of photosynthetically active radiation in winter wheat

Some current growth and yield models for wheat require methodology to estimate the daily quantity of photosynthetically active radiation ( PAR) captured by the canopy. The interception of PAR was examined throughout the growing season in two varieties of winter wheat ( Triticum aestivum L.) planted at three densities. Daily values of percent of incident PAR intercepted were influenced by cloud cover, when leaf area index ( LAI) values were small. At high values of LAI, the percent interception values were large and independent of the quality of incident PAR. A relationship was determined between daily percent of incident PAR intercepted and LAI, which is valid up to the time of rapid leaf senescence. A mathematical function was developed to quantify the relationship.

Characterization of light climate under canopies of intensively-cultured hybrid poplar plantations

Photosynthetically active radiation (PAR) was measured by quantum sensors in the open and in young hybrid poplar plantations. On clear summer days the incoming PAR approached 2000 μEm −2 s −1. From May to October, daily PAR in the open averaged 30–33 E m −2. Fully-foliated canopies transmitted only a fraction of 1% of the incoming PAR and leafless canopies transmitted less than 50%. Foliated canopies depleted the blue and red portions of the spectrum and allowed more green and near-infrared light to reach the forest floor. Sunflecks provided 66–89% of the total PAR under foliated canopies and may be important for the survival and longevity of the shaded leaves. During the growing season (May–September), hybrid poplar plantations may convert up to 8.5% of the incident PAR into dry matter.

LEAF ORIENTATION, RADIATION INTERCEPTION, AND NOCTURNAL ACIDITY INCREASES BY THE CAM PLANT AGAVE DESERTI (AGAVACEAE)

Both field measurements and a computer model were used to study the interception of photosynthetically active radiation (PAR) by Agave deserti (Engelm.), a desert CAM plant with a basal rosette of massive opaque leaves. PAR interception was determined in the winter and the summer for upper and lower leaf surfaces on a plant with about 60 leaves. Total daily PAR on the leaf surfaces was approximately 10 mol m‐2 for a winter day and 20 mol m‐2 for a summer day. For a PAR of 15 mol m‐2, the nocturnal increase in acidity was about 0.6 mol m‐2 for both leaf surfaces and various leaf orientations, except for the oldest most horizontal leaves where the increase was less than half as large. The acidity increase measured in the field was 90% saturated at 25 mol m‐2. Thus, daytime PAR in the desert is often limiting for the nocturnal acidity increase, especially for the lower leaf surfaces. Simulated tilting of the plant by 55° so that the vertical axis pointed to the sun at solar noon on a winter day increased the PAR incident on the upper surfaces of the leaves, but did not affect the total nocturnal increase in acidity by the whole plant. Although simulated removal of alternate leaves increased the PAR per unit leaf area for the remaining leaves, it reduced the total increase in nocturnal acidity of the whole plant by 31%. PAR interception by plants on slopes facing steeply north, east, or west was substantially reduced compared to the horizontal. Thus, the model proved to be quite useful for quantifying the relation between leaf orientation, PAR interception, and nocturnal increases in acidity by A. deserti, and it indicated that the lower frequency of plants on north‐ compared to south‐facing slopes was due to PAR limitations.

Interception of photosynthetically active radiation by cacti of different morphology.

Ribbing, orientation of cladodes, and variations in stem height were examined theoretically and in the field to see whether the morphology of various cacti could be interpreted as adaptations for intercepting photosynthetically active radiation (PAR). The total daytime PAR incident on different parts of the stem was related to the nocturnal increase in tissue acidity of these Crassulacean acid metabolism plants. The acidity change was 90% saturated for a total daytime PAR of about 22 mol m-2, which indicates that the vertical stems of cacti are often light-limited in the desert.PAR interception by a cactus with ribs of various depths was simulated. Ribbing led to more surface area but a lower PAR per unit area, so net carbon gain was little influenced.Although the cladodes of Opuntia basilaris Engelm. and Bigel. var. basilaris were randomly oriented, those of another platyopuntia, O. chlorotica Engelm. and Bigel, preferentially faced northsouth. Compared to facing east-west, cladodes facing N-S would receive 52% more PAR at the winter solstice, a time when water is generally available and overheating is not a problem in the Mojave desert habitat of O. chlorotica.The maximum height of Stenocereus gummosus (Engelm.) Gibs. & Horak varied from 0.7 m in northern Baja California (31°52'N) to 4.1 m at 23°47'N. Stem height was positively correlated (r 2=0.93) with the height of the surrounding vegetation and negatively correlated (r 2=0.91) with the PAR 1 m above the ground. The great plasticity in stem height permits S. gummosus to obtain sufficient PAR in the presence of subtropical trees in the southern part of its range and yet expend less biomass to reach unobstructed PAR in the northern part of its range, where the surrounding vegetation is much shorter.

Photosynthesis and Related Processes : Rabinowitch, Eugene I. Vol. 2, pt. 2: Kinetics of Photosynthesis (Continued); Addenda to Vol. 1 and Vol. 2, pt. 1. N.Y., Interscience, 1956. pp. 1209–2088

Accurate estimation of carbon fluxes across space and time is of great importance for quantifying global carbon balances. Current production efficiency models for calculation of gross primary production (GPP) depend on estimates of light-use-efficiency (LUE) obtained from look-up tables based on biome type and coarse-resolution meteorological inputs that can introduce uncertainties. Plant function is especially difficult to parameterize in the savanna biome due to the presence of varying mixtures of multiple plant functional types (PFTs) with distinct phenologies and responses to environmental factors. The objective of this study was to find a simple and robust method to accurately up-scale savanna GPP from local, eddy covariance (EC) flux tower GPP measures to regional scales utilizing entirely remote sensing oservations. Here we assessed seasonal patterns of Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation products with seasonal EC tower GPP (GPPEC) at four sites along an ecological rainfall gradient (the North Australian Tropical Transect, NATT) encompassing tropical wet to dry savannas.The enhanced vegetation index (EVI) tracked the seasonal variations of GPPEC well at both site- and cross-site levels (R2 = 0.84). The EVI relationship with GPPEC was further strengthened through coupling with ecosystem light-use-efficiency (eLUE), defined as the ratio of GPP to photosynthetically active radiation (PAR). Two savanna landscape eLUE models, driven by top-of-canopy incident PAR (PARTOC) or top-of-atmosphere incident PAR (PARTOA) were parameterized and investigated. GPP predicted using the eLUE models correlated well with GPPEC, with R2 of 0.85 (RMSE = 0.76 g C m− 2 d− 1) and 0.88 (RMSE = 0.70 g C m− 2 d− 1) for PARTOC and PARTOA, respectively, and were significantly improved compared to the MOD17 GPP product (R2 = 0.58, RMSE = 1.43 g C m− 2 d− 1). The eLUE model also minimized the seasonal hysteresis observed between green-up and brown-down in GPPEC and MODIS satellite product relationships, resulting in a consistent estimation of GPP across phenophases. The eLUE model effectively integrated the effects of variations in canopy photosynthetic capacity and environmental stress on photosynthesis, thus simplifying the up-scaling of carbon fluxes from tower to regional scale. The results from this study demonstrated that region-wide savanna GPP can be accurately estimated entirely with remote sensing observations without dependency on coarse-resolution ground meteorology or estimation of light-use-efficiency parameters.