Invasive grasses worldwide are causing declines in diversity, abundance, and fitness of their new ecological communities. Although the grass-fire cycle is one well-established mechanism by which invasive grasses transform vegetation communities, increasing fire prevention budgets is an insufficient tactic to prevent species loss. New research suggests a grass currently spreading throughout the Sonoran Desert, Pennisetum ciliare, decreases abundance and diversity of native shrubs, trees, and succulents. Further demographic studies indicate recruitment especially may be inhibited. To demonstrate whether P. ciliare inhibits native perennial seedling germination and survival, we monitored seedling emergence and persistence during the later portion of the 2011 summer rainy season in the foothills surrounding Tucson, Arizona. Nine of 17 one meter square plots containing P. ciliare and native shrubs were randomly selected to have the invasive grass removed with a combination of glyphosate application and mowing. An additional 8 plots were established in nearby uninvaded vegetation as a further comparison.
Results/Conclusions
Strong germination was observed on plots with P. ciliare removed compared with paired control plots with P. ciliare. However, nearby germination on unmapiulated plots without P. ciliare was not significantly different than on plots with P. ciliare. Seedling survival, however, generated an intriguing pattern. Only two native seedlings (3.7%) survived in native vegetation, which is similar to long term seedling survival studies conducted nearby. Three native seedlings (2.1%) survived on the plots where P. ciliare had been removed. No seedlings survived where P. ciliare remained. While not statistically significant, these results do suggest that the invasive grass inhibits what is already a sensitive and low survival life stage, potentially creating – or at least contributing to – the observed declines in native shrub and succulent abundance and diversity following invasion. Further studies should increase the power to detect such an effect, and additionally determine the specific pathway(s) by which seedling survival is inhibited. A more thorough understanding of the mechanisms by which plant invasions impact their new environments can help prioritize management of ongoing invasions, respond rapidly to new and analogous invasions, and reveal the underlying ecological dynamics of the invaded system.