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Forage quality is an important regulator of livestock performance also determining the grazing capacity in grasslands and pastures. The objective of this work was to develop spectral normalized indices to accurately predict canopy nitrogen (N), neutral detergent fibre (NDF), and acid detergent fibre (ADF) concentrations and in vitro dry matter digestibility (IVDMD) in three forage species, at two phenological stages and under two fertilization conditions. To select indices with the highest possible independence from canopy structure, we prioritized the selection of indices that were stable at both leaf and canopy scales and evaluated if the best selected indices were correlated with selected leaf and canopy structural traits and leaf water content. All possible normalized indices, based on the reflectance and the first difference reflectance, for the 400–2400 nm spectral range were related through simple regression models with N, NDF, and ADF concentrations and IVDMD. The index that combined the first difference reflectance in the 685 and 1770 nm wavelengths was found to be a potentially useful index to predict canopy N concentration under different field conditions. The best indices selected to predict canopy NDF and ADF concentration and IVDMD, based on the reflectance around 2120–2145 and 2250–2260 nm, had limited application and appeared to be suitable only to identify gross differences in fibre and IVDMD. Future studies should analyse how the best selected indices behave under field lighting conditions and for a wide range of species, phenological stages, and variations in canopy structural traits. [ABSTRACT FROM AUTHOR]

Accurately assessing terrestrial gross primary productivity (GPP) is crucial for characterizing the climate-carbon cycle. Remotely sensing the photochemical reflectance index (PRI) across vegetation functional types and spatiotemporal scales has received increasing attention for monitoring photosynthetic performance and simulating GPP over the last two decades. The factors confounding PRI variation, especially on long timescales, however, require the improvement of PRI understanding to generalize its use for estimating carbon uptake. In this review, we summarize the most recent publications that have reported the factors affecting PRI variation across diurnal and seasonal scales at foliar, canopy and ecosystemic levels; synthesize the reported correlations between PRI and ecophysiological variables, particularly with radiation-use efficiency (RUE) and net carbon uptake; and analyze the improvements in PRI implementation. Long-term variation of PRI could be attributed to changes in the size of constitutive pigment pools instead of xanthophyll de-epoxidation, which controls the facultative short-term changes in PRI. Structural changes at canopy and ecosystemic levels can also affect PRI variation. Our review of the scientific literature on PRI suggests that PRI is a good proxy of photosynthetic efficiency at different spatial and temporal scales. Correcting PRI by decreasing the influence of physical or physiological factors on PRI greatly strengthens the relationships between PRI and RUE and GPP. Combining PRI with solar-induced fluorescence (SIF) and optical indices for green biomass offers additional prospects. [ABSTRACT FROM AUTHOR]

Question How is the magnitude and seasonality of carbon uptake affected by the replacement of native grasslands by eucalyptus plantations? Location Río de la Plata Grasslands in Argentina and Uruguay. Methods A total of 115 paired sites of fast-growing Eucalyptus grandis plantations and adjacent grasslands were used to characterize the magnitude and seasonality of (1) radiation interception by canopies and (2) above-ground net primary productivity based on a time series of MODIS-derived normalized difference vegetation index ( NDVI). The response of NDVI to precipitation was explored across temporal scales. Results NDVI in afforested vs. grassland plots presented higher annual averages (1.3-fold), lower seasonal ranges (average relative range of 0.11 vs. 0.29) and delayed growing seasons (2-month shift). Temporally, NDVI was positively associated with precipitation input, showing a correlation with longer periods of precipitation accumulation in tree plantations compared to grasslands (> 7 vs. 2-3 months). Estimated average annual above-ground net primary productivity ( ANPP) almost quadrupled as a consequence of replacing grasslands by tree plantations (∼4 vs. ∼17 Mg dry matter. ha−1·yr−1), and this difference was evidenced throughout the whole study period. Conclusions Afforested grasslands intercept more radiation and have higher and more stable ANPP throughout the year, probably as a result of major changes in leaf phenology and root distribution patterns, which in turn allowed better access to water. Changes in carbon uptake can influence climate/biosphere feedbacks and should be considered in land-use planning, especially when grassland afforestation is recommended as a tool to mitigate global warming. [ABSTRACT FROM AUTHOR]

