Regional Variation in Phragmites australis Reproductive Traits and Seedling Performance in North America

Abstract

Seedling recruitment is an important mode of spread utilized by many invasive plants. In widespread invasive plants, regional variation in the rates of seedling recruitment can contribute to differences in invasion intensity across regions. In this study we examined regional variation in reproductive traits and seedling performance in a cosmopolitan invasive wetland grass, Phragmites australis. We tested whether nitrogen levels and regions with different histories and intensities of invasion would affect reproductive traits and seedling performance. We sampled invasive Phragmites inflorescences from 34 populations across three regions in North America: The Northeast (oldest most intense invasion), the Midwest (recent intense invasion), and Southeast (recent sparse invasion). We hypothesized that Northeast Phragmites populations would have the highest reproductive output and seedling performance, and that populations experiencing high nitrogen pollution would have higher reproductive output and seedling performance under high nitrogen conditions. We found that populations in the Northeast had the highest inflorescence mass, as expected. We also found that despite sparse distribution of Phragmites in the Southeast, populations from the Southeast displayed a high potential for sexual reproduction. However, increasing watershed-level nitrogen decreased percent seed germination in Southeastern populations, suggesting that Southeastern populations are sensitive to rising nitrogen levels. While elevated nitrogen increased belowground growth in Southeastern Phragmites seedlings, elevated nitrogen decreased belowground growth in Midwestern seedlings. These results suggest that the southeastern region of North America may become the next invasion front of Phragmites, warranting more research into the possible management of Phragmites spread in the region.