Absorbed Photosynthetically Active Radiation Research Articles

Light absorption and light-use efficiency in mixtures of Abies alba and Picea abies along a productivity gradient

There is great interest in how species interactions change along spatial gradients in resource availability and climatic conditions. Many studies have shown that facilitation or complementary effects increase as growing conditions become harsher, however, several studies in forests have recently shown the opposite trend. Increasing complementary effects with improving growing conditions may result when species interactions influence light absorption or use. This hypothesis was tested in mixed-species forests of Abies alba Mill. and Picea abies (L.) Karst. in south-western Germany where complementary effects on growth increased as climatic conditions improved. The absorption of photosynthetically active radiation (APAR) by individual trees was predicted using a detailed light model. Light-use efficiency (LUE) of individual trees was estimated as basal area growth per unit APAR. APAR and LUE were modelled as functions of crown size, climatic variables and the species composition and density of the neighbourhood of individual trees. For a given tree size, APAR (for both species) and LUE (of A. alba trees) were greater for trees in mixed-species neighbourhoods than monospecific neighbourhoods and this complementary effect increased as climatic conditions improved and on sites with faster growing trees. Increases in APAR for A. alba probably resulted because shading from P. abies trees was less intense than that from other A. alba trees on the more productive site. The species composition of individual tree neighbourhoods did not influence relationships between stem diameter and crown diameter or height to the crown base, for either species. However, the height of each species, for a given stem diameter, increased as the proportion of a given tree’s neighbourhood basal area that was composed of the same species increased. This change in crown architecture could also have contributed to the complementarity effects in these stands. Increasing complementary effects with improving growing conditions is consistent with the stress-gradient hypothesis if competition for light is considered to be the stressor. This study shows that such a spatial pattern in species interactions could be associated with increasing differences in APAR or LUE between mixtures and monocultures as climatic conditions become more favourable.

Potentials and limitations for estimating daytime ecosystem respiration by combining tower-based remote sensing and carbon flux measurements

Vegetation carbon uptake and respiration constitute the largest carbon cycle of the planet with an annual turnover in the order of 120 GT. Currently, neither ecosystem carbon uptake (through photosynthesis) nor ecosystem carbon release (through respiration) can be measured directly during the daytime. Instead, flux-tower measurements rely on nighttime respiration based on the assumption of zero carbon uptake which are then projected to daytime using an exponential relationship to soil temperature at shallow soil depth. As an alternative to this approach, R could possibly also be determined from combining daytime eddy covariance measurements of net ecosystem production (NEP) and spectral observations of gross primary production (GPP). In previous work, we have shown that multi-angular observations can be used to determine GPP from the absorbed photosynthetically active radiation (APAR) and spectrally obtained observations of light-use efficiency (ε). The difference of NEP and GPP suggests that daytime respiration is greater and more dynamic than conventional estimates derived from nighttime flux values. Our findings also suggest that an accelerated ecosystem metabolism results in an exponential increase in respiration which eventually diminishes net ecosystem production. Respiration was also closely related to air and soil temperature. We conclude that tower-level spectral measurements provide considerable new insights into ecosystem fluxes as they allow independent yet complementary measurements of different aspects of the carbon and energy cycle.

Using a stand-level model to predict light absorption in stands with vertically and horizontally heterogeneous canopies

BackgroundForest ecosystem functioning is strongly influenced by the absorption of photosynthetically active radiation (APAR), and therefore, accurate predictions of APAR are critical for many process-based forest growth models. The Lambert-Beer law can be applied to estimate APAR for simple homogeneous canopies composed of one layer, one species, and no canopy gaps. However, the vertical and horizontal structure of forest canopies is rarely homogeneous. Detailed tree-level models can account for this heterogeneity but these often have high input and computational demands and work on finer temporal and spatial resolutions than required by stand-level growth models. The aim of this study was to test a stand-level light absorption model that can estimate APAR by individual species in mixed-species and multi-layered stands with any degree of canopy openness including open-grown trees to closed canopies.MethodsThe stand-level model was compared with a detailed tree-level model that has already been tested in mixed-species stands using empirical data. Both models were parameterised for five different forests, including a wide range of species compositions, species proportions, stand densities, crown architectures and canopy structures.ResultsThe stand-level model performed well in all stands except in the stand where extinction coefficients were unusually variable and it appears unlikely that APAR could be predicted in such stands using (tree- or stand-level) models that do not allow individuals of a given species to have different extinction coefficients, leaf-area density or analogous parameters.ConclusionThis model is parameterised with species-specific information about extinction coefficients and mean crown length, diameter, height and leaf area. It could be used to examine light dynamics in complex canopies and in stand-level growth models.

