A study by Liu and colleagues published in PNAS has found that where human-introduced non-native tree species establish and spread in eastern US forests, native tree diversity decreases, altering forest community composition and biodiversity patterns. The study may help identify ecosystems at risk.
The study is the result of over 5 million tree measurements across eastern US forests, over almost 30 years. The results revealed a consistent pattern. In areas with introduced non-native trees, the diversity of those trees increased while native tree species diversity decreased over time.
“There’s this assumption that introduced species are not a good thing, but we don’t always know what that means,” said study co-author Doug Soltis. “People have tried to get at their impact using fine-scale studies. What this paper does is take a more macro-level approach.”
The team found the nature of new trees in an area mattered. If they were non-native, native biodiversity fell. But if the trees were native, they found that native tree species richness tended to increase beyond just the added species. The native trees created conditions for other natives to arrive.
The native trees that disappear after the arrival of non-natives tended to be species that were rare locally and regionally. It suggests that interactions may disproportionately affect rarer trees, which has implications for forest resilience.
The key to co-survival is to be niche differentiation. Native trees persist when they are functionally distinct. “Phylogenetic and trait-space distances between different species groups may also help explain why certain native species survived nonnative invasion while others went locally extinct.”
Despite diversity decreasing when exotic trees arrive, biomass did not. Indeed, “the biomass of both nonnatives and natives increased significantly over time in invaded plots.” This might be a hangover of other human factors, such as increased CO₂, and more nitrogen in the soil from fertilisers.
The rate of change is accelerating. In a press release the team highlight Chinese tallow tree (Triadica sebifera) as a problem. They comment on how it “quickly and efficiently went about converting prairies into woodlands made primarily of tallow trees,” as well as taking over dormant pasture.
The problem today is the result of deliberate human choices in the past. The U.S. Department of Agriculture incentivized growing tallow trees, to try creating a soap manufacturing industry, and nurseries have sold trees like tree-of-heaven and Chinese tallow as ornamentals.
The authors aren’t offering a quick fix for the problem, but they believe their research will help protect some species. “What we can offer is a risk map,” Dr Yunpeng Liu says. “We can tell people which region or ecosystem they should pay more attention to in the future.”
Liu, Y., Scheiner, S.M., Hogan, J.A., Thomas, M.B., Soltis, P.S., Guralnick, R.P., Soltis, D.E., & Lichstein, J.W. (2025). Nonnative tree invaders lead to declines in native tree species richness. Proceedings of the National Academy of Sciences, 122(17), e2424908122. https://doi.org/ph36
Cross-posted to Bluesky & Mastodon.
Cover: Chinese tallow tree (Triadica sebifera). Image: Canva.
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