Historical Aerial Surveys Map Long-Term Changes of Forest Cover and Structure in the Central Congo Basin

Koen Hufkens,Hans Beeckman,Elizabeth Kearsley,Filip Vandelook,Thalès De Haulleville,Piet Stoffelen,Michael Amara,Lisa Wingate,Kim Jacobsen,Sofie Meeus,Jan Van Den Bulcke,Leen Van Hirtum,Hans Verbeeck

doi:10.3390/rs12040638

Abstract

Given the impact of tropical forest disturbances on atmospheric carbon emissions, biodiversity, and ecosystem productivity, accurate long-term reporting of Land-Use and Land-Cover (LULC) change in the pre-satellite era (<1972) is an imperative. Here, we used a combination of historical (1958) aerial photography and contemporary remote sensing data to map long-term changes in the extent and structure of the tropical forest surrounding Yangambi (DR Congo) in the central Congo Basin. Our study leveraged structure-from-motion and a convolutional neural network-based LULC classifier, using synthetic landscape-based image augmentation to map historical forest cover across a large orthomosaic (~93,431 ha) geo-referenced to ~4.7 ± 4.3 m at submeter resolution. A comparison with contemporary LULC data showed a shift from previously highly regular industrial deforestation of large areas to discrete smallholder farming clearing, increasing landscape fragmentation and providing opportunties for substantial forest regrowth. We estimated aboveground carbon gains through reforestation to range from 811 to 1592 Gg C, partially offsetting historical deforestation (2416 Gg C), in our study area. Efforts to quantify long-term canopy texture changes and their link to aboveground carbon had limited to no success. Our analysis provides methods and insights into key spatial and temporal patterns of deforestation and reforestation at a multi-decadal scale, providing a historical context for past and ongoing forest research in the area.

Highlights

Tropical ecosystem services are severely impacted by deforestation and forest degradation [1,2,3]
Our analysis provides a first spatially explicit historical composite of aerial survey images in support of mapping land-use and land-cover within the Congo Basin
The orthomosaic served as input for all subsequent Land-Use and Land-Cover Change (LULCC) analysis with all derived maps provided with the manuscript repository

Summary

Introduction

Tropical ecosystem services are severely impacted by deforestation and forest degradation [1,2,3]. Estimates show that 31% of carbon emissions are caused by edge effects alone [6]. Accurate estimates of LULCC and forest canopy structure are imperative to estimate carbon emissions and other ecosystem services [1,2]. High-resolution aerial images provide scientists with tools to monitor forest extent, structure, and carbon emissions as canopy texture is linked to aboveground biomass [14,15,16]. Most of these estimates are limited in time to recent decades [1,2,17,18]

Methods

Results

Discussion

Conclusion