Simulating spatiotemporal dynamics of sichuan grassland net primary productivity using the CASA model and in situ observations.

Net primary productivity (NPP) is an important indicator for grassland resource management and sustainable development. In this paper, the NPP of Sichuan grasslands was estimated by the Carnegie-Ames-Stanford Approach (CASA) model. The results were validated with in situ data. The overall precision reached 70%; alpine meadow had the highest precision at greater than 75%, among the three types of grasslands validated. The spatial and temporal variations of Sichuan grasslands were analyzed. The absorbed photosynthetic active radiation (APAR), light use efficiency (ε), and NPP of Sichuan grasslands peaked in August, which was a vigorous growth period during 2011. High values of APAR existed in the southwest regions in altitudes from 2000 m to 4000 m. Light use efficiency (ε) varied in the different types of grasslands. The Sichuan grassland NPP was mainly distributed in the region of 3000–5000 m altitude. The NPP of alpine meadow accounted for 50% of the total NPP of Sichuan grasslands.

江苏省农作物最大光能利用率时空特征及影响因子分析

基于遥感数据的光能利用率模型被广泛应用于计算陆地生态系统的生产力，但其结果对最大光能利用率（<em>ε</em>_max）参数非常敏感。由于该参数存在明显的时空变异性，在区域尺度上难以确定。利用MODIS遥感数据、分县产量统计数据和VPM模型推算了2001-2010年江苏省各县逐年的农田<em>ε</em>_max，分析了其时空变化特征及其可能原因。结果表明，在2001-2010年期间，江苏省61个县区农田<em>ε</em>_max平均值的变化范围为0.757-3.435 g C/MJ，呈现北高南低、中间高四周较低的空间分布特征；各县区农田的<em>ε</em>_max都呈现出上升趋势，但在2001-2006年期间存在明显的年际波动，2002年、2004年和2006年的<em>ε</em>_max相对较低，2007年后全省农田<em>ε</em>_max稳定上升；<em>ε</em>_max的年际波动呈现由北向南递减的趋势；全省大部分地区<em>ε</em>_max的年际变化与单位耕地面积农用化肥施用量呈正相关性，苏北北部尤为明显；同时也与C₄作物产量所占比例相关。研究表明在利用光能利用率模型计算农田生产力时，需要发展能考虑<em>ε</em>_max时空变化的参数化方案。;The productivity is a critical component of the terrestrial carbon cycle. Remote sensing driven light use efficiency models have been widely utilized to calculate the productivity of terrestrial ecosystems. They calculate the productivity as a linear product of maximum light use efficiency <em>ε</em>_max, scaling factors of environmental stress, and absorbed photosynthetically active radiation (APAR). The outputs of these models are very sensitive to t maximum light use efficiency (<em>ε</em>_max), which represents the utilization rate of photosynthetically active radiation under ideal conditions without restrictions. However, it is difficult to determine this parameter properly since it is affected by numerous factors, including canopy nitrogen content, atmospheric concentration of CO₂, plant types and species, and exhibit significant spatial and temporal variations. In currently used light use efficiency models, this parameter only changes with plant types. Such simplified parameterization might induce large uncertainties in calculated productivity of terrestrial ecosystems at regional and global scales. In this study, MODIS reflectance data, county\level yield census data, locally observed meteorological data, and the vegetation photosynthesis model (VPM) were employed to derive annual mean county\level cropland <em>ε</em>_max in Jiangsu province for the period from 2001 to 2010. The county\level yield census data was converted into total net primary productivity of each county for estimating the <em>ε</em>_max parameter in the VPM model. Then, the spatial and temporal variations of <em>ε</em>_max and possible driving factors were analyzed were analyzed. The results show that the county\level means of cropland <em>ε</em>_max in 61 counties of Jiangsu province varied between 0.757-3.435 g C/MJ during the period from 2001 to 2010, with an overall gradient decreasing from the north to the south. It is higher in the central areas than in the surrounding areas. Annual mean cropland <em>ε</em>_max increased during the period from 2001 to 2010 in all 61 counties, but showed obvious interannual fluctuations during the period from 2001 to 2006. It was relatively lower in years 2002, 2004 and 2006 and steadily increased in all counties since 2007. The interannual fluctuations of <em>ε</em>_max were normally higher in the north than in the south. The annual means of <em>ε</em>_max were positively related to the amount of fertilizer used in per unit area of cultivated cropland in most counties, particularly in the northern areas of Jiangsu Province. The increasing rates of <em>ε</em>_max were positively correlated with the increasing rates of fertilizer utilization per unit area. The interannual fluctuations of <em>ε</em>_max were also related to the yield fraction of C₄ crops (corn). This study proves that it is of importance to develop a parameterization scheme accounting for the temporal and spatial variations of <em>ε</em>_max for improving the calculation of productivity in croplands using light use efficiency models and remote sensing data. It should be kept in mind that some factors might induce uncertainties estimated <em>ε</em>_max, such as the uncertainties in yield census data, the parameters used to converts yield into net primary productivity, methodologies used to calculate APAR and scaling factors of environmental stress. The underlying mechanisms for driving significant spatial and temporal variations of <em>ε</em>_max need further thorough investigation. Nevertheless, the findings in this study can provide a value reference for optimizing the calculation of carbon fluxes between the atmosphere and terrestrial ecosystems.

