Nitrogen addition, but not pulse frequency, shifts competitive interactions in favor of exotic invasive plant species

Temporally dynamic resource supplies may alter or lead to fluctuations in competitive outcomes. Resource pulses have been theorized to promote incursion by exotic species with environments prone to higher resource fluctuations being more susceptible to invasion than those with more stable resource supplies. This is thought to be due, in part, to the ability of invasive species, especially those that are fast growing, to utilize available resources more rapidly than natives. We compared the effects of high-frequency (more but smaller pulses) versus low-frequency pulses of nitrogen (N) on interactions among two native grasses and two exotic invasive forbs planted alone and in pairs. The total quantity of N added was the same. Nitrogen pulse frequency had no effect on the biomass of exotics or natives when grown without competition. In all treatments combined, the competitive effect of exotics on natives was roughly three times higher than the competitive effect of natives on exotics. The competitive effect of exotics on natives was not affected by N additions, but N additions weakened the competitive effect of natives on exotics. Species-specific patterns in our results suggest that N pulses sometimes impacted competitive intensity, but N pulse frequency did not alter competitive intensity. Our results are inconsistent with the idea that fluctuations in resource supply ubiquitously favor invaders, as native species were as responsive to pulse frequency as exotics. However, increased N, regardless of pulse frequency, favored exotic species, consistent with a large literature showing that high resource supplies favor exotic invaders.