These two reports to aid in the protection of plants; Species at Risk In SK and Rare Plant Survey Guidelines. Saskatchewan has implemented "The Weed Control Act"  to control plants introduced to Saskatchewan which have become a threat to the natural biodiversity such as leafy spurge ("Euphorbia esula"). There are two reports in this regard; Invasive Species and Noxious Weeds of Saskatchewan. The tree which was designated in 1988 as a symbol of Saskatchewan is the Paper Birch "Betula papyrifera". Saskatchewan's provincial flower is the Western red lily "Lilium philadelphicum" var. "andinum" ("a protected species") designated in 1941. Needle-and-thread grass "Hesperostipa comata" is Saskatchewan's provincial grass declared in 2001. Saskatchewan is within the Holarctic Kingdom. There are two regions within this kingdom, the Circumboreal floristic region or which provides a cool northern temperate zone and the North American Atlantic Region in Southern Saskatchewan which is part of a warmer Midwestern Plains zone. These zones are characterized by a certain degree of endemism. An ecoregion encompasses soil types and landform similarities. The Taiga Shield ecozone in the far north includes the Selwyn Lake upland and Tazin Lake Upland ecoregion. This would have vegetation generally corresponding to the Subarctic Woodland. The Boreal Shield ecozone is further divided into the Athabasca Plain and Churchill River Upland, and this area of Northern Saskatchewan has been described by the World Wildlife Fund as part of the Midwestern Canadian Shield forests ecoregion. The ecozone Boreal Plains comprises the Mid-Boreal Upland, Mid-Boreal Lowland and Boreal Transition ecoregions. Further south is the Prairie ecozone which consists of the Aspen Parkland, Moist Mixed Prairie, Northern Mixed Grassland and Cypress Upland ecoregions. The ecoregions are further divided into Landscape Areas. Several biogeographic factors contribute to the richness and diversity of Saskatchewan flora. From north to south there are a variety of vegetative zones. To the far north are the Subarctic Woodland and Northern Boreal Forest. The Southern Boreal Forest is south of the treeline. The Prairie is divided into the Aspen Parkland, Moist Mixed Grassland, Mixed Grassland, Cypress Upland and Fescue Grassland.