Tropical tree crops. From a driver of deforestation to a restoration opportunity

We live in an era of unprecedented environmental change, motivating unprecedented global actions to both protect and restore forest ecosystems. Tropical deforestation is responsible for about 11% of global greenhouse gas emissions. Reducing emissions from deforestation and sequestering carbon by restoring tropical forests are cost-efficient measures and potentially applicable on a large scale; they therefore take a relevant place in global decarbonisation scenarios. Tree crops play a potentially important role as both driver of deforestation and restoration.

This PhD research aims to provide building blocks to turn tree crops from causes of deforestation to means of enhancing forest restoration, within a theory of change framework. The chapters of this PhD research differ in scope, scale, geography and methodology. The common theme throughout the chapters is the forest transition curve, where each chapter represents a different stage, ranging from likely future deforestation in forest-rich Southern Cameroon to the transitioning forest plantation landscapes of Sumatra and Kalimantan.

Chapter 1 introduces basic concepts, relates tree crops to the main production factors (land, labour, capital). It sketches out the objectives and overall approach of this PhD research in a theory of change framework. Chapter 2 presents a global assessment of bio-physical suitability for oil palm cultivation and relates it to a set of spatially-explicit criteria to direct future production to lands where more environmentally friendly palm oil production is possible in the future. Main findings show that from a bio-physical viewpoint 1.37 billion hectares concentrated in twelve countries are suitable for growing palms. With sustainability criteria being applied, the area reduces to 234 million hectares or 17% of worldwide suitable areas potentially available for less harmful production. However, of these lands many are located far away from consumption centres where oil palm cultivation may be a barely economically viable activity. Chapter 3 zooms into Southern Cameroon whose remaining forest-rich landscape is threatened by agricultural expansion. The chapter describes the development of a land use model used to design a forward-looking forest reference level to serve as benchmark for receiving payments under the REDD mechanism. Results show that future emissions might increase by 47% as compared to a historical baseline, but uncertainties are high as a Monte Carlo analysis of the model shows. Tree crops do play a role as deforestation driver but annual crops outweigh them, and statistics about tree crops are very poor which makes it difficult to consider them in for reliable GHG reporting. Further down the slope of the forest transition curve, Chapter 4 presents a novel approach to automatically mapping both the extent and the age of oil palm plantations in South-East Asia combined with a citizen-science approach to map validation. Findings (mapped oil palm area: 11.66 ± 2.10 million hectares) have high policy relevance as they show that the productive oil palm area has likely been over-estimated by existing mapping products; plantations in the established production centres are moving beyond the prime age of high oil productivity, suggesting that the sector should take action for gradual plantation renewal. On a cautiously upwards-facing slope of the forest transition curve, Chapter 5 emerges against the backdrop of a species shift from nitrogen-fixing acacias to eucalypts in the Indonesian pulp and paper sector. This unprecedented shift in the history of plantation forestry raises questions about the future nitrogen supply of plantations. Results from an ecosystem model show that eucalypts will soon require fertilization to maintain a high level of productivity. Simulations confirm cross-fertilisation effects of alternating rotations of acacia and eucalypt but the effect is not strong enough to maintain productivity of unfertilised eucalypts by itself. Further, the fertilization effect on plantation economics will likely not suffice to incentivise a shift to this novel mixed-species plantations scheme.   

Chapter 6 first discusses to what extent the theory of change is confirmed by the four substantive chapters of this PhD. It then concludes with policy-relevant recommendations and sheds light on the applicability of major policy types to the four substantive chapters: regulatory policies, land sparing, informational policies and investment. This chapter closes by proposing promising avenues for future research, notably highlighting potential connections between the existing chapters.