Variation of absorbed photosynthetically active radiation (APAR) and yield of different kharif rice cultivars influenced by date of transplanting

A field experiment was conducted at BCKV, Kalyani, West Bengal (Latitude 22°58’60 N, Longitude 88°28’60 E) to evaluate the performance of different rice cultivars grown during kharif season of 2010 and 2011. The experiment was laid down in a split-plot design where three dates of transplanting (D1: 15th June; D2: 29th June and D3: 13th July) for both the years were allotted as main plot treatment and three rice cultivars (V1: Satabdi ; V2: Baishmukhi ; V3: Pre-Release) was considered as sub-plot treatment. Date of transplanting significantly influenced the grain yield. It was at the highest under D1 which reduced gradually with delay in transplanting. Among different varieties, Satabdi produced the highest yield (4079 kg ha-1). Irrespective of variety, absorbed photosynthetically active radiation (APAR) value increased with the advancement of crop growing season and it was at the highest level during panicle initiation stage. In general, the highest APAR value (90.2 μMol m-2s-1) was recorded under pre-released variety during panicle initiation stage due to its leaf orientation which was declined by 14% and 19.5% respectively under Baishmukhi and Satabdi variety.

Ecological consequences of variation in precipitation: separating short‐ versus long‐term effects using satellite data

AbstractAimPrecipitation controls the production of semi‐arid plants through various mechanisms that operate at a range of time‐scales. Short‐term variation in precipitation affects vegetation through adjustments in plant physiology and leaf phenology, whereas long‐term effects are mediated by plant establishment and mortality, community composition and disturbance regimes. Our goal is to use remote sensing observations to separate the short‐ and long‐term effects of variation in precipitation on ecosystem production.LocationCalifornia, USA.MethodsWe used time series of gridded absorbed photosynthetically active radiation (APAR) to quantify the short‐ and long‐term responses of diverse ecosystems to variation in precipitation across large productivity and precipitation gradients. We investigated the relationships between temporal sensitivity of APAR to interannual variation in precipitation and mean annual precipitation (MAP), ecosystem properties and disturbance.ResultsAPAR increased with precipitation both interannually within locations and across locations with MAP. The slope of the interannual relationship, which reflects the sensitivity of APAR to short‐term fluctuations in precipitation, varied with climate, vegetation type and structure, and time since disturbance. The interannual APAR sensitivity decreased from c. 0.5 MJ m−2 mm−1 at a MAP of 300 mm year−1 to less than 0.05 MJ m−2 mm−1 at 1000 mm year−1. The slope of the spatial relationship, which reflects the long‐term sensitivity of APAR to climate, decreased from c. 2.5 MJ m−2 mm−1 at 300 mm year−1 MAP to c. 0.6 MJ m−2 mm−1 at 1000 mm year−1. The initial physiological and leaf area effects of a precipitation shift were amplified five‐fold over time by gradual changes in population density and species composition.Main conclusionThe impact of a hydroclimatic shift on the primary production, structure and function of California's terrestrial ecosystems depends heavily on time‐scale and how rapidly changes in plant population density and community composition can occur.

