156
Results
The spectral library for the five dominant shrub species is presented in Jiménez and
Díaz-Delgado (2015), and shows the different spectral response for the species in
both seasons taking into account LAI range variation along the ecosystem. Looking
at the shrub spectra presented in Fig. 3, it is immediately apparent that all species
appear to be very similar, as might be expected for a group of species that populate
an environment with very similar conditions. Despite the increased variability
between the species in the dry season, the comparison of the plant spectral curves
between seasons revealed that most marked differences were the reduction in the
water absorption bands in the NIR and the increase reflectance in lignin and cellulose
absorption bands (centered at 2100 and 2310 nm) for the dry season. These wavebands are important for species discrimination. In Jiménez and Díaz-Delgado (2015),
the separability test was applied over spectra in both seasons to determine whether
significant differences existed in intra-species variability and the degree of inter-species similarity. T-tests comparing the spectral libraries for both seasons indicated that
the dry season had slightly better levels of discrimination than the wet season
(p < 0.05), where all the species are in the same physiological state. In the estimation
of similarity index between species, low and significant values were found for E.
scoparia, H. halimifolium, and R. officinalis, with very high values found between
the legume species U. australis and S. genistoides.
The airborne spectroscopy imagery mosaics processed for both flight campaigns
are shown in Fig. 3. The ground reflectance was obtained with less than 5%
reflectance accuracy for both dates, when compared to the field spectra acquired
over sand dunes. The geolocation error for both imagery mosaics was below two
pixels, as estimated with Ground Control Points located in the field using GPS.
Plant cover measured in the 50 ground-truth plots was used to assess the classified AHS images. The distribution of the plots covers all of the altitude variability
along the stabilised dunes ecosystem, with plots situated in both the Naves and
Manto Arrasado zones.
In Fig. 4, we show the distribution maps for the five dominant shrub species in
the RBD stabilized ecosystem. The species distribution maps presented were generated from the 2008 aerial imagery. The map color scale represents the cover fraction
values estimated by AHS, with this ranging from red for lower values to orange for
higher values, according to dominant species. The background image for the map
representation is the band 15 of AHS. In addition, in Fig. 4, we present the correlation and RMSE values obtained for the AHS imagery cover fractions from the
ground survey on both dates.
Even through a visual inspection of the species distribution maps, it is easy to
describe the different spatial distribution of species. E. scoparia and U. australis
were mainly confined to Manto Arrasado areas whilst H. halimifolium and R. officinalis were more widely dispersed. E. scoparia was found mainly in the lower altitude and wetter areas of the Manto Arrasado, with up to 90 % cover recorded. H.
halimifolium shows a broad spatial distribution, colonizing both the Naves and
Manto Arrasado areas and attained a fraction cover of up to 80 % in the Manto
M. Jiménez and R. Díaz-Delgado
Results
The spectral library for the five dominant shrub species is presented in Jiménez and
Díaz-Delgado (2015), and shows the different spectral response for the species in
both seasons taking into account LAI range variation along the ecosystem. Looking
at the shrub spectra presented in Fig. 3, it is immediately apparent that all species
appear to be very similar, as might be expected for a group of species that populate
an environment with very similar conditions. Despite the increased variability
between the species in the dry season, the comparison of the plant spectral curves
between seasons revealed that most marked differences were the reduction in the
water absorption bands in the NIR and the increase reflectance in lignin and cellulose
absorption bands (centered at 2100 and 2310 nm) for the dry season. These wavebands are important for species discrimination. In Jiménez and Díaz-Delgado (2015),
the separability test was applied over spectra in both seasons to determine whether
significant differences existed in intra-species variability and the degree of inter-species similarity. T-tests comparing the spectral libraries for both seasons indicated that
the dry season had slightly better levels of discrimination than the wet season
(p < 0.05), where all the species are in the same physiological state. In the estimation
of similarity index between species, low and significant values were found for E.
scoparia, H. halimifolium, and R. officinalis, with very high values found between
the legume species U. australis and S. genistoides.
The airborne spectroscopy imagery mosaics processed for both flight campaigns
are shown in Fig. 3. The ground reflectance was obtained with less than 5%
reflectance accuracy for both dates, when compared to the field spectra acquired
over sand dunes. The geolocation error for both imagery mosaics was below two
pixels, as estimated with Ground Control Points located in the field using GPS.
Plant cover measured in the 50 ground-truth plots was used to assess the classified AHS images. The distribution of the plots covers all of the altitude variability
along the stabilised dunes ecosystem, with plots situated in both the Naves and
Manto Arrasado zones.
In Fig. 4, we show the distribution maps for the five dominant shrub species in
the RBD stabilized ecosystem. The species distribution maps presented were generated from the 2008 aerial imagery. The map color scale represents the cover fraction
values estimated by AHS, with this ranging from red for lower values to orange for
higher values, according to dominant species. The background image for the map
representation is the band 15 of AHS. In addition, in Fig. 4, we present the correlation and RMSE values obtained for the AHS imagery cover fractions from the
ground survey on both dates.
Even through a visual inspection of the species distribution maps, it is easy to
describe the different spatial distribution of species. E. scoparia and U. australis
were mainly confined to Manto Arrasado areas whilst H. halimifolium and R. officinalis were more widely dispersed. E. scoparia was found mainly in the lower altitude and wetter areas of the Manto Arrasado, with up to 90 % cover recorded. H.
halimifolium shows a broad spatial distribution, colonizing both the Naves and
Manto Arrasado areas and attained a fraction cover of up to 80 % in the Manto
M. Jiménez and R. Díaz-Delgado