Net primary productivity of aquatic vegitation of the Amazon floodplain : a multi-SAR satellite approach
Date
2018-06-04
Authors
Costa, Maycira
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Abstract
Field measures were combined with synthetic aperture radar (SAR) images to evaluate the use of radar for estimating temporal biomass and mapping of aquatic vegetation in the lower Amazon. A SAR-based methodology was developed for quantification of the annual net primary productivity (NPP) of aquatic vegetation. The predictable monomodal flooding cycle of the floodplain is the primary control of the growth pattern of the aquatic vegetation. The total biomass increased steadily from November to August following the hydrological cycle. However, the above water biophysical properties of the canopy remained constant all year around, except in November. By November, when the water level started to rise, new leaves and nodes were formed; the backscattering values were on average -12 and -l4dB for RADARS AT and JERS-1, respectively. By April, a full canopy was developed, remaining constant due to the high turn over rate of leaves. By August, the water level quickly receded, the senescent stage began, the plant water content decreased, and the stems bent, changing from an almost vertical orientation. From April onwards the backscattering coefficientes were on average -7 and -9.5 dB, respectively. The spatial variability of the canopy biophysical properties was detectable with radar data. Significant correlation existed between backscattering coefficients and above water dry biomass, height, and percentage of canopy cover.
The logarithmic relationship between backscattering coefficients and biomass suggested that ( 1 ) at low biomass, high transmissivity of the microwave radiation through the vegetation canopy occurred and the backscattering was a result of quasi-specular reflection of both C and L bands and a minor contribution of canopy volume scattering from C band; (2) at intermediate levels of biomass, moderate changes in backscattering values occurred and the backscattering saturation point was reached at 470, 660, and 620 gm⁻², for C band, L band, and the index, respectively; and (3) at high biomass, the transmissivity of C and L band radiation was equally attenuated and backscattering approached similar values for both. The derived index [special characters omitted] combines the capabilities of both C and L bands providing an empirical model for estimating above water biomass [special characters omitted] with the highest R² (0.67), the lowest root mean square error (34%), and an intermediate saturation point. The despeckled composite SAR images (C and L bands from the same season) were classified using a region-based approach.
Complementary information of the satellites yielded classification accuracy higher than 95% for vegetated areas of the floodplain. The seasonal thematic classification yielded an estimate of the length of inundation of different regions of the floodplain. Regions under flooded conditions of at least 300 days yr⁻¹ were colonized predominantly by the aquatic vegetation, Hymenachne amplexicaulis; the tree-like aquatic plant, Montrichartia arborescens; and some shrub-like trees. Secondary colonizers such as Cecropia sp., Pseudobombax munguba, and Astrycaryum jauari, which are tall well-developed flooded forest, colonized regions with inundation periods of approximately 150 days yr⁻¹. Climax forest colonized regions with inundation periods of approximately 60 days yr⁻¹.
The combination of the mapped area of seasonal aquatic vegetation with the SAR derived-biomass estimation allowed the calculation of the seasonal total biomass. By November, the new generation of aquatic vegetation started to develop; total biomass in the area was O.l x lO⁻¹² g. The steady growth of vegetation yielded a total biomass of 1.5 x 10⁻¹² g in an area of 395 km² in May. From May onwards, with the water receding, some plants detached from the sediment and were carried towards the Amazon River. Consequently, by August, both area and total biomass decreased to 281km² and 5 x lO⁻¹¹g, respectively. Any estimate of total biomass had a margin of error of at least 18%. After correction for seasonal biomass loss, the estimated annual NPP was 6350gm⁻² or 4.l x l0⁻¹²g for the entire area. Despite the smaller dimensions and the C3 photosynthetic pathway of the dominant H. amplexicaulis, its estimated productivity was comparable to the values reported for the most productive aquatic vegetation of the Amazon floodplain, and other aquatic plants colonizing wetlands worldwide. The estimated NPP of the aquatic vegetation yielded a total carbon uptake of 1.9 x l0⁻¹² g C yr⁻¹. Calculations based on the estimated area of each habitat of the floodplain, and the productivity data suggested in the literature, resulted in a net carbon productivity from flooded forest, phytoplankton, and periphyton of 0.35 x l0⁻¹²gC yr⁻¹, 0.22 x l0⁻¹²g C yr⁻¹, 0.07 x 10⁻¹² g C yr⁻¹, respectively. The total combined autochthonous annual net productivity
of the study area was 2.5 x 10⁻¹² g C, of which 75% was from C3 aquatic plants. This study represents the first attempt to develop a method to use SAR and field data for estimating spatial and temporal variations in biomass of aquatic vegetation from a natural floodplain.
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Keywords
Vegetation mapping, Remote sensing