Landscape characteristics of disturbed shrubsteppe habitats in southwestern Idaho (USA)

We compared 5 zones in shrubsteppe habitats of southwestern Idaho to determ ine the effect of differing disturbance combinations on landscapes that onc e shared historically similar disturbance regimes. The primary consequence of agriculture, wildfires, and extensive fires ignited by the military duri ng training activities was loss of native shrubs from the landscape. Agricu lture created large square blocks on the landscape, and the landscape conta ined fewer small patches and more large shrub patches than non-agricultural areas. In contrast, fires left a more fragmented landscape. Repeated fires did not change the distribution of patch sizes, but decreased the total ar ea of remaining shrublands and increased the distance between remaining shr ub patches that provide seed sources. Military training with tracked vehicl es was associated with a landscape characterized by small, closely spaced, shrub patches. Our results support the general model hypothesized for conversion of shrubl ands to annual grasslands by disturbance. Larger shrub patches in our regio n, historically resistant to fire spread and large-scale fires because of a perennial bunchgrass understory, were more fragmented than small patches. Presence of cheatgrass (Bromus tectorum), an exotic annual, was positively related to landscape patchiness and negatively related to number of shrub c ells. Thus, cheatgrass dominance can contribute to further fragmentation an d loss of the shrub patch by facilitating spread of subsequent fires, carri ed by continuous fuels, through the patch. The synergistic processes of fra gmentation of shrub patches by disturbance, invasion and subsequent dominan ce by exotic annuals, and fire are converting shrubsteppe in southwestern I daho to a new state dominated by exotic annual grasslands and high fire fre quencies.