IMPACTS OF GRASSLAND HABITAT ON YELLOW STARTHISTLE (CENTAUREA-SOLSITIALIS L) INVASION

Yellow starthistle, a Mediterranean winter annual introduced in southe astern Washington around the turn of the century, expanded rapidly in foothills rangeland of the Blue Mountains in the early 1960s, invading and dominating bluebunch wheatgrass communities on deeper soils of so uth-facing slopes. Examination of site differences (soil depth and asp ect) revealed a high correlation of yellow starthistle with stored soi l moisture on south-facing slopes. Yellow starthistle roots grew rapid ly and continuously during the winter giving it access to moisture sto red deep in the soil profile not depleted by the native plant associat ion. Yellow starthistle seedlings grown under reduced light produced s horter roots, larger leaves, more erect rosettes and fewer flowers tha n plants grown in full sunlight. Interaction of shade and moisture str ess appeared to reduce yellow starthistle invasion of four species of perennial grass treated with four seasonal clipping regimes: May, July , October, and no clipping. The two sod forming cultivars, ''Oahe'' in termediate wheatgrass and ''Luna'' pubescent wheatgrass were generally less susceptible to starthistle invasion under all clipping regimes t han the two bunchgrasses, ''Nordan'' crested wheatgrass and ''Whitmar' ' bluebunch wheatgrass. Yellow starthistle invaded Whitmar bluebunch w heatgrass clipped any time of year. All four grass species resisted st arthistle invasion if left unclipped. Two critical factors limiting st arthistle invasion in eastern Washington appeared to be light intensit y at the soil surface during the winter and residual soil moisture dur ing the summer.