The Mixedwood Plains Ecozone is the Canadian ecozone with the most southern extent, covering all of southwestern Ontario, and parts of central and northeastern Ontario and southern Quebec along the Saint Lawrence River. It was originally dominated by temperate deciduous forest growing mostly on limestone covered by glacial till. It is the smallest ecozone in Canada, but it includes the country's most productive industrial and commercial region, and is home to nearly half of Canada's population, including its two largest cities, Toronto and Montreal. Hence, little of the original forest cover remains, making protection of the remaining forests a high conservation priority. This ecozone includes two regions described by J.S. Rowe in his classic Forest Regions of Canada: the entire Deciduous Forest Region, and the southern portions of the Great Lakes-St. Lawrence Forest Region. In the province of Ontario, the Ontario Ministry of Natural Resources maps this area as Site Regions 6E and 7E.
The United States uses a different terminiology; the corresponding Level I ecoregion of the United States Environmental Protection Agency system is the Eastern Temperate Forest ecoregion. To deal with the differences in names between Canada and the United States, the Commission for Environmental Cooperation has produced a continental atlas. In this atlas, the Mixed Wood Plains is a Level II Ecoregion 8.1, which includes the above areas within Canada, as well as adjoining parts of the United States.
The Mixedwood Plains stretch along the Quebec City-Windsor corridor. At its western end, it encompasses all of Southwestern Ontario, and is bounded by three Great Lakes and their connecting waterways. To its north is Lake Huron, and to the south Lake Erie. Further east, it has boundaries with Lake Ontario to the south and Lake Simcoe to the north, before continuing east along a narrow strip of the Saint Lawrence River coast toward Quebec. It covers approximately 9% of the total surface of Canada and has a geographic area of 175,963 km².