Athletic Field Paint Color Differentially Alters Light Spectral Quality and Bermudagrass Photosynthesis

ABSTRACTPainting of athletic fields is widespread throughout the world and can often cause declines in turfgrass health. Visible light and photosynthesis share the same wavelengths (400–700 nm), and it was hypothesized that alterations in visible light to produce specific colors would lead to reductions in photosynthetically active radiation (PAR) and total canopy photosynthesis (TCP). Lab experiments using a spectroradiometer and LICOR 1800‐12 integrating sphere examined the impacts of 10 colors of athletic field paint on PAR as well as wavelengths within PAR. These colors were then applied weekly for 5 wk to ‘Tifway’ bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt Davy], and TCP was measured using a gas exchange system 24 h after each application. Spectroradiometry analyses revealed the significant effects of paint color (P ≤ 0.001) on reflection, transmission, and absorption of PAR. Lighter colors including white, yellow, orange, and red reflected 47 to 92% of PAR while darker colors including green, black, and dark blue absorbed 87 to 95% of PAR. Accompanying gas exchange measurements revealed that TCP was most negatively correlated with absorption of PAR (r = −0.959, P ≤ 0.001) and that darker colors negatively impact TCP more than lighter colors. The results clearly indicate that damage to turfgrasses with long‐term painting will be difficult to avoid, and this is particularly true with darker colors of paint.

Variations in fraction of absorbed photosynthetically active radiation and comparisons with MODIS data in burned black spruce forests of interior Alaska

Absorption of photosynthetically active radiation (PAR) by vegetation was observed in two burned black spruce forests, one and seven years after wildfire, in interior Alaska along with several vegetation properties. This study considered PAR absorption by mosses by examining the relationship between PAR transmittance and fractional coverage of green vegetation. Our results suggest that mosses absorbed a considerable fraction of incoming PAR in the burned forests, which cannot be neglected in evaluating the fraction of absorbed PAR (FPAR). The relationships between FPAR and vegetation indices revealed that enhanced vegetation index (EVI) may be suitable for expressing the spatial and temporal variation of FPAR, regardless of stand age after wildfire. The comparison between the observed in situ FPAR and FPAR derived from Moderate Resolution Imaging Spectroradiometer (MODIS FPAR) clearly showed that MODIS FPAR was highly overestimated. The most likely reason for the overestimation was identified as misclassification of land cover type. The current regional estimation of photosynthesis in boreal region based on the light-use efficiency approach and MODIS FPAR is probably overestimated, and an accurate distribution of FPAR is desired for clarifying the regional carbon exchange in boreal forests.

Fertilization and irrigation effects on tree level aboveground net primary production, light interception and light use efficiency in a loblolly pine plantation

Fertilization and irrigation may substantially increase productivity of forests by increasing stand leaf area index and the efficiency of converting intercepted light into wood biomass. This stand-level growth response is the summation of individual tree responses, and these tree-level responses are often non-linear, resulting from shifting in the intensity of competition and dominance. We examined tree-level responses of aboveground net primary production (ANPP), absorbed photosynthetically active radiation (APAR) and (light use efficiency) LUE in relation to tree size class to explore how stand-level outcomes depend on shifting patterns among trees. We evaluated the production ecology of a nine-year-old loblolly pine (Pinus taeda L.) plantation, 2years after the initiation of treatments: control, irrigation, fertilization and irrigation+fertilization. We measured tree level ANPP, simulated APAR for individual tree crowns using the MAESTRA process-based model and calculated LUE (ANPP/APAR) in relation to tree size to explore the influence of tree dominance on both light capture and light use efficiency. Fertilization and irrigation+fertilization strongly increased both APAR and LUE, in contrast to little effect of irrigation alone. Tree size had a strong influence on APAR and LUE across all treatments; the largest 20% trees showed 3.4 times greater ANPP when compared to the smallest 20% trees, with 66% resulting from higher APAR, and 34% from higher LUE, than the smallest 20% of trees. Fertilization increased the growth of the largest 20% trees 2-fold (8.6kg tree−1year−1), with 29% of the increase resulting from higher APAR (13.7GJ tree−1year−1), and 71% from higher LUE (0.63gMJ−1), relative to the largest trees in the control treatment (4.3kg tree−1year−1, 11GJ tree−1year−1 and 0.39gMJ−1, respectively). Irrigation and fertilization tripled production (13.2kg tree−1year−1) of the largest trees with an even greater proportional contribution from increased LUE (15.1GJ tree−1year−1, 85% response contribution; APAR 0.87gMJ−1, 15% response contribution). Overall, large trees grow faster than smaller trees because of greater light capture, whereas the greater response of large trees to treatments resulted more from increased efficiency of using light.

