Calcareous Loam

Mechanism

Transitions from State to State T1 Result: transition from Reference State to State 2 Annual Invasive Species. Primary Trigger: under heavy grazing pressure winterfat plants become hedged but persist remarkably well. Then Sandberg bluegrass becomes heavily grazed but does not fare well and invasive species invade. Secondary Trigger: high moisture year that causes a micro-flush of cheatgrass Ecological process: with consistent defoliation pressure winterfat plants become hedged while Sandberg bluegrass plants decline with poor vigor, shrinking crowns and mortality. Colonization by invasive species: a high moisture year that causes a micro-flush of cheatgrass and broadleaf weeds and is the principle means of colonization. Most sites in the Reference State receive invasive weed seed annually. This seed is waiting for enough moisture to germinate and to compete with the native species for space, light and moisture. When the right year happens even pristine communities in the Reference State are susceptible to colonization by invasive species. In normal and dry years invasive species are all but nonexistent. Expansion of invasive species: as Sandberg bluegrass continue to decline invasive species become co-dominant and then dominant. It is the declining Sandberg bluegrass cover that ensures invasive species will colonize successfully and expand. Indicators: The occurrence of invasive species on sites where they had been absent. Declining cover and increasing distance between Sandberg bluegrass plants.