Many anthropogenic global changes not only alter environmental conditions, but also increase the availability of plant resources, potentially promoting plant invasion. In semi-arid ecosystems, predicted changes include increases in carbon availability with elevated CO2 (eCO2), nitrogen (N) availability with deposition, and water availability with both increased precipitation (at high latitudes) and eCO2. Conversely, warming may reduce water availability. Over the past 14 years, we have conducted a series of experiments to test whether global changes alter invasion in mixed-grass prairie, the largest remaining grassland in North America. We asked (1) how and why do key invasive species respond to global changes? (2) do global changes interact with one another or with soil disturbance to influence invasion? (3) do global change responses depend on species origin or life history? To answer these questions, we added invasive species to plots of mixed-grass prairie treated with combinations of altered precipitation and nitrogen addition (experiment 1) or combinations of free air CO2 enrichment, infrared warming, and disturbance (experiments 2-4).
Results/Conclusions
Each global change increased invasion, but for different reasons. For example, snow-addition increased recruitment of Linaria dalmatica (Dalmatian toadflax), Centaurea diffusa (Diffuse knapweed), and Gypsophila paniculata (baby’s breath). Simulated N-deposition increased both recruitment and growth of L. dalmatica. Elevated CO2 increased L. dalmatica growth 13-fold through a combination of C-fertilization and increased soil water availability. Warming hastened the phenology of the winter-annual grass Bromus tectorum (downy brome), allowing it to grow larger in the early spring while competitors were inactive. Warming, which decreased soil water availability, was also the only global change observed to inhibit invasion of some species. While effects of different global changes were usually additive rather than interactive, N responses occurred only in combination with added snow. Across species, perennials responded more positively to eCO2 and annuals responded more positively to warming. Irrespective of life history, invasive species responded more positively to eCO2 and warming than did natives. We conclude that additive effects of multiple global changes may greatly increase invasion within the historically invasion-resistant mixed-grass prairie. While the mechanisms vary among global changes and species, they are consistent with the hypothesis that effects of global changes on invasion often mirror their effects on plant resource availability.