What is the name of the broad expanse of flat land in the western United States and western Canada  where the species of plant called the Polygonum sawatchense of the buckwheat family be found?
Polygonum sawatchense Polygonum sawatchense is a North American species of plants in the buckwheat family. It in the western United States and western Canada, from the Pacific Coast to the western Great Plains, so from British Columbia south to California and east as far as New Mexico, Colorado, Saskatchewan, and the Dakotas.
Spiraea splendens Spiraea splendens is a shrub of the Rose Family (Rosaceae) native to the western mountains of North America, from California to British Columbia, commonly known as dense-flowered spiraea, rose meadowsweet, rosy spiraea, subalpine spiraea, and mountain spiraea. It is commonly found at elevations between 2,000 and 11,000 feet on inland mountain ranges. The plant is adapted to cold, moist, rocky slopes, subalpine forests and meadows.