Nutrient enrichment and herbivore exclusion disrupt the climate-driven balance between C 3 and C 4 plants in grasslands

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Abstract The distribution of plants with different photosynthetic pathways is strongly structured by climate, with C 3 plants favoured in cooler temperate regions and C 4 plants in hotter, high-light conditions. The relative abundance of C 3 and C 4 plants across the world has cascading impacts on local food webs, decomposition, productivity and other vital ecosystem processes. Human impacts, including climate change, changes to herbivore assemblages, and increased nutrient availability, are shifting the optimal conditions for important C 3 and C 4 -dominated ecosystems and crops. Using 3,184 plot-level observations from 112 sites across six continents, we reveal how chronic nutrient enrichment disrupts the climate-driven balance between C 3 and C 4 plants in grasslands. We found that, consistent with expectations, the global distribution of C 4 plants was strongly related to climate. However, experimental nutrient addition reduced the relative cover of C 4 species, with the strongest declines found when nitrogen and phosphorus were added together. Herbivore exclusion had no consistent effect on C 4 plants. Our results provide global experimental evidence that elevated nutrients, particularly nitrogen, alter competitive outcomes among plant functional types to suppress C 4 grasses, even in climatically optimal conditions. This has major implications for predicting vegetation responses to global change, with consequences for carbon cycling, primary productivity, herbivore dynamics, and food security.

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