A systematic global analysis has revealed that more than half of the world’s tree species – some 26,096 distinct types – have been introduced to regions outside their natural habitats. The research, published in the journal Current Biology today, provides the first comprehensive checklist and mapping of global tree flows, drawing on a massive dataset of over 31 million occurrence records covering 86.3% of all known tree species.
The study, led by researchers including Professor David Edwards from the Department of Plant Sciences at the University of Cambridge, identifies the complex environmental and socioeconomic factors driving this unprecedented redistribution of botanical life. The findings suggest that as humans continue to move species across borders for timber, food, and ornamentation, the global forest landscape is being fundamentally reshaped.
A continent of newcomers
The scale of these introductions varies significantly across the globe. Europe stands out with the highest proportion of non-native trees, where a staggering 88.4% of tree species are introduced. This high percentage is partly due to Europe’s relatively low native tree diversity compared to other continents.
In contrast, South America hosts the highest overall richness (total number of species) of non-native trees, followed by North America and temperate Asia. The research highlights that intracontinental flows – the movement of species within the same continent – dominate tree introductions. For instance, Brazil is both the largest donor and the largest recipient of non-native tree species when considering movements within the South American continent.
Historical legacies also play a significant role. Europe has exported the highest proportion of its own native tree species (72.7%) to other parts of the world, a trend likely reflecting its long history of global trade and colonial-era plant transfers.
The traits of a successful traveller
The study found that tree introductions are not random; they are phylogenetically conserved, meaning successful introductions tend to cluster within specific plant families. The researchers identified specific ecological traits that make a tree species more likely to be moved and established in a new range.
Taller tree species, those with a high specific leaf area, and those with lower leaf phosphorus content are more frequently introduced. These characteristics are often associated with fast-growing, competitive species that are desirable for forestry or urban greening. However, the prevalence of these traits also underscores a potential risk; species with highly competitive dominance can eventually lead to biotic homogenization, where diverse local ecosystems are replaced by a few widespread, non-native species.
Socioeconomics over geography
While climate remains a factor – with non-native richness generally increasing in warmer, wetter environments – the study found that human activity is the primary driver of tree redistribution. Regions with higher Gross Domestic Product (GDP) and greater levels of human modification to the landscape show significantly higher numbers of non-native trees.
Interestingly, the research found that islands host a comparable number of non-native species to mainland areas, despite their geographic isolation. This suggests that high niche availability and lower biotic resistance from native species on islands may make them particularly susceptible to successful introductions.
The paradox of the ‘genetic ark’
One of the most striking findings involves threatened species. The study identified 4,766 tree species that are introduced as non-natives while being classified as vulnerable, endangered, or critically endangered in their home ranges.
These introduced populations can act as ‘genetic arks’, providing a safety net against total extinction when wild populations in their native ranges collapse due to habitat loss or climate change. For example, the Ginkgo biloba, though rare in the wild in China, is now common in cultivation globally.
This conservation benefit comes with a caveat. Some threatened species can transition from being a conservation asset to an invasive threat in their new environment. The researchers cite the example of Pinus radiata, which, while of conservation concern in parts of its native range, has become a major invasive species in New Zealand.
Managing the invasion debt
The researchers warn of a phenomenon known as ‘invasion debt’, where the negative ecological impacts of an introduced species may not manifest for several decades. A species introduced today for its perceived climate resilience or rapid growth may only become a problematic invader in the future.
The study concludes that proactive policy interventions and international cooperation are essential. While introduced trees provide undeniable socioeconomic benefits, the authors urge for biosecurity strategies that prioritize native biodiversity and include rigorous risk assessments before new species are introduced.
By providing the first global map of where these trees are and why they were moved, this research offers a vital foundation for balancing the economic utility of trees with the urgent need to preserve the world’s ecological integrity.
Reference: Zhu, Y., et al. ‘Global patterns and drivers of tree introductions.’ Current Biology, May 2026. DOI:10.1016/j.cub.2026.04.044.
Image: Radiata pine plantation in New Zealand. Credit: Photon-Photos / Getty Images.