Drivers of 30 years of ionomic change on a North American tallgrass prairie

Plant ionomes-the suite of chemical elements making up plant tissue-constrain plant performance and the nutrition of consumers. The possible mechanisms driving Nutrient Dilution-the globally distributed decline of essential element density (parts per million [ppm]) in plant tissue-are rarely evaluated together. Toward a remedy, we explored a 30+ year record of 17 elements in the grasses, forbs, and woody plants across three burn frequencies on Konza, a North American tallgrass prairie. About one quarter of ionomic variation arose among Konza's three plant functional groups, which differed in ppm and its regulation over three decades. Nutrient-poor grass biomass increased steadily with CO₂, encroaching woody plant biomass accelerated over the same period, and nutrient-rich forb biomass decreased. Each functional group revealed its own pattern of nutrient dilution across the ionome, where it was more widespread in grasses (12/17 elements) than forbs (5/17) or woody plants (2/17). Competition with other functional groups regularly depleted the ionome ppm of grasses (9 elements) and forb and woody plants (4 elements). Unexpectedly, nutrient densities often increased in response to higher CO₂ (especially in forbs), suggesting photosynthate was invested in nutrient harvest. Fire suppression had fewer, and more idiosyncratic effects. In an era of herbivore declines, grass-feeding herbivores in this tallgrass prairie are experiencing more abundant but lower quality food. Forb feeders, in contrast, must search for less abundant but sometimes enriched food.