We now know that at least 17,510 tree species are threatened with extinction, which represents nearly a third of the world's tree species. Over 100 tree species are already extinct in the wild, and many more will soon become extinct unless urgent action is taken, as human impacts on the biosphere continue to intensify. Currently, around 15.3 billion trees are being destroyed each year as a result of harvesting, deforestation, land use change and other forms of disturbance (Crowther et al., 2015). Trees are of immense importance to ecological systems and the global economy and to human livelihoods and culture throughout the world. Loss of tree species adversely affects human health and well-being from local to global scales and undermines the resilience of ecological systems on which human livelihoods depend. Extinction of tree species therefore represents a critical element of the global biodiversity crisis, which has not received sufficient attention in previous discourse (e.g. IPBES, 2019b). A strong and urgent response is required, both to prevent further tree species extinctions and to restore the damaged and degraded ecosystems of which they form a part. Such actions will simultaneously help to address both the global biodiversity crisis and the climate change emergency.

We endorse the call for actions in other ‘warning to humanity’ papers (Albert et al., 2021; P. Cardoso et al., 2020, 2021; Cavicchioli et al., 2019; Heleno et al., 2020; Jenny et al., 2020; Pyšek et al., 2020; Ripple et al., 2017, 2020) and support the suggestions that humanity can take to transition to sustainability, including the need to shift to a green economy and to address climate change. We also identify the following seven key actions needed to avoid the catastrophic implications for humanity that could result from the loss of tree species.

Tree species are essential elements of global biodiversity, comprising the key components of forest ecosystems. Yet trees are often not appreciated as individual species but are more often seen as interchangeable elements of a particular habitat, community or ecosystem. Monitoring of environmental trends focuses on assessing changes in forest cover or tree density (Crowther et al., 2015; FAO, 2020; IPBES, 2019b), while ignoring the vital role of individual tree species and changes in tree species richness. All tree species have unique functions and ecological roles, while providing essential habitat for distinct communities of other species. Prevention of the extinction of individual tree species is paramount, together with maintenance of tree species richness within ecological communities. Greater recognition and understanding of the specific importance of the roles of different tree species is needed from individuals, conservation organisations, governments and the international policy community, in order to mobilise and act to prevent the further extinction of tree species.

Two thirds of tree species are found in protected areas (BGCI, 2021). We call for the effective conservation of threatened tree species within the global protected area network by strengthening local knowledge of their status and distribution, improving the effectiveness of conservation management, monitoring populations of threatened species and where necessary increasing enforcement of controls on illegal or nonsustainable harvesting of threatened species. We also need to extend protected area coverage, to include those threatened tree species and species assemblages that are currently not adequately represented in protected areas. We call for the information about tree species to be factored in to conservation prioritisation and decision-making, for example, by strengthening existing Key Biodiversity Areas (KBAs) (Eken et al., 2004) and in the designation of new KBAs. Restoration of degraded forests should focus on restoring the species composition and richness of tree communities as well as associated ecological processes and ecosystem functions (Aerts & Honnay, 2011).

Results of the GTA have provided insights into which factors are threatening individual tree species (Table 2; BGCI, 2021). Action needs to be taken to address each of the threats affecting the most threatened species, supported by legal measures where necessary. Recognising that habitat loss from agricultural expansion and associated land cover change is the principal threat to most tree species, steps need to be taken to ensure that trees are conserved within agricultural landscapes and that rates of land cover change are reduced. Recognising that many tree species are threatened by overexploitation, we advocate measures to ensure that management of natural forests is environmentally sustainable, whether for timber production, nonwood products or multiple uses. We also need a greater understanding of the impact of harvesting on the population dynamics of individual tree species, through improved inventory, monitoring, and research. Efforts are also needed to address illegal logging more vigorously and to strengthen legal compliance and verification. In areas affected by invasive species, or by spread of pests and diseases, we need improved early warning of these threats by monitoring and understanding their spread and impacts on tree species, while developing improved controls and management practices.