Tree and stand light use efficiencies over a full rotation of single- and mixed-species Eucalyptus grandis and Acacia mangium plantations

Understanding the light absorption and light use efficiency of each species at the tree scale is essential to disentangle the effects of intra- and inter-species interactions on productivity in mixed-species forest plantations. A complete randomized block design was set up using Eucalyptus grandis (E) and Acacia mangium (A), which is a N2-fixing species, planted in monospecific stands (100A, 100E) and in additive (25A:100E, 50A:100E, 100A:100E) and replacement (50A:50E) mixtures. Tree size and biomass were monitored over the complete rotation (6years). The absorbed photosynthetically active radiation (APAR) for each tree in the experiment was simulated over the full rotation with the MAESTRA model. Measurements of tree leaf area, leaf angle distributions, leaf area density, and leaf and soil optical properties were performed to parameterize this model. The APAR and the ratio of stem biomass increment divided by the APAR (which is a measure of the Light Use Efficiency [LUE] for stem production) were calculated at tree and plot scales for each year of the rotation. The LUE of the 100E stand increased with age until stabilizing at 4years of age, while the LUE of the 100A stand decreased between 2 and 4years of age and increased between the two last years of the rotation. Eucalyptus trees dominated Acacia trees in mixed plantations. The stratification of the canopy led to an increase of stand Leaf Area Index (LAI) and APAR compared to monospecific plantations. However, both Eucalyptus and Acacia LUE decreased at the end of the rotation in the mixed-species stands, with the decrease occurring more markedly in Acacia, and the final stem biomass of the stand was not enhanced in mixed-species plantations compared with the average of the pure stands. Our results indicate that a stratified canopy may offer the potential benefit of capturing more light in mixed-species forests, but this may be negated if another resource deficiency prevents trees from converting intercepted radiation into dry matter. Mixed-species plantations should be established at sufficiently rainfed sites to maximize LUE, and appropriate fertilizer regimes should be applied.

The use of precipitation intensity in estimating gross primary production in four northern grasslands

Remote sensing is a useful tool for the estimation of gross primary production (GPP) in terrestrial ecosystems at regional to global scales. One limitation of remote sensing based GPP models is the inappropriate characterizing of precipitation impacts. In this study, we showed positive relationship between the monthly flux-measured GPP of four grasslands ecosystems and the precipitation intensity, which was calculated from dividing the monthly sums of precipitation by the half-hourly precipitation frequency. Suggested by this finding, two remote sensing based GPP models, i.e. the greenness and radiation model (GR) and the temperature and greenness (TG) model, were selected to test the potential of incorporating this precipitation intensity for the estimation of monthly GPP. A scaled precipitation intensity was proposed by normalizing a multi-year maximum precipitation intensity, considering its dynamical ranges across sites and regions. Results indicated that by adding of this scalar, the revised models can provide better monthly GPP estimates with average 10% improvements in precisions compared to their original outputs. A further analysis showed that such better performances of the revised models can be attributed to the positive relationship between precipitation intensity and the absorbed photosynthetically active radiation (APAR). However, no evident response has been observed on the light use efficiency (LUE), indicating the LUE and precipitation intensity relationship may differ across species and ecoregions. To the best of our knowledge, this is the first report of the potential use of precipitation intensity in the remote sensing based GPP models and it will be useful for the development of future models that can better predict GPP in the context of future precipitation regimes.

Data assimilation of photosynthetic light-use efficiency using multi-angular satellite data: I. Model formulation

