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Nitrogen-fixing plants are vanishing from temperate forests, not due to climate change, but because of human-caused nitrogen pollution.
Climate change is altering plant populations across the world, but something peculiar is happening in the temperate forests of Europe and the USA. A new study by Moreno-García and colleagues shows that nitrogen-fixing plants are in decline. These are plants that are able to take nitrogen from the atmosphere and ‘fix’ it into compounds that they and other plants can use. The team found that human deposition of nitrogen compounds as pollution is to blame.
The researchers used the forestREplot database to track changes in the composition of forests. Taking over 900 pairs of survey and resurvey plots from fifty-three sites enabled them to compare change over time, with the earliest surveys being from 1940 and the latest from 2019. They could then see how the proportion of nitrogen-fixing species changed over time, as well as see what had changed in the plots.
Contrary to predictions, climate change didn’t benefit nitrogen fixers, with temperature changes having no significant effect on nitrogen-fixer diversity. Aridity changes showed only a marginal effect. The big difference was the effect of nitrogen deposition. So why aren’t the nitrogen-fixers suffering from climate effects?
One possibility put forward in the paper is that the plants would be affected, if they had a chance but the effects are relatively slow compared to the impact of the massive rise in pollution. Temperature and aridity had risen, but nitrogen deposition was almost ten times higher between surveys.
No nitrogen fixer species seem to be coming out as consistent winners or losers across all sites, with all species losing out somewhere. In addition, it seems we’re not just losing species, but whole branches of phylogenetic diversity. This is particularly concerning because evolutionarily divergent species often play unique roles in ecosystems. Their loss could mean the loss of unique functions or adaptations that might be important for ecosystem resilience, especially in the face of environmental changes.
Moreno-García, P. et al., 2024. Long-term nitrogen deposition reduces the diversity of nitrogen-fixing plants. Science Advances, 10(42), p.eadp7953. https://doi.org/10.1126/sciadv.adp7953 (OA)
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