Read here. Forests respond to warmer temperatures by advancing the forest tree line farther north, and when it is colder, the tree line retreats - it's as simple as that. During the Medieval Warming, Canadian forests were established much further north versus the forests of today.
"Starting from the northern edge of continuous forest, which in the early 1960s crossed Canada's Ennadai Lake at about 60°45'N, 101°W, the authors presented evidence -- based on buried soil and charcoal characteristics and their radiocarbon dating -- "of forests at least 280 km north of the present tree line about 3500 years ago and at least 90 km [55 miles] north about 900 years ago," as well as evidence that forests were "farther south than at present" during the intervening interval [cooling period] of time, going on to state that they "correlate the first advance with the Climatic Optimum and the second with the Little Climatic Optimum," which is otherwise known as the Medieval Warm Period."