Forest photosynthetic exchange rates at landscape scales have proven difficult to either accurately measure or estimate. Recent developments (Hall et al., 2011, 2008; Hilker et al., 2011a, 2010a) permit us to infer photosynthetic forest light use efficiency (ε) using multi-angle measurements of photochemical reflectance index (PRI) from the CHRIS/PROBA satellite imaging spectrometer, thus completing a long sought-after capability to remotely sense the major inputs driving gross primary production GPP i.e., ε and absorbed photosynthetically active radiation (APAR). In this first of two companion papers we introduce the theoretical underpinnings of an innovative approach that utilizes our recent developments to produce remotely sensed and spatially explicit maps of ε and GPP from space, and a data assimilation approach to extend the spatially explicit maps to diurnal, daily and annual time scales. We quantify GPP using the traditional radiation-limited approach of Monteith (1972); however we apply it in an innovative way. [I] Using CHRIS/PROBA we quantify ε at each satellite overpass for a 625km2 area at 30m resolution. [II] We use a novel physiologically-based multivariate function of APAR, temperature and water vapor pressure deficit model (described herein) and use it to down-regulate ε at 30 minute intervals. [III] We use the CHRIS/PROBA images of spatial variation in ε, and NDVI to quantify APAR, hence produce snapshots of GPP. We use a data assimilation approach to extend ε and GPP to temporally continuous and spatially contiguous maps of vegetation carbon uptake.In the second part of this study (Hilker et al., 2011b) we demonstrate and validate our approach over eight different forest flux tower sites in North America.

Estimating fraction of photosynthetically active radiation of corn with vegetation indices and neural network from hyperspectral data

The fraction of photosynthetically active radiation (FPAR) is a key variable in the assessment of vegetation productivity and land ecosystem carbon cycles. Based on ground-measured corn hyperspectral reflectance and FPAR data over Northeast China, the correlations between corn-canopy FPAR and hyperspectral reflectance were analyzed, and the FPAR estimation performances using vegetation index (VI) and neural network (NN) methods with different two-band-combination hyperspectral reflectance were investigated. The results indicated that the corn-canopy FPAR retained almost a constant value in an entire day. The negative correlations between FPAR and visible and shortwave infrared reflectance (SWIR) bands are stronger than the positive correlations between FPAR and near-infrared band reflectance (NIR). For the six VIs, the normalized difference vegetation index (NDVI) and simple ratio (SR) performed best for estimating corn FPAR (the maximum R2 of 0.8849 and 0.8852, respectively). However, the NN method estimated results (the maximum R2 is 0.9417) were obviously better than all of the VIs. For NN method, the two-band combinations showing the best corn FPAR estimation performances were from the NIR and visible bands; for VIs, however, they were from the SWIR and NIR bands. As for both the methods, the SWIR band performed exceptionally well for corn FPAR estimation. This may be attributable to the fact that the reflectance of the SWIR band were strongly controlled by leaf water content, which is a key component of corn photosynthesis and greatly affects the absorption of photosynthetically active radiation (APAR), and makes further impact on corn-canopy FPAR.

Effect of thinning, pruning and nitrogen fertiliser application on light interception and light-use efficiency in a young Eucalyptus nitens plantation

Thinning, pruning and fertiliser application are used by forest managers to increase growth rates of retained trees and to improve log quality. Understanding whether growth responses are due to changes in light interception, light-use efficiency (LUE) or both, and whether they are subject to treatment interactions could assist the design of more productive and light-use efficient plantations. Two levels each of thinning, pruning and nitrogen fertiliser treatments were applied in factorial design to a Eucalyptus nitens plantation in south-eastern Australia at age 3.2years, and responses measured to age 8.1years. Treatments were: unthinned, or thinned from ca. 900 to 300treesha−1; unpruned, or 50% of the live crown length pruned of the largest-diameter 300 potential sawlog crop treesha−1; and nil, or 300kgha−1 N fertiliser. Light-use efficiency was defined as annual above-ground biomass (AGB) or stem-wood growth per unit of absorbed photosynthetically active radiation (APAR). Growth responses were associated with changes in both light interception and LUE. There were interactions between thinning and pruning: thinning effects were weaker for APAR, and stronger for LUE for pruned than unpruned trees. During the first year after treatment, thinning increased AGB growth of the 200 largest-diameter sawlog crop trees (SCT200) by 34% (7.1–9.5Mgha−1year−1), APAR200 by 24% (0.77–0.95GJm−2year−1), and LUE200 by 13% (0.93–1.05gMJ−1); fertiliser application increased AGB200 growth by 23% (7.4–9.1Mgha−1year−1), APAR200 by 6% (0.83–0.89GJm−2year−1), and LUE200 by 13% (0.93–1.05gMJ−1). Thinning and fertiliser application both increased APAR200 by increasing tree leaf areas, and increases in LUE were associated with increases in photosynthesis. Pruning reduced tree leaf area by about 75%, but AGB200 growth and APAR200 were only reduced by 12% (8.8–7.7Mgha−1year−1) and 37% (1.06–0.67GJm−2year−1), respectively, and LUE200 increased by 36% (0.84–1.14gMJ−1) during the first year after treatment. Pruning increased LUE by removing the least efficient lower canopy foliage, and by increasing the efficiency of the remaining foliage. This study shows how changes in stand structure, crown architecture and tree nutrition can be used to alter APAR and LUE, and improve our understanding of responses to silvicultural interventions in eucalypt plantations managed for solid-wood products.

