Biotic resistance, disturbance, and mode of colonization impact the invasion of a widespread, introduced wetland grass

Abstract

Disturbance and biotic resistance are important factors driving plant invasions, but how these factors interact for plants with different modes of colonization (i.e., sexual and asexual) is unclear. We evaluated factors influencing the invasion of nonnative Phragmites australis, which has been rapidly expanding in brackish tidal wetlands in Chesapeake Bay. We conducted a survey of naturally occurring small-scale disturbances (removal of vegetation and/or sediment deposition) across four plant communities; determined the effects of small-scale disturbance and biotic resistance on P. australis seedling and rhizome emergence; and tested the effects of size and frequency of small-scale disturbances on seedling emergence and survival of transplanted seedlings. The results of our study demonstrate that the invasion window for seeds is in disturbed areas in high-marsh plant communities that flood less frequently; seedling emergence in undisturbed areas was negligible. Establishment of shoots from rhizome segments was low in all plant communities. Disturbance size and frequency had no significant impact on seed germination and seedling survival. Our findings provide evidence that small-scale within-wetland disturbances are important for the invasion of the nonnative lineage of P. australis by seeds in brackish tidal wetlands in Chesapeake Bay. Efforts to reduce disturbances, large and small, in wetlands can be used to limit P. australis invasion by seed, but invasion by rhizome is still likely to occur across many plant communities irrespective of the presence of disturbance.

Management of invasive Phragmites australis in the Adirondacks: a cautionary tale about prospects of eradication

“…In our study only the smallest sites expanded rapidly and we attribute failure of P. australis to achieve higher expansion rates to lack of disturbed habitat in the Adirondacks. Except along major roadways, lack of human disturbances is limiting P. australis seedling establishment (Byun et al 2015;Kettenring et al 2015) and regional encroachment (Bertness et al 2002;Silliman and Bertness 2004). Large populations may have also reached local limits of available wetland habitat within the forest matrix despite [240,000 ha of wetlands in the Adirondacks.…”

Life on the edge: reproductive mode and rate of invasive Phragmites australis patch expansion

“…Expansion of a newly formed patch into the surrounding matrix may also occur clonally or by seed. The composition of the surrounding matrix is a strong determinant of seedling recruitment, with seedling success increasing with disturbance to matrix vegetation, hydrologic drawdowns, and other favorable abiotic conditions (e.g., suitable salinity levels) (Weisner et al 1993;ter Heerdt and Drost 1994;Armstrong et al 1999;Clevering and Lissner 1999;Mauchamp et al 2001;Chambers et al 2003;Alvarez et al 2005;Baldwin et al 2010;Wilcox 2012;Kettenring et al 2015). Patch expansion through rhizome extension would be expected to occur under a broader range of environmental conditions than seedling establishment, including flooding, since ramets of the same genet would be subsidized through a network of rhizome connections (Brisson et al 2010;Bhattarai and Cronin 2014).…”

“…Expansion of existing patches can also occur via seedling recruitment at patch edges, particularly with drawdowns and disturbances to matrix vegetation. Patch edge expansion by seed would be expected to increase levels of genet richness at patch edges relative to patch interiors (Koppitz et al 1997;Clevering and Lissner 1999;Koppitz 1999;Kettenring et al 2015).…”

Local and regional disturbances associated with the invasion of Chesapeake Bay marshes by the common reed Phragmites australis

“…Elevated nitrogen increases P. australis sexual reproduction and expansion in Chesapeake Bay (Kettenring et al 2011); increases P. australis density, height, and above-ground shoot biomass (Bastlova et al 2004;Engloner 2009); and allows seedlings to rapidly escape from a vulnerable life stage (Saltonstall and Stevenson 2007;Kettenring et al 2015;Hazelton et al 2014). P. australis stands have larger nitrogen pools than other marsh communities (Meyerson et al , 2000Windham and Meyerson 2003).…”

“…These positive associations likely arise because the construction of riprap creates disturbed sites for colonization, and wave action can continue to disturb vegetation and substrate at the end of the protected shoreline. Phragmites australis stands often grow at the edges of structures such as riprap and bulkhead (Melissa McCormick, unpublished data), and disturbances as small as 30 cm in diameter can promote P. australis invasion (Kettenring et al 2015).…”