Topographic restrictions on land-use practices: Consequences of different pixel sizes and data sources for natural forest management policies in the tropics

Abstract

Much of the tropical forest that will escape conversion is on steep slopes. Land uses in steep areas disproportionately affect environmental processes, especially hydrology (e.g., peak flows, suspended sediment loads). We use data from East Kalimantan, Indonesia, to demonstrate why slope measurements used for planning and regulatory purposes should be based on digital elevation models (DEMs) constructed with small pixel data and ground-based or canopy-penetrating remote sensing, and not just mean slopes calculated for large areas with passive remote sensing. For five logging concessions, the proportion of the forest on slopes >40% (21.8°) ranged 35–85% with crown penetrating airborne lidar pixels of 1 m, but only 13–69% when pixel size was increased to 30 m. With passive satellite-based remotely sensed 30 m pixels, estimates of land on slopes >40% were even lower (11–56%). Policies based on DEMs with underestimated slopes contribute to the misuses of steep areas and the consequent deleterious in-forest and downstream impacts. The energy costs of forest operations increase with slope, which decreases the financial costs of compliance with environmentally motivated policies for the protection of steep terrain.