No abstract

No abstract

No abstract

No abstract

No abstract

Solar-induced chlorophyll fluorescence (SIF), which can be used as a novel proxy for estimating gross primary production (GPP), can be effectively retrieved using ground-based, airborne and satellite measurements. Absorbed photosynthetically active radiation (APAR) is the key bridge linking SIF and GPP. Remotely sensed SIF at the canopy level (SIF canopy ) is only a part of the total SIF emission at the photosystem level. An SIF-based model for GPP estimation would be strongly influenced by the fraction of SIF photons escaping from the canopy ( f esc ). Understanding the response of SIF canopy to the absorbed photosynthetically active radiation absorbed by chlorophyll (APAR chl ) is a key step in estimating GPP but, as yet, this has not been well explored. In this study, we aim to investigate the relationship between remotely sensed SIF canopy and APAR chl based on simulations made by the Soil Canopy Observation Photosynthesis Energy fluxes (SCOPE) model and field measurements. First, the ratio of the fraction of the absorbed photosynthetically active radiation absorbed by chlorophyll (fPAR chl ) to the fraction of absorbed photosynthetically active radiation absorbed by green leaves (fPAR green ) is investigated using a dataset simulated by the SCOPE model. The results give a mean value of 0.722 for Cab at 5 µg cm −2 , 0.761 for Cab at 10 µg cm −2 and 0.795 for other Cab content (ranging from 0.71 to 0.81). The response of SIF canopy to APAR chl is then explored using simulations corresponding to different biochemical and biophysical conditions and it is found that SIF canopy is well correlated with APAR chl . At the O 2 -A band, for a given plant type, the relationship between SIF canopy and APAR chl can be approximately expressed by a linear statistical model even for different values of the leaf area index (LAI) and chlorophyll content, whereas the relationship varies with the LAI and chlorophyll content at the O 2 -B band. Finally, the response of SIF canopy to APAR chl for different leaf angle distribution (LAD) functions is investigated using field observations and simulations; the results show that f esc is larger for a planophile canopy structure. The values of the ratio of SIF canopy to APAR chl are 0.0092 ± 0.0020, 0.0076 ± 0.0036 and 0.0052 ± 0.0004 µm −1 sr −1 for planophile vegetables/crops, planophile grass and spherical winter wheat, respectively, at the O 2 -A band. At the O 2 -B band, the ratios are 0.0063 ± 0.0014, 0.0049 ± 0.0030 and 0.0033 ± 0.0004 µm −1 sr −1 , respectively. The values of this ratio derived from observations agree with simulations, giving values of 0.0055 ± 0.0002 and 0.0068 ± 0.0001 µm −1 sr −1 at the O 2 -A band and 0.0032 ± 0.0002 and 0.0047 ± 0.0001 µm −1 sr −1 at the O 2 -B band for spherical and planophile canopies, respectively. Therefore, both the simulations and observations confirm that the relationship between SIF canopy and APAR chl is species-specific and affected by biochemical components and canopy structure, especially at the O 2 -B band. It is also very impo...

Both the net primary productivity (NPP) and the normalized difference vegetation index (NDVI) are commonly used as indicators to characterize vegetation vigor, and NDVI has been used as a surrogate estimator of NPP in some cases. To evaluate the reliability of such surrogation, here we examined the quantitative difference between NPP and NDVI in their outcomes of vegetation vigor assessment at a landscape scale. Using Landsat ETM+ data and a process model, the Boreal Ecosystem Productivity Simulator, NPP distribution was mapped at a resolution of 90 m, and total NDVI during the growing season was calculated in Heihe River Basin, Northwest China in 2002. The results from a comparison between the NPP and NDVI classification maps show that there existed a substantial difference in terms of both area and spatial distribution between the assessment outcomes of these two indicators, despite that they are strongly correlated. The degree of difference can be influenced by assessment schemes, as well as the type of vegetation and ecozone. Overall, NDVI is not a good surrogate of NPP as the indicators of vegetation vigor assessment in the study area. Nonetheless, NDVI could serve as a fairish surrogate indicator under the condition that the target region has low vegetation cover and the assessment has relatively coarse classification schemes (i.e., the class number is small). It is suggested that the use of NPP and NDVI should be carefully selected in landscape assessment. Their differences need to be further evaluated across geographic areas and biomes.

No abstract