8.3.4 Modeling Results Evaluation
Above ground biomass data collected from the study area in the peak growing
season of 2005 were converted to NPP for the purpose of model evaluation. This
conversion is needed because simulated NPP is the total annual production which
includes both above ground and below ground production. Specifically, total
biomass was calculated using a ratio of 0.57 between above ground NPP and
total NPP. This ratio is adapted from Bradford et al. (2005), whereby above ground
NPP of native Northern American vegetation in the Great Plains was estimated to
be 164 g C/m
2 , while below ground NPP averaged 122 g C/m
2 . The assumptions
behind the above-ground biomass and NPP conversion are: (1) any standing dead or
litter was carried over from the previous year, and dead vegetation in the current
year is negligible; and (2) live biomass was not carried over from previous years.
To ensure that the biomass data collected in 100 Â 100 m
2 sites could be used to
evaluate simulated NPP at a 250 Â 250 m
2 scale, SPOT 5 imagery acquired in the
summer of 2005 was utilized to investigate the homogeneity of each site and for
visual comparison between vegetation density and modeling results. Mean NDVI
values were calculated from the SPOT image for the 100 Â 100 m
2 sites and also for
the surrounding buffered 250 Â 250 m
2 areas. If a significant difference of NDVI
values was found between 100 Â 100 m
2 and 250 Â 250 m
2 areas, the site was ruled
out for model evaluation.
8.4 Results and Discussion
8.4.1 MODIS-Based Foliar C:N Ratio
Using the two MODIS images, a foliar C:N ratio map was produced for the entire
study area – the West Block of the GNP (Fig. 8.3). In the park, the foliar C:N ratio
values range from 9.5 to 25.4 with a low ratio in the majority of the park area and a
high ratio along the Frenchman River. The descriptive statistical analysis of the
map indicates that mean C:N ratio in the park is 15.8 with a standard deviation of
5.5. The mean C:N ratio of 15.8 is much lower than the model default C:N ratio of
24 for C3 grass. Visually comparing the C:N ratio map with the SPOT 5 image
(Fig. 8.3) indicates that foliar C:N ratio is high in areas with high vegetation cover
(e.g. river bank), and low in areas with low vegetation cover.
8.4.2 Sensitivity of the Model to Foliar C:N Ratio
Figure 8.4 shows averaged model-simulated NPP, the highest simulated NPP
values, and the lowest simulated NPP values from the study area in 2005, as a
8 Grassland Productivity Simulation: Integrating Remote Sensing. . .
161
Above ground biomass data collected from the study area in the peak growing
season of 2005 were converted to NPP for the purpose of model evaluation. This
conversion is needed because simulated NPP is the total annual production which
includes both above ground and below ground production. Specifically, total
biomass was calculated using a ratio of 0.57 between above ground NPP and
total NPP. This ratio is adapted from Bradford et al. (2005), whereby above ground
NPP of native Northern American vegetation in the Great Plains was estimated to
be 164 g C/m
2 , while below ground NPP averaged 122 g C/m
2 . The assumptions
behind the above-ground biomass and NPP conversion are: (1) any standing dead or
litter was carried over from the previous year, and dead vegetation in the current
year is negligible; and (2) live biomass was not carried over from previous years.
To ensure that the biomass data collected in 100 Â 100 m
2 sites could be used to
evaluate simulated NPP at a 250 Â 250 m
2 scale, SPOT 5 imagery acquired in the
summer of 2005 was utilized to investigate the homogeneity of each site and for
visual comparison between vegetation density and modeling results. Mean NDVI
values were calculated from the SPOT image for the 100 Â 100 m
2 sites and also for
the surrounding buffered 250 Â 250 m
2 areas. If a significant difference of NDVI
values was found between 100 Â 100 m
2 and 250 Â 250 m
2 areas, the site was ruled
out for model evaluation.
8.4 Results and Discussion
8.4.1 MODIS-Based Foliar C:N Ratio
Using the two MODIS images, a foliar C:N ratio map was produced for the entire
study area – the West Block of the GNP (Fig. 8.3). In the park, the foliar C:N ratio
values range from 9.5 to 25.4 with a low ratio in the majority of the park area and a
high ratio along the Frenchman River. The descriptive statistical analysis of the
map indicates that mean C:N ratio in the park is 15.8 with a standard deviation of
5.5. The mean C:N ratio of 15.8 is much lower than the model default C:N ratio of
24 for C3 grass. Visually comparing the C:N ratio map with the SPOT 5 image
(Fig. 8.3) indicates that foliar C:N ratio is high in areas with high vegetation cover
(e.g. river bank), and low in areas with low vegetation cover.
8.4.2 Sensitivity of the Model to Foliar C:N Ratio
Figure 8.4 shows averaged model-simulated NPP, the highest simulated NPP
values, and the lowest simulated NPP values from the study area in 2005, as a
8 Grassland Productivity Simulation: Integrating Remote Sensing. . .
161