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

Abstract

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 AmericanSpartina densiflorain 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 comparedS. densifloraseedling establishment with and without competition withPhragmites australisandTypha domingensis, two hydrophytes differing in clonal architecture. We hypothesized that seedlings of the stress tolerantS. densiflorawould be out-competed by stands ofP. australisandT. domingensis. Growth, survivorship, biomass patterns and foliar nutrient content were recorded in a common garden experiment to determine the effect of matureP. australisandT. domingensison the growth and colonization ofS. densifloraunder fresh water conditions where invasion events are likely to occur. MatureP. australisstands prevented establishment ofS. densifloraseedlings andT. domingensisreducedS. densifloraestablishment by 38%. Seedlings grown withP. australisproduced 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 ofS. densiflora. The presence of healthy stands ofP. australis, and to some extentT. domingensis, along river channels and in brackish marshes may prevent the invasion ofS. densifloraby stopping the establishment of its seedlings.