Tropical forest loss in the early twenty-first century : patterns, drivers, and implications for the carbon cycle

Abstract

Forests are the major land cover of the Earth's land surface. Changes in forest resources, including location, distribution, and composition, affect various aspects of the Earth system. This thesis focuses on three studies that aim to reveal the patterns, drivers, and consequences of forest loss across the tropics, the global hotspots of forest clearance. The first study shows large uncertainties in forest changes identified by different forest maps and land-cover products. These uncertainties were, in part, attributed to the scale issue for detecting small-scale forest loss and the varied definitions of forest (changes) that determine forest (changes) inherently with different tree cover thresholds and canopy heights. Overall, the Global Forest Change loss products have reliable forest loss detection and can identify the increasing trends in gross forest loss, which can be used for accurately monitoring tropical forest loss. The second reveals accelerating forest loss and dynamic topographical patterns in the tropics during the first two decades of the 21st century. The shift in forest loss topography is characterized by an upward expansion in the frontier of forest exploitation, particularly in tropical Asia where forest loss predominantly occurred in the lowlands in the 2000s then increasingly encroached forests at higher elevations in the 2010s. Large- and small-scale agriculture expansion dominated tropical forest loss, contributing to 43.9% (108.8 Mha) and 43.4% (107.5 Mha) of forest loss during 2001–2019, respectively. Further visual interpretation of post-forest-loss-land-cover using very-high-resolution Planet imagery reveals that ~70% of former forest lands converted to agriculture in 2001–2019 remained so in 2020, confirming a dominant role of agriculture in long-term pan-tropical carbon reductions on formerly forested landscapes. The third study uncovers a doubling of tropical forest carbon loss from 0.97 ±0.16 Pg C yr-1 during 2001–2005 to 1.99 ±0.13 Pg C yr-1 during 2015–2019. Most of the increases occurred in Brazil's central Para and Maranhão states, the Congo Basin, the Gulf of Guinea coast, and mainland Southeast Asia. Attribution analysis shows that most of the forest carbon loss (82%) is at some stages associated with large-scale commodity or small-scale agriculture activities, particularly in Africa and Southeast Asia. These three studies provide new insight regarding tropical forest dynamics that have not been recognized in prior climate change assessments or parameterized in current model configurations simulating impacts. Such exclusion of forest loss patterns misrepresents regional biophysical and biochemical feedbacks of deforestation. These results also demonstrate the immense challenge posed by the Glasgow Leaders’ Declaration on Forests and Land Use which pledges to halt forest loss by 2030. These findings imply an urgent need for renewed and sustained action to achieve commitments to reduce forest loss, which must include efficient and legal strategies to produce commodities and food without compromising tropical forests.