Abstract
Global-scale forest/non-forest (FNF) maps are of crucial importance for applications like biomass estimation and deforestation monitoring. Global FNF maps based on optical remote sensing data have been produced by the wall-to-wall satellite image analyses or sampling strategies. The German Aerospace Center (DLR) and the Japan Aerospace Exploration Agency (JAXA) also made available their global FNF maps based on synthetic aperture radar (SAR) data. This paper attempted to answer the following scientific question: how comparable are the FNF products derived from optical and SAR data? As test sites we selected the Amazon (tropical rainforest) and Cerrado (tropical savanna) biomes, the two largest Brazilian biomes. Forest estimations from 2015 derived from TanDEM-X (X band; HH polarization) and ALOS-2 (L band; HV polarization) SAR data, as well as forest cover information derived from Landsat 8 optical data were compared with each other at the municipality and image sampling levels. The optical-based forest estimations considered in this study were derived from the MapBiomas project, a Brazilian multi-institutional project to map land use and land cover (LULC) classes of an entire country based on historical time series of Landsat data. In addition to the existing forest maps, a set of 1619 Landsat 8 RGB color composites was used to generate new independent comparison data composed of circular areas with 5-km diameter, which were visually interpreted after image segmentation. The Spearman rank correlation estimated the correlation among the data sets and the paired Mann–Whitney–Wilcoxon tested the hypothesis that the data sets are statistically equal. Results showed that forest maps derived from SAR and optical satellites are statistically different regardless of biome or scale of study (municipality or image sampling), except for the Cerrado´s forest estimations derived from TanDEM-X and ALOS-2. Nevertheless, the percentage of pixels classified as forest or non-forest by both SAR sensors were 90% and 80% for the Amazon and Cerrado biome, respectively, indicating an overall good agreement.
Highlights
Forests cover approximately 4060 million ha in the world [1]
88% of this biome was originally constituted by dense ombrophylous forest (48%), open ombrophylous forest (25%), and ecotones (15%; transition between forest and savannah, mostly along the boundary between Amazon and Cerrado biomes) [25]
The conversion is occurring rapidly, especially in the northern part of the biome, where we find a new agricultural frontier known as MATOPIBA
Summary
Forests cover approximately 4060 million ha in the world [1]. Since 1990, the world has lost about 178 million ha of forests [1], regardless of increasing monitor-. The largest losses occurred in the tropical regions of South America and Africa. The increasing demand for food, products, and energy, either from forests themselves (e.g., timbers for exportation) or from the lands replacing the forests (e.g., water reservoirs for hydroelectric power generation and clear-cut terrains for livestock and cropland) is the primary cause of forest loss [3,4]. Government settlement programs, expansion of urban centers, and mining activities are other major contributors for the forest reduction [5,6]. The construction of new roads contributes to deforestation, as it increases forest access along the forest edges [7]
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