Floristic Plant Geography of Eastern Washington and Northern Idaho

The origins of the present vascular flora of eastern Washington and northern Idaho have been traced from the early Cenozoic era to the present by means of published fossil floras, supplemented by interpretations of modern species' ranges. The flora is shown to bepolyphyletic, resulting from three major immigrations coming from different sources at different times. Along a vegetation transect from steppe to subalpine forest, the contribution of these three floristic elements to the series of modern plant associations is expressed quantitatively, showing that each association contains species from at least two of the three elements.