Modified light use efficiency model for assessment of carbon sequestration in grasslands of Kazakhstan: combining ground biomass data and remote-sensing

A modified light use efficiency (LUE) model was tested in the grasslands of central Kazakhstan in terms of its ability to characterize spatial patterns and interannual dynamics of net primary production (NPP) at a regional scale. In this model, the LUE of the grassland biome (ϵn) was simulated from ground-based NPP measurements, absorbed photosynthetically active radiation (APAR) and meteorological observations using a new empirical approach. Using coarse-resolution satellite data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), monthly NPP was calculated from 1998 to 2008 over a large grassland region in Kazakhstan. The modelling results were verified against scaled up plot-level observations of grassland biomass and another available NPP data set derived from a field study in a similar grassland biome. The results indicated the reliability of productivity estimates produced by the model for regional monitoring of grassland NPP. The method for simulation of ϵn suggested in this study can be used in grassland regions where no carbon flux measurements are accessible.

How does agricultural management modify ecosystem services in the argentine Pampas? The effects on soil C dynamics

Crop management modifies the structure and functioning of the ecosystems. C dynamics has been identified as a key intermediate or support ecosystem service that is profoundly altered by agricultural practices. The temperate grasslands of Argentina, the Pampas, are one of the main crop production regions of the world. Crop sequence, tillage and fertilization change inputs and outputs and, consequently the whole C dynamics. Our objectives in this article were (i) to provide a spatially explicit characterization, based on remotely sensed data, of crop sequences and tillage systems in the Rolling Pampas, (ii) to evaluate changes in C gains by computing the absorbed photosynthetic active radiation (APAR) from NDVI data of different crop, (iii) to evaluate the soil organic carbon (SOC) balance of different management schemes (crop sequence, conventional tillage vs. no till and three levels of nitrogen fertilization) using the CENTURY model, and (iv) to estimate the changes in SOC at a regional level. The results showed that 54% of the area was under continuous agriculture, with only two crop rotations occupying 61% of the area, and the main tillage system was no-tillage (73% of the area analyzed). Annual APAR was lower in crops than in rangelands, except for wheat-soybean double crop. Based on CENTURY simulations the crop management which had a most negative SOC balance (SOC reference value (100%)=79tha−1) was crop sequence “maize/soybean” under conventional tillage and with no fertilizer application (37% losses of SOC in 60 years). The management that presented the most positive SOC balance was “soybean/wheat-soybean double crop (6 years) pasture (4 years)” under no till and with high fertilization (10% increase of SOC in 60 years). A positive and linear relationship was found between APAR estimates derived from satellite data and simulated SOC providing basis for a quantitative hypothesis on the importance of C inputs on SOC's dynamics. At regional scale, if crop sequences proportions remain constant, the lost of SOC would average a 15% in 60 years.

Spatial and Temporal Variability of Net Primary Productivity (NPP) over Terrestrial Biosphere of India Using NOAA-AVHRR Based GloPEM Model

The monitoring of terrestrial carbon dynamics is important in studies related with global climate change. This paper presents results of the inter-annual variability of Net Primary Productivity (NPP) from 1981 to 2000 derived using observations from NOAA-AVHRR data using Global Production Efficiency Model (GloPEM). The GloPEM model is based on physiological principles and uses the production efficiency concept, in which the canopy absorption of photosynthetically active radiation (APAR) is used with a conversion “efficiency” to estimate Gross Primary Production (GPP). NPP derived from GloPEM model over India showed maximum NPP about 3,000 gCm−2year−1 in west Bengal and lowest up to 500 gCm−2year−1 in Rajasthan. The India averaged NPP varied from 1,084.7 gCm−2year−1 to 1,390.8 gCm−2year−1 in the corresponding years of 1983 and 1998 respectively. The regression analysis of the 20 year NPP variability showed significant increase in NPP over India (r = 0.7, F = 17.53, p < 0.001). The mean rate of increase was observed as 10.43 gCm−2year−1. Carbon fixation ability of terrestrial ecosystem of India is increasing with rate of 34.3 TgC annually (t = 4.18, p < 0.001). The estimated net carbon fixation over Indian landmass ranged from 3.56 PgC (in 1983) to 4.57 PgC (in 1998). Grid level temporal correlation analysis showed that agricultural regions are the source of increase in terrestrial NPP of India. Parts of forest regions (Himalayan in Nepal, north east India) are relatively less influenced over the study period and showed lower or negative correlation (trend). Finding of the study would provide valuable input in understanding the global change associated with vegetation activities as a sink for atmospheric carbon dioxide.

