Question: 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?

full_context: Document 1: Depopulation of the Great Plains: The depopulation of the Great Plains refers to the large-scale migration of people from rural areas of the Great Plains of the United States to more urban areas and to the east and west coasts during the 20th century. This phenomenon of rural-to-urban migration has occurred to some degree in most areas of the United States, but has been especially pronounced in the Great Plains states, including Texas, Oklahoma, Kansas, Nebraska, South Dakota, North Dakota, Montana, Wyoming, Colorado, and New Mexico where many counties have lost more than 60 percent of their former populations.
Document 2: Tallgrass Prairie National Preserve: Tallgrass Prairie National Preserve is a United States National Preserve located in the Flint Hills region of Kansas, north of Strong City. The preserve protects a nationally significant example of the once vast tallgrass prairie ecosystem. Of the 400,000 square miles (1,000,000 km) of tallgrass prairie that once covered the North American continent, less than 4% remains, primarily in the Flint Hills. Since 2009, the preserve has been home to the growing Tallgrass Prairie bison herd.
Document 3: Polygonum californicum: Polygonum californicum is a species of flowering plant in the buckwheat family known by the common name California knotweed. It is native to the west coast of the United States from Washington, Oregon, and northern and central California in the Coast Ranges and Sierra Nevada foothills as far south as Napa and Tulare Counties. It can be found in many types of open habitats.
Document 4: High Plains (United States): The High Plains are a subregion of the Great Plains mostly in the Western United States, but also partly in the Midwest states of Nebraska, Kansas, and South Dakota, generally encompassing the western part of the Great Plains before the region reaches the Rocky Mountains. The High Plains are located in southeastern Wyoming, southwestern South Dakota, western Nebraska, eastern Colorado, western Kansas, eastern New Mexico, western Oklahoma, and south of the Texas Panhandle. The southern region of the Western High Plains ecology region contains the geological formation known as Llano Estacado which can be seen from a short distance or from on satellite maps. From east to west, the High Plains rise in elevation from around 1160 ft to over 7800 ft .
Document 5: 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.
Document 6: Polygonum majus: Polygonum majus is a North American species of flowering plant in the buckwheat family known by the common name wiry knotweed. It grows in the western United States and western Canada, from British Columbia south as far as the Sierra Nevada of northwestern Inyo County in California, east as far as Montana.
Document 7: Polygonum erectum: Polygonum erectum is a North American species of annual plant species in the buckwheat family, with upright or ascending stems, called erect knotweed. It was once cultivated for food by Native Americans as part of the group of crops known as the Eastern Agricultural Complex. It is found primarily in the northeastern and north-central parts of the United States, but with scattered populations in other parts of the US and also in Canada.
Document 8: Polygonum austiniae: Polygonum austiniae, common name Mrs. Austin's knotweed, is a plant species in the buckwheat family. It is native to western Canada and the western United States, from Alberta and British Columbia south as far as California, Nevada, and Wyoming.
Document 9: Northern short grasslands: The Northern short grasslands is one of 867 terrestrial ecoregions defined by the World Wide Fund for Nature. This ecoregion includes parts of the Canadian provinces of Alberta and Saskatchewan, and the American Great Plains states of Montana, North Dakota, Wyoming, South Dakota and Nebraska. The United States Environmental Protection Agency (EPA) further breaks this ecoregion into the Northwestern Glaciated Plains and Northwestern Great Plains.
Document 10: Tallgrass Prairie Preserve: The Tallgrass Prairie Preserve, located in Osage County, Oklahoma near Foraker, Oklahoma, is owned and managed by The Nature Conservancy. It is protected as the largest tract of remaining tallgrass prairie in the world. The preserve contains 39000 acre owned by the Conservancy and another 6000 acre leased in what was the original tallgrass region of the Great Plains that stretched from Texas to Manitoba.
Document 11: Osage Plains: The Osage Plains are a physiographic section of the larger Central Lowland province, which in turn is part of the larger Interior Plains physiographic division. The area is sometimes called the Lower Plains, North Central Plains, and the Rolling Plains. The Osage Plains, covering west-central Missouri, the southeastern third of Kansas, most of central Oklahoma, and extending into north-central Texas, is the southernmost of three tallgrass prairie physiographic areas. It grades into savanna and woodland to the east and south, and into shorter mixed-grass prairie to the west. The Osage Plains consist of three subregions. The Osage Plains proper occupy the northeast segment. Although sharply demarcated from the Ozark uplift, the Plains are nonetheless a transitional area across which the boundary between prairie and woodland has shifted over time. In the central portion of the physiographic area lies the second subregion, the Flint Hills, commonly called "The Osage" in Oklahoma. This large remnant core of native tallgrass prairie is a rocky rolling terrain that run from north to south across Kansas and extends into Oklahoma. To the west and south of these hills are the Blackland Prairies and Cross Timbers. This vegetatively complex region of intermixed prairie and scrubby juniper-mesquite woodland extends into north-central Texas. Bluestem prairies and oak-dominated savannas and woodlands characterize the natural vegetation in the Cross Timbers. Much of the area has been converted to agriculture, although expanses of oak forest and woodland are still scattered throughout the eastern portion of the subregion.
Document 12: Great Plains: The Great Plains (sometimes simply "the Plains") is the broad expanse of flat land (a plain), much of it covered in prairie, steppe and grassland, that lies west of the Mississippi River tallgrass prairie states and east of the Rocky Mountains in the United States and Canada. It embraces
Document 13: Polygonum engelmannii: Polygonum engelmannii, common name Engelmann's knotweed, is a North American species of plants in the buckwheat family. It is native to western Canada and the western United States, usually at high elevations in the mountains. It has been found in Alberta, British Columbia, Idaho, Montana, Wyoming, Nevada, Utah, and Colorado.
Document 14: Tallgrass Prairie National Preserve: Tallgrass Prairie National Preserve is a United States National Preserve located in the Flint Hills region of Kansas, north of Strong City. The preserve protects a nationally significant example of the once vast tallgrass prairie ecosystem. Of the 400,000 square miles (1,000,000 km) of tallgrass prairie that once covered the North American continent, less than 4% remains, primarily in the Flint Hills. Since 2009, the preserve has been home to the growing Tallgrass Prairie bison herd.
Document 15: Polygonum spergulariiforme: Polygonum spergulariiforme is a North American species of flowering plants in the buckwheat family known by the common name spurry knotweed or fall knotweed. It grows in western Canada (British Columbia and Saskatchewan) and the western United States (primarily Washington, Oregon, and northern and central California but with a few isolated populations in Idaho, Wyoming and Montana).
ground_truth: The Great Plains