PARENT SESSION
Oral Session #49: Restoration Ecology. Presiding: J. Callaway.
Wednesday, August 8, 2001. 1:00 PM to 4:30 PM. Madison Ballroom D.
The replacement of wetland vegetation by reed canary grass (Phalaris arundinacea) .
Kercher, Suzanne1, Lindig-Cisneros, Roberto1, Maurer, Debbie1, Werner, Katy1, Kercher, Suzanne1, Miller, Becky1, Niendorf, Maria1, Zedler, Joy1, 1
ABSTRACT- Although invasions are becoming understood, the complete replacement of native species by invasive monotypes is not well explained. We hypothesize that environmental changes that increase both community invasibility and invader aggressiveness allow the replacement of native vegetation, based on our studies of Phalaris arundinacea (Pa, a clonal grass) in Wisconsin. We found that Pa is limited by its requirement for light, which is needed by seeds to germinate and rhizome fragments to sprout. Species-rich canopies (e.g., fens, Carex stricta meadows) and mosses intercept most of the light, reducing invasibility. When disturbance opens a gap in plant cover, light (red>far red) stimulates seed germination. A common disturbance to sedge meadows is stormwater inflow, which adds sediments that reduce the surface area of sedge tussocks and reduce species richness (a 33-cm tall tussock would lose 1.5 species per 10 cm of sediment accretion). With fewer species, the canopy transmits more light, increasing invasibility. Simultaneously, stormwater adds nutrients, increasing Pa aggressiveness. Pa is a good invader because its seeds germinate rapidly (6 d), and it grows earlier in spring and later in fall than most native species. We found that parent plants subsidize the growth of tillers into the dark understory. The hollow stems grow twice as long per unit biomass as a native grass. If blown down, side branches grow upward to compensate. Pa becomes even more aggressive with nutrient inflows; we found that it increases tiller density and length and allocates more biomass to height growth than to roots. Thus, influxes of sediment-rich stormwater simultaneously make native vegetation more invasible and Pa more aggressive. The result is a Pa monotype.
KEY WORDS: wetlands, invasive species, reed canarygrass, Phalaris arundinacea