Development of a model for the estimation of photosynthetically active radiation from geostationary satellite data in a tropical environment

This paper presents a model for the estimation of photosynthetically active radiation (PAR) from geostationary satellite data. The model is aimed to estimate the monthly average hourly PAR in a tropical environment. This model represents a physical relation of PAR incident on the earth's surface and satellite-derived earth-atmospheric albedo together with the absorption and scattering coefficients of various atmospheric constituents. The earth-atmospheric albedo was obtained from the Multifunctional Transport Satellite-1R (MTSAT-1R). The absorption of PAR by water vapor, an important process for the tropics, was computed from the ambient temperature and relative humidity. The absorption of PAR by aerosols was estimated by using the visibility data and aerosol optical properties obtained from the Aerosol Robotic Network (AERONET) of NASA in this region. The total column ozone from the Ozone Monitoring Instrument onboard of AURA satellite (OMI/AURA) was used for the estimation of the absorption of PAR by ozone. The model was validated against the monthly average hourly PAR from measurements at four solar radiation measuring stations situated in the tropical environment of Thailand. The values of the monthly average hourly PAR estimated from the model and those obtained from the measurement were in good agreement, with the root mean square error (RMSE) and mean bias error (MBE) of 9.8% and 0.6%, respectively. After the validation, the model was employed to estimate the monthly average hourly PAR over Thailand using a 4-year period of data from MTSAT-1R and other ancillary surface data. Values of the monthly average hourly PAR were presented as maps showing the geographical distribution of PAR. These maps reveal the diurnal and seasonal variation of PAR over the country.

Spectral attenuation of ultraviolet and visible radiation in lakes in the Yunnan Plateau, and the middle and lower reaches of the Yangtze River, China

Solar radiation spectra were measured in China in 2006 and 2007 in situ in 16 lakes with 42 stations in the Yunnan Plateau (YP) and in 4 lakes with 41 stations in the middle and lower reaches of the Yangtze River (YR) to determine the principal factors influencing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). The diffuse attenuation coefficient for PAR (K(d)(PAR)) varied from 0.19 to 2.88 m(-1) in the YP, which was significantly lower than the values that were measured in the YR, which ranged from 0.55 to 4.52 m(-1) (t-test, p < 0.001). In the UV-B region (320 nm), K(d)(320) ranged from 0.59 to 18.56 m(-1) in the YP, significantly lower than that in the YR, which ranged from 4.80 to 20.64 m(-1) (t-test, p < 0.001). The corresponding 1% attenuation depths ranged from 0.25 to 7.81 m in the YP lakes and 0.22 to 0.96 m in the YR lakes (t-test, p < 0.001). Partial correlation analyses were used to partition the importance of every optically-active substance on UVR and PAR attenuation in the two different regions. In the YP lakes, chromophoric dissolved organic matter (CDOM) absorption explained most of the variability in the UV-B (320 nm) and UV-A (360 nm) attenuations (partial correlation: r = 0.911, p<0.001; r = 0.820, p<0.001, respectively). Tripton and phytoplankton made no significant contributions to UVR attenuation. Tripton, phytoplankton and CDOM all made significant contributions to PAR attenuation with the highest partial correlation corresponding to tripton. In the YR lakes, tripton, phytoplankton, and CDOM all made significant contributions to UVR attenuation. The contribution of CDOM to UVR decreased with increasing wavelength, but the contribution of tripton and phytoplankton to UVR increased with increasing wavelength. Tripton contributed the most to UV-A attenuation. Tripton and phytoplankton made significant contributions to PAR attenuation with a higher partial correlation for tripton but CDOM had no significant contribution to PAR attenuation. The absorption and attenuation data of UVR and PAR in this study showed a shift from absorption-dominated K(d) in the YP lakes to scattering-dominated K(d) in YR lakes.