Adam M. Lambert

Ecology and Impacts of the Large-Statured Invasive Grasses Arundo donax and Phragmites australis in North America

Abstract Large-statured invasive grasses (LSIGs) constitute a distinct functional group with characteristic life history traits that facilitate colonization and aggressive growth in aquatic ecosystems, particularly those modified by human activities. These species typically form monocultures in the systems they invade and have wide-ranging and negative impacts on biodiversity and ecosystem processes. In March 2008, a special symposium was held as part of the Western Society of Weed Scientists annual meeting to synthesize our current knowledge of the ecological impacts and management of two notorious LSIGs: Arundo donax and Phragmites australis. In this volume of Invasive Plant Science and Management, symposium participants provide articles summarizing existing knowledge, recent research progress, and research needs for these two taxa. Here, we summarize the basic biology of these species and suggest the use of a more holistic approach to deal with the effects and management of LSIG invasions. Nomenclature: Giant reed, Arundo donax L.; common reed, Phragmites australis (Cav.) Trin. ex Steud.

Genetics and Reproduction of Common (Phragmites australis) and Giant Reed (Arundo donax)

Abstract Genetic diversity and reproductive characteristics may play an important role in the invasion process. Here, we review the genetic structure and reproductive characteristics of common reed and giant reed, two of the most aggressive, large-statured invasive grasses in North America. Common reed reproduces both sexually and asexually and has a complex population structure, characterized by three subspecies with overlapping distributions; of which, one is introduced, one native, and the third is of unknown origins. These three subspecies show varying levels of genetic diversity, with introduced common reed having high levels of nuclear diversity, indicating that multiple introductions have likely occurred. In contrast, giant reed has low genetic diversity and appears to reproduce solely via asexual fragments yet is highly aggressive in parts of its introduced range. Both species are well-adapted for growth in human-dominated landscapes, which is presumably facilitated by their rhizomatous growth habit. Nomenclature: Common reed, Phragmites australis (Cav.) Trin. ex Steudel; giant reed, Arundo donax L

A Tale of Three Lineages: Expansion of Common Reed (Phragmites australis) in the U.S. Southwest and Gulf Coast

Abstract The common reed invasion in North America has spanned two centuries and is still ongoing. This expansion comprises two main forms: an introduced Eurasian lineage (identified here as “Introduced Phragmites”) and a Gulf Coast lineage of unknown origin (identified here as “Gulf Coast Phragmites”). Both lineages are spreading beyond their current ranges and are colonizing Southwestern and Gulf Coast ecosystems where they have not previously existed. As a result, the native North American lineage of common reed (hereafter “native Phragmites”) has declined in many places. The recent invasion of the U.S. Southwest by Introduced and Gulf Coast Phragmites lineages has made this the only region in the world colonized by all three lineages. Along the central Gulf Coast where Gulf Coast Phragmites remains the dominant form, Introduced Phragmites has also recently invaded the Mississippi River delta. The consequences of these new invasions are uncertain, but a rapid response is needed to protect native species and ecosystems and reduce future control costs. Nomenclature: Common reed, Phragmites australis (Cav.) Trin. ex Steud

Distribution of native and exotic Phragmites australis in Rhode Island.

Abstract Exotic populations of Phragmites australis (common reed) are now present in southern New England wetland habitats where native populations were once abundant. We surveyed Rhode Island to determine the distribution of native and exotic P. australis, and used this information to build a publicly accessible Geographic Information System (GIS) database. All P. australis populations sampled on the mainland were exotic. We only found native populations growing throughout a network of tidal marshes and ponds on Block Island, and several of these populations are being overrun by expanding exotic populations. The GIS database from this survey can be expanded to other regions, and can be used for the conservation of the native subspecies and for ensuring that control efforts target only exotic populations.

Exotic wildland weeds serve as reservoirs for a newly introduced cole crop pest, Bagrada hilaris (Hemiptera: Pentatomidae)

The role of wildland weeds that serve as alternate hosts for insect pests has not been directly examined for the potential to sustain pest populations or facilitate pest outbreaks. The development of weed biological control programmes is also complicated by weed species that are closely related to economically important or native plants, especially rare or special status species. In recent surveys in southern California, USA, we found a newly introduced insect pest of cole crops, Bagrada hilaris Burmeister (Bagrada bug), feeding on Brassicaceae weeds in riparian areas adjacent to agricultural fields where cole crops are routinely grown. Insect populations grew to levels well over action thresholds and caused severe damage to populations of the invasive weed, Lepidium latifolium (perennial pepperweed). The numerical response of B. hilaris on L. latifolium and other Brassicaceae weeds in natural areas may pose a significant challenge to effectively managing pest populations in crops. However, the accidental introduction of this insect provides the opportunity to examine plant–insect interactions with important implications for development of biologically based control methods for weeds.

No Evidence of Fungal Endophytes in Native and Exotic Phragmites australis

Abstract Native and exotic haplotypes of Phragmites australis show differential susceptibility to herbivores, but the mechanisms behind these differences are not known. Endophytic fungi are common in the grass family and confer resistance against insects through the production of toxic chemicals. We used both a common endophyte-staining technique and interference contrast microscopy to analyze leaf sheaths of native and exotic P. australis haplotypes from several populations in the northeastern United States to determine if the various haplotypes were infected with endophytes. No endophytes were found in any of the native and exotic haplotypes using procedures that consistently detected endophytes in infected rye grass.

Competition from native hydrophytes reduces establishment and growth of invasive dense-flowered cordgrass (Spartina densiflora)

Experimental studies to determine the nature of ecological interactions between invasive and native species are necessary for conserving and restoring native species in impacted habitats. Theory predicts that species boundaries along environmental gradients are determined by physical factors in stressful environments and by competitive ability in benign environments, but little is known about the mechanisms by which hydrophytes exclude halophytes and the life history stage at which these mechanisms are able to operate. The ongoing invasion of the South American Spartina densiflora in European marshes is causing concern about potential impacts to native plants along the marsh salinity gradient, offering an opportunity to evaluate the mechanisms by which native hydrophytes may limit, or even prevent, the expansion of invasive halophytes. Our study compared S. densiflora seedling establishment with and without competition with Phragmites australis and Typha domingensis, two hydrophytes differing in clonal architecture. We hypothesized that seedlings of the stress tolerant S. densiflora would be out-competed by stands of P. australis and T. domingensis. Growth, survivorship, biomass patterns and foliar nutrient content were recorded in a common garden experiment to determine the effect of mature P. australis and T. domingensis on the growth and colonization of S. densiflora under fresh water conditions where invasion events are likely to occur. Mature P. australis stands prevented establishment of S. densiflora seedlings and T. domingensis reduced S. densiflora establishment by 38%. Seedlings grown with P. australis produced fewer than five short shoots and all plants died after ca. 2 yrs. Our results showed that direct competition, most likely for subterranean resources, was responsible for decreased growth rate and survivorship of S. densiflora. The presence of healthy stands of P. australis, and to some extent T. domingensis, along river channels and in brackish marshes may prevent the invasion of S. densiflora by stopping the establishment of its seedlings.