Critical role and collapse of tropical mega-trees: A key global resource

Very large tropical forest trees (‘mega-trees’) represent an irreplaceable habitat associated with large benefits in terms of biodiversity and ecosystem services. Here we provide a comprehensive overview of the importance of tropical mega-trees relative to biodiversity persistence, ecosystem services, and sociocultural value. We describe all contemporary threats to mega-trees as a result of the conversion of old-growth forests into human-degraded landscapes exposed to climate change. We reveal a myriad of taxa, functional groups and ecosystem services that are directly or indirectly supported by tropical mega-trees, such as highly illuminated crowns for epiphytes and large boles and branches sheltering many vertebrate and invertebrate taxa. Mega-trees also amplify the resource spectra associated with non-redundant niche space, including thermal buffering and extended vertical microclimates above the forest canopy. Mega-trees also make the largest contribution to forest productivity, aboveground biomass, and timber. However, protecting this resource is a major challenge due to the natural rarity of mega-trees, illegal logging, old-growth forest conversion and climate change. We argue for ‘productive forest landscapes’ as a strategy to protect mega-trees and highlight their connections with sustainable development goals and other global targets. This strategy poses economic, political, technological and social challenges, as sparing tropical forests still incurs high opportunity costs that few stakeholders can realistically afford. Despite these challenges, the future of tropical forests mega-trees and local livelihoods are inextricably linked, and productive forest landscapes would bring clear unanticipated benefits for future generations and tropical biodiversity.