Mapping and quantification of ferruginous outcrop savannas in the Brazilian Amazon: A challenge for biodiversity conservation

The eastern Brazilian Amazon contains many isolated ferruginous savanna ecosystem patches (locally known as ‘canga vegetation’) located on ironstone rocky outcrops on the top of plateaus and ridges, surrounded by tropical rainforests. In the Carajás Mineral Province (CMP), these outcrops contain large iron ore reserves that have been exploited by opencast mining since the 1980s. The canga vegetation is particularly impacted by mining, since the iron ores that occur are associated with this type of vegetation and currently, little is known regarding the extent of canga vegetation patches before mining activities began. This information is important for quantifying the impact of mining, in addition to helping plan conservation programmes. Here, land cover changes of the Canga area in the CMP are evaluated by estimating the pre-mining area of canga patches and comparing it to the actual extent of canga patches. We mapped canga vegetation using geographic object-based image analysis (GEOBIA) from 1973 Landsat-1 MSS, 1984 and 2001 Landsat-5 TM, and 2016 Landsat-8 OLI images, and found that canga vegetation originally occupied an area of 144.2 km2 before mining exploitation. By 2016, 19.6% of the canga area was lost in the CMP due to conversion to other land-use types (mining areas, pasturelands). In the Carajás National Forest (CNF), located within the CMP, the original canga vegetation covered 105.2 km2 (2.55% of the CNF total area), and in 2016, canga vegetation occupied an area of 77.2 km2 (1.87%). Therefore, after more than three decades of mineral exploitation, less than 20% of the total canga area was lost. Currently, 21% of the canga area in the CMP is protected by the Campos Ferruginosos National Park. By documenting the initial extent of canga vegetation in the eastern Amazon and the extent to which it has been lost due to mining operations, the results of this work are the first step towards conserving this ecosystem.

巴西亚马逊东部区域分布有诸多孤立的铁质稀树草原生态斑块，当地将其称为“坎加植被（canga vegetation）”，这类斑块分布于高原顶部与山脊的铁质岩层出露区域，四周被热带雨林环绕。在卡拉加斯矿产省（Carajás Mineral Province, CMP）范围内，这些岩层出露区域蕴藏着大型铁矿石储量，自20世纪80年代起，当地便采用露天开采方式对其进行开发。坎加植被受采矿活动的影响尤为显著，因为赋存的铁矿石与该类植被伴生；而截至目前，学界对采矿活动启动前坎加植被斑块的分布范围仍知之甚少。该信息不仅可为量化采矿活动的生态影响提供关键依据，也能为保护规划的制定提供有力支撑。本研究通过估算采矿活动启动前坎加植被斑块的分布面积，并将其与当前坎加植被斑块的实际覆盖范围进行对比，评估了CMP范围内坎加区域的土地覆盖变化格局。研究团队借助1973年Landsat-1 MSS、1984年与2001年Landsat-5 TM以及2016年Landsat-8 OLI影像，采用面向对象影像分析（geographic object-based image analysis, GEOBIA）技术完成了坎加植被的制图工作，结果表明，采矿活动启动前，坎加植被的原始覆盖总面积达144.2 km²。至2016年，CMP范围内已有19.6%的坎加植被区域因被转换为采矿区、牧场等其他土地利用类型而遭到损毁。位于CMP范围内的卡拉加斯国家森林（Carajás National Forest, CNF）中，原始坎加植被覆盖面积为105.2 km²（占CNF总面积的2.55%）；至2016年，该区域内坎加植被的覆盖面积仅剩77.2 km²（占比1.87%）。综上，历经三十余载的矿产开发，CMP范围内的坎加植被总面积损失比例仍不足20%。目前，CMP范围内21%的坎加植被区域受到费鲁吉诺索斯高原国家公园（Campos Ferruginosos National Park）的保护。本研究厘清了亚马逊东部地区坎加植被的初始分布范围，以及其因采矿活动而遭受的流失规模，此项研究成果可为该生态系统的保护工作奠定首个基石。