Environmental limits on the spread of invasive Phragmites australis into upland forests with marine transgression.
Phragmites australis is a cosmopolitan grass that has transformed tidal wetlands in regions where it is invasive. The position of P. australis at the upland margin of tidal wetlands makes it likely to be among the earliest species to colonize uplands in marshes responding to sea-level rise; P. australis has been spreading in step with transgressive forest retreat. Considered a high light demanding species, the forest shade has been thought to keep this prolific species in check. In this study, we surveyed the distribution of P. australis across the marsh-forest ecotone, with particular attention to low-light areas in the forest understory, an environmental setting that has been largely overlooked in investigations of this well-studied species. We found P. australis to be present and well-established in forest canopy cover of up to 87.4%. However, canopy cover was significantly and negatively related to P. australis density in a multivariate model describing the species' distribution in the ecotone. To test the species' growth in shade conditions corresponding to the forest understory, we performed a greenhouse shading experiment. While photosynthetic parameters and growth rates were reduced in shade treatments, shade-grown individuals exhibited relatively high rates of photosynthesis in light-response tests, and there was no difference between the species' performance in partial and deep shade treatments. We conclude that, while P. australis prefers high light conditions, it is able to colonize and survive low-light conditions in closed forest. By colonizing forest understory areas prior to other salt-tolerant species, P. australis is prepositioned to be the dominant species during saltwater intrusion and forest retreat.