Degradation as a catalyst for primary forest conversion in Indonesia

Abstract:

Natural, old-growth tropical forests provide critically important ecosystem services, both locally and globally, but these valuable landscapes have been rapidly cleared in recent years. Disturbance events such as logging and fire make tropical forests more vulnerable to conversion by changing forest structure and increasing accessibility. However, these events often leave only ephemeral impacts on the spectral signatures of forest canopies. This makes degradation difficult to map using remote sensing data, which in turn makes it difficult to fully understand the relationship between degradation and forest conversion in the tropics. Here, we use a sample-based technique to track forest degradation and clearing over a 29-year period in Indonesia, the country with the third largest tropical forest area in the world. To identify degradation and clearing events, we visually interpreted annual and bimonthly (6 per year) Landsat composite images for the entire study period (1990-2019) for 10,000 randomly selected sites. Using bimonthly in addition to annual data allowed us to capture logging and fire events that could not be identified using annual composites alone, improving our ability to accurately detect forest disturbance. For each site, we recorded all change events, including in cases where multiple degradation or clearing events took place over the course of the study. We then used propensity score matching methods to compare the fates of intact and degraded forest sites that had otherwise similar risk factors for conversion (elevation, soil type, distance to cities, etc.), in order to better understand the extent to which degradation facilitates or increases the likelihood of forest clearing in Indonesia. This information can inform policy and conservation strategies as it helps untangle the relationship between forest degradation and conversion, allowing practitioners to identify areas at high risk of deforestation and to develop policies to limit degradation events that might leave tropical forests more vulnerable.