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Societal Impact Statement Trees play vital roles in many of the world's ecosystems while providing many benefits to people. New evidence indicates that a third of tree species are threatened with extinction, representing a tree extinction crisis. Here we demonstrate how tree species extinction will lead to the loss of many other plants and animals and significantly alter the world's ecosystems. We also show how tree extinction will negatively affect billions of people through loss of livelihoods and benefits. We highlight a series of urgent actions needed to avert an ecological, cultural and socio‐economic catastrophe caused by widespread extinction of tree species. Summary Trees are of exceptional ecological importance, playing a major functional role in the world's ecosystems, while also supporting many other plants, animals and fungi. Many tree species are also of direct value to people, providing a wide range of socio‐economic benefits. Loss of tree diversity could lead to abrupt declines in biodiversity, ecosystem functions and services and ultimately ecosystem collapse. Here we provide an overview of the current knowledge regarding the number of tree species that are threatened with extinction, and the threats that affect them, based on results of the Global Tree Assessment. This evidence suggests that a third of the world's tree species are currently threatened with extinction, which represents a major ecological crisis. We then examine the potential implications of tree extinctions, in terms of the functioning of the biosphere and impacts on human well‐being. Large‐scale extinction of tree species will lead to major biodiversity losses in other species groups and substantially alter the cycling of carbon, water and nutrients in the world's ecosystems. Tree extinction will also undermine the livelihoods of the billions of people who currently depend on trees and the benefits they provide. This warning to humanity aims to raise awareness of the tree extinction crisis, which is a major environmental issue that requires urgent global attention. We also identify some priority actions that need to be taken to reduce the extinction risk of tree species and to avert the ecological and socio‐economic catastrophe that will result from large‐scale extinction of tree species.