1. Precipitation is considered to be a key driver of ecosystem processes in mesic grasslands, and climate models predict changes in the amount and intensity of precipitation under future global change scenarios. Although most experimental rainfall studies decrease precipitation, seasonal rainfall is predicted to increase in the northern Great Plains under climate change.
2. We analysed changes in community composition and structure of upland and lowland native tallgrass prairie in central Kansas, USA, subjected to 19 years of irrigation designed to eliminate moisture stress throughout the growing season.
3. Irrigation had limited effects on species richness in both upland and lowland prairie. Total cover increased significantly and consistently with irrigation in drier uplands and in more mesic lowlands. Abundance of rhizomatous, tall, perennial species as well as C3 forbs increased with irrigation.
4. The strongest response to irrigation came within the dominant functional type, C4 perennial grasses. Panicum virgatum became the dominant species in irrigated lowlands, whereas Andropogon gerardii remained the dominant species in irrigated uplands and in control plots. Overall, irrigation had less effect on community composition and structure than other known drivers of grassland structure and function.
5. In comparison with other studies, our results demonstrate that water addition has less of an impact than fire, grazing or nitrogen addition on composition and dynamics in this mesic grassland. The strongest response to long-term irrigation occurred within the dominant functional type: tall, perennial, rhizomatous, C4 grasses. Thus, functional redundancy will buffer this ecosystem from potential increases in rainfall due to climate change. Finally, our results highlight the limited utility of qualitative functional traits to predict how this mesic grassland will respond to climate change.