Aim The controls of gross radiation use efficiency (RUE), the ratio between gross primary productivity (GPP) and the radiation intercepted by terrestrial vegetation, and its spatial and temporal variation are not yet fully understood. Our objectives were to analyse and synthesize the spatial variability of GPP and the spatial and temporal variability of RUE and its climatic controls for a wide range of vegetation types. Location A global range of sites from tundra to rain forest. Methods We analysed a global dataset on photosynthetic uptake and climatic variables from 35 eddy covariance (EC) flux sites spanning between 100 and 2200 mm mean annual rainfall and between −13 and 26°C mean annual temperature. RUE was calculated from the data provided by EC flux sites and remote sensing (MODIS). Results Rainfall and actual evapotranspiration (AET) positively influenced the spatial variation of annual GPP, whereas temperature only influenced the GPP of forests. Annual and maximum RUE were also positively controlled primarily by annual rainfall. The main control parameters of the growth season variation of gross RUE varied for each ecosystem type. Overall, the ratio between actual and potential evapotranspiration and a surrogate for the energy balance explained a greater proportion of the seasonal variation of RUE than the vapour pressure deficit (VPD), AET and precipitation. Temperature was important for determining the intra-annual variability of the RUE at the coldest energy-limited sites. Main conclusions Our analysis supports the idea that the annual functioning of vegetation that is adapted to its local environment is more constrained by water availability than by temperature. The spatial variability of annual and maximum RUE can be largely explained by annual precipitation, more than by vegetation type. The intra-annual variation of RUE was mainly linked to the energy balance and water availability along the climatic gradient. Furthermore, we showed that intra-annual variation of gross RUE is only weakly influenced by VPD and temperature, contrary to what is frequently assumed. Our results provide a better understanding of the spatial and temporal controls of the RUE and thus could lead to a better estimation of ecosystem carbon fixation and better modelling. [ABSTRACT FROM AUTHOR]

The estimation of the carbon balance in ecosystems, regions, and the biosphere is currently one of the main concerns in the study of the ecology of global change. Current remote sensing methodologies for estimating gross primary productivity are not satisfactory because they rely too heavily on (i) the availability of climatic data, (ii) the definition of land-use cover, and (iii) the assumptions of the effects of these two factors on the radiation-use efficiency of vegetation (RUE). A new methodology is urgently needed that will actually assess RUE and overcome the problems associated with the capture of fluctuations in carbon absorption in space and over time. Remote sensing techniques such as the widely used reflectance vegetation indices (e.g. NDVI, EVI) allow green plant biomass and therefore plant photosynthetic capacity to be assessed. However, there are vegetation types, such as the Mediterranean forests, with a very low seasonality of these vegetation indices and a high seasonality of carbon uptake. In these cases it is important to detect how much of this capacity is actually realized, which is a much more challenging goal. The photochemical reflectance index (PRI) derived from freely available satellite information (MODIS sensor) presented for a 5-year analysis for a Mediterranean forest a positive relationship with the RUE. Thus, we show that it is possible to estimate RUE and GPP in real time and therefore actual carbon uptake of Mediterranean forests at ecosystem level using the PRI. This conceptual and technological advancement would avoid the need to rely on the sometimes unreliable maximum RUE. [ABSTRACT FROM AUTHOR]

Estimates of carbon uptake at the continental scale become urgently needed as the role of countries as net sinks or sources of carbon gains political and economic importance. Despite uncertainties related to radiation use efficiency, the amount of photosynthetically active radiation (PAR) intercepted by the canopy is a reliable estimator of primary production. Theoretical and empirical data support the relationship between the Normalized Difference Vegetation Index (NDVI) derived from the Advanced Very High Resolution Radiometer sensor on National Oceanic & Atmospheric Administration satellites and the fraction of PAR intercepted by green canopies. It is shown, for the period 1981-2000, that there is an overall increase in the radiation intercepted by the canopy over South America by 1.3%, with rainforests making the largest absolute contribution (45%), followed by savannas (23%). Under conditions of minimal agricultural use, disturbance and anthropogenic N deposition, humid temperate forests showed the highest proportional increase in NDVI during the last two decades (4.9%). Deserts showed a net reduction in NDVI relative to the 1981-1985 average (-4.4%). The expansion of agriculture over the last two decades was associated with NDVI reductions over subtropical forests. NDVI trends in South American region highlight a biome-dependent imprint of major global change noticeable in only two decades. [ABSTRACT FROM AUTHOR]