Miller and Halpern reconstructed patterns of tree invasion at 17 meadow sites in the central Cascade Range of Oregon-sites representing diverse physical environments and vegetation types and experiencing different histories of recent anthropogenic disturbance (sheep grazing). Spatial distributions and age structures of invasive tree populations were analyzed with respect to climate records and grazing history. Removal of sheep in the montane zone was synchronous with tree invasion on south facing slopes and steep, highly dissected slopes, but poorly correlated in hydric meadow basins. In the sub-alpine region correlations between invasions and cessation of grazing were generally poor (especially on south facing slopes), and Festuca viridula (a bunchgrass) remained a dominant species in these meadows, which suggests limited range deterioration due to grazing. Generally, severe disturbance followed by complete removal of grazers contributes to increased forestation, but on a regional scale, climate and fire will be more dominant factors.
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