Abstract
Tropical forest disturbances linked to fire usage cause large amounts of greenhouse gas (GHG) emissions and environmental damages. Supporting precise GHG estimations and counteracting illegal fire usages in the tropics require timely and thematically detailed large-scale information on fire-related forest disturbances. Multi-sensor optical and radar detection and ranging (radar) remote sensing data combined with active fire alerts shows the potential for a more in-depth characterization of fire-related forest disturbances. We utilized dense optical (Landsat-7, Landsat-8 and Sentinel-2) and radar (Sentinel-1) time series to individually map forest disturbances in the province of Riau (Indonesia) for 2018–2019. We combined the sensor-specific optical and radar forest disturbance maps with daily active fire alerts and classified their temporal relationship (predating, coinciding, postdating) into seven so-called archetypes of fire-related forest disturbances. The archetypes reflect sensor-specific sensitives of optical (e.g., changes in tree foliage) and radar (e.g., changes in tree structure) data to detect varying types of forest disturbances, ranging from either a loss of tree foliage and/or structure predating, coinciding or postdating fires. These can be related to different magnitudes of fire-related forest disturbances and burn severities and can be associated with specific land management practices, such as slash-and-burn agriculture and salvage logging. This can support policy development, local and regional forest management and law enforcement to reduce illegal fire usage in the tropics. Results suggest that a delayed or opposing forest disturbance detection in the optical and radar signal is not only caused by environmental influences or different observation densities but, in some cases, such as fire-related forest disturbances, can be related to their different sensitives to detect changes in tree foliage and structure. Multi-sensor-based forest monitoring approaches should, therefore, not simply combine optical and radar time series on a data level, as it bears the risk of introducing artefacts.
Highlights
Indonesia is globally one of the main contributors of forest carbon emissions in the21st century as a result of large-scale forest disturbances including deforestation and forest degradation [1,2]
79.3% of mapped forest disturbances were not co-located with active fire alerts and indicated no fire-related forest disturbance
The remaining one fifth (20.7%) of mapped forest disturbances were co-located with active fire alerts indicating fire-related disturbances, with secondary forest showing the highest proportion (24.1%) compared to primary (9.6%) and plantation forest (5.2%) (Figure 3)
Summary
Indonesia is globally one of the main contributors of forest carbon emissions in the21st century as a result of large-scale forest disturbances including deforestation and forest degradation [1,2]. Forest disturbances in Indonesia are caused primarily by smallholder or commercial agriculture crop expansion and timber production, of which many are illegal or unsustainable [3,4]. These disturbances are strongly linked to fire use [5]. While fire use for land management is forbidden by Indonesian law, a wide range of fire-related practices are still used today [6] These practices traditionally include limited and controlled burning of forests to, for example, clear understory providing access prior to logging operations, burn forest directly or burn remaining material at previously logged patches in preparation of agricultural use [7,8]. Escaped land use fires cause large-area forest fires in dry El Nino years (e.g., 2015) [9]
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