Giant reed is an extremely aggressive riparian invader in California. Little is known about its response to nitrogen, which is often elevated in watersheds downstream from agricultural fields and wastewater treatment facilities. Two pot-experiments were conducted to quantify physiological responses of giant reed, and a co-occurring riparian species, common threesquare, to added nitrogen and to investigate a possible enhancement effect of nitrogen on the ability of giant reed to spread laterally belowground into a competitive environment. The first experiment measured shoot height, tissue biomass, and leaf area of giant reed and common threesquare, both herbaceous perennials, grown in pots with and without added nitrogen. The second experiment measured lateral rhizome growth, tissue biomass, and tiller production of giant reed in planters subjected to four possible treatments: with or without competition and with or without added nitrogen. Competition planters had previously been colonized by common threesquare and no-competition planters were unoccupied. Nitrogen-treated plants from the first experiment had greater overall shoot height. With added nitrogen, giant reed produced more root and shoot biomass, whereas common threesquare produced more rhizome and shoot biomass. In the second experiment, added nitrogen resulted in significantly greater rhizome length and greater production of tillers by giant reed regardless of competition. In competition plantings without added nitrogen, giant reed tiller production was reduced, whereas the addition of nitrogen nearly restored tiller production to levels attained without competition. Neither nitrogen nor competition significantly affected giant reed biomass production. Results of these experiments indicate the positive response of giant reed and a native riparian species to nitrogen enrichment and suggest that nitrogen can compensate for the effects of competition on giant reed in some cases. As a result, this species might be able to penetrate some environments without negative impacts from competing vegetation.

Nomenclature: Common threesquare, Scirpus americanus Pers. SCPAM, giant reed, Arundo donax L. ABKDO