Currently, there are more threatened tree species listed on the IUCN Red List than the number of threatened mammals, birds, reptiles and amphibians combined, yet it is these species groups that are typically used as flagships for biodiversity conservation. We suggest that tree species can usefully be considered as ‘charismatic mega-flora’ and as conservation flagships. As resources are limited, we need to prioritise conservation action for the most threatened trees. Information from the GTA (BGCI, 2021, 2022b) including conservation status, distribution and conservation action can inform conservation planning, prioritisation and action at local, national and international scales. To date, conservation action has been undertaken for over 400 of the world's threatened tree species through the Global Trees Campaign (BGCI and FFI, 2021). However, the vast majority of threatened tree species are still lacking a conservation action plan or any practical conservation measures. These are now needed to ensure that no tree species is forgotten, with funding, attention and action directed to tree species and sites that are in greatest need of conservation.

Action in support of tree conservation needs to be mandated by policies and legislation from local to international scales. Those policies and mechanisms that are currently in place that affect populations of tree species, such as those relating to forestry, biodiversity conservation, land use and climate change, should include specific measures supporting conservation action for threatened tree species. International policies should be implemented and applied to tree species with much greater resolve and commitment. The UN Convention on Biological Diversity (CBD) should acknowledge and address the specific conservation needs of tree species, for example, by encouraging Parties to develop action plans for those species that are threatened. Data describing the conservation status of tree species should be integrated into biodiversity indicators for monitoring implementation success. The policies and mechanisms of the UN Framework Convention on Climate Change (UNFCCC) designed to reduce deforestation should be rigorously supported, with adequate provision of funding. We support the important pledge made at UNFCCC COP26 to halt and reverse deforestation and land degradation by 2030 and encourage all Parties to ensure that the pledge is achieved in practice. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) includes over 900 tree species in its Appendices, including species traded for timber, medicinal and aromatic products. Implementation needs to be urgently strengthened for tree species, with measures extended to include a wider range of species. We welcome The New York Declaration on Forests, a voluntary and nonbinding international declaration to take action to halt global deforestation, which specifically includes commitments from the private sector to eliminate deforestation from the supply chains of major agricultural commodities. We also support the UN Strategic Plan for Forests 2017–2030, which calls for forest law enforcement and governance to be enhanced, and for illegal logging and associated trade to be significantly reduced worldwide. We strongly encourage stakeholders in these policy processes to ensure that these commitments are met in full.

Conservation of tree species is crucial to retaining future options to support human well-being. As noted by Miller et al. (2020), trees are critical to global efforts aimed at ending poverty; they play a crucial role in supporting the livelihoods of people in rural communities. The contribution of trees to supporting the well-being of hundreds of millions of people around the world, particularly those in rural areas, needs to be recognised. The value of trees to people needs to be reflected in policies and actions aimed at achieving poverty alleviation, such as the Sustainable Development Goals. Tree species need to be appreciated as valuable assets for the poor; these values are often overlooked. Policies that conserve and sustainably manage tree species need to be implemented, so that they can directly benefit the poor (Miller et al., 2020; Newton, 2008), and support sustainable development.

None of these suggested actions can be achieved by individuals, conservation organisations, businesses or governments acting in isolation. In order to prevent a tree extinction crisis, we need to develop a much wider partnership, including local communities, government agencies, forestry organisations, business communities, conservation NGOs, botanic gardens, universities and all other stakeholders that depend on trees in a myriad of different ways.

We all need to take action for the world's tree species, both collectively and individually. As individuals, we need to encourage our representatives and decision-makers to take action to protect tree biodiversity. We also need to ensure that products derived from trees are sourced sustainably, especially when this involves harvesting from natural forest. We can achieve this through our consumption preferences and the demands we place on the businesses that provide goods for our consumption. We also need to limit our consumption of products that are destroying natural forests (e.g. some sources of soya, oil palm and tropical hardwoods). We each need to support practical conservation initiatives throughout the world that are helping to conserve tree species, and to protect and restore natural forests. When supporting tree planting schemes, we need to make sure we choose schemes that include planting of native species, and ideally threatened tree species.

Our message for humanity is to remember how trees enrich and support our lives, as they have throughout human history. Yet we need to acknowledge that these values are at risk if we fail to consider the impacts of our actions and to change our collective behaviour in relation to trees. Although there is still much to learn about the biology, ecology and wonder of trees, we know how to conserve them. We also know that now is the time to act.


*Based on paper by Adrian C. Newton, Sara Oldfield, Global Tree Assessment Contributors (https://doi.org/10.1002/ppp3.10314)