The examples of Bromus japonicus and Poa pratensis (Table 6) showed that exotic species cannot be viewed as a group of species with similar responses to the same level of disturbance.
For example, Bromus japonicus, which we found in the northern mixed prairie site, increases litter buildup and decreases soil evaporation, which favors its germination and establishment (Whisenant 1990).
One of the most ubiquitous introduced species is Bromus japonicus,(2) a cool-season, annual grass, that has successfully invaded large portions of grazed as well as ungrazed North American Great Plains, mixed-grass prairie (e.g., see Whisenant, 1990; Haferkamp et al., 1993; Heitschmidt et al., 1987, 1995).
We know from general observation that Bromus japonicus can modify the physiognomy of mixed-grass prairie communities.
Other important species were Bromus japonicus, a cool-season, annual grass; Poa secunda, a cool-season, perennial grass; Sporobolus cryptandrus, a warm-season, perennial grass; Festuca octoflora, a cool-season, annual grass; Plantago patagonica, a cool-season, annual forb; and Opuntia polyacantha, a warm-season, succulent.
Because we could not discriminate between Bromus japonicus and B.
Seed densities peaked in late spring and were dominated by the annual grasses (12,435 [m.sup.-2]) with the dominant species being Bromus japonicus (10,430 [m.sup.-2]).
Annual grasses were the dominate species [ILLUSTRATION FOR FIGURE 4 OMITTED] with Bromus japonicus seedling density peaking at 2080 seedlings [m.sup.-2] in late September 1991.
Patterns of emergence for annual grasses and forbs were similar between September 1991 and July 1992 although Bromus japonicus seedlings emerged about 1 wk earlier than other annual grasses and forbs in 1991.
However, close examination revealed only a minor portion of this pulse was from newly emergent seedlings; rather most of the pulse was attributable to regrowth of previously defoliated annual grass plants, particularly Bromus japonicus plants.
Similarly, seedling densities were sharply less in the fall and winter of 1992-1993 than 1991-1992 following a peak annual-grass seedling density in late August of about 750 Bromus japonicus seedlings [m.sup.-2].
Bromus japonicus seedling densities declined 80% between late summer 1992 and early spring 1993 [ILLUSTRATION FOR FIGURE 4 OMITTED].