Abstract
A humid tropical forest disturbance alert using Sentinel-1 radar data is presented for the Congo Basin. Radar satellite signals can penetrate through clouds, allowing Sentinel-1 to provide gap-free observations for the tropics consistently every 6–12 days at 10 m spatial scale. In the densely cloud covered Congo Basin, this represents a major advantage for the rapid detection of small-scale forest disturbances such as subsistence agriculture and selective logging. Alerts were detected with latest available Sentinel-1 images and results are presented from January 2019 to July 2020. We mapped 4 million disturbance events during this period, totalling 1.4 million ha with nearly 80% of events smaller than 0.5 ha. Monthly distribution of alert totals varied widely across the Congo Basin countries and can be linked to regional differences in wet and dry season cycles, with more forest disturbances in the dry season. Results indicated high user’s and producer’s accuracies and the rapid confirmation of alerts within a few weeks. Our disturbance alerts provide confident detection of events larger than or equal to 0.2 ha but do not include smaller events, which suggests that disturbance rates in the Congo Basin are even higher than presented in this study. The new alert product can help to better study the forest dynamics in the Congo Basin with improved spatial and temporal detail and near real-time detections, and highlights the value of dense Sentinel-1 time series data for large-area tropical forest monitoring. The research contributes to the Global Forest Watch initiative in providing timely and accurate information to support a wide range of stakeholders in sustainable forest management and law enforcement. The alerts are available via the https://www.globalforestwatch.org and http://radd-alert.wur.nl.
Highlights
The Congo Basin rainforest is the second-largest in the world, covering almost 200 million ha of humid tropical forest
Forest disturbance alerts Disturbance alerts for the humid tropical forest of the Congo Basin for the period between 1 January 2019 and 1 July 2020 are depicted in figure 4
We examined the 12 samples labeled as commission error and identified three primary sources of false detections: dynamics in non-forested swamp and savannah areas that remained in our benchmark forest map (7 samples), swamp forest dynamics such as seasonal inundation changes that cause strong radar backscatter variations (2 samples), and unidentifiable causes of falsely detected small-scale events (3 samples)
Summary
The Congo Basin rainforest is the second-largest in the world, covering almost 200 million ha of humid tropical forest. It plays a crucial role in the global climate cycle and provides local livelihoods and resources for more than 100 million people across six countries: Cameroon, Central African Republic, the Democratic Republic of the Congo, Equatorial Guinea, Gabon and the Republic of the Congo (Somorin et al 2012). Artisanal and industrial logging account for 10% of the total forest disturbance across the Congo Basin, but up to 60% in individual countries, such as Gabon (Tyukavina et al 2018). A large part of the forest disturbances in the Congo Basin is considered unsustainable and/or illegal (Lescuyer et al 2014, Kleinschroth et al 2019, Umunay et al 2019)
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