To-day they are covered with farms. The cause of the treelessness has been much discussed.
The treelessness of the prairies cannot be due to insufficient time for tree invasion since glacial evacuation; for forests cover the rocky uplands of Canada, which were occupied by ice for ages after the prairies were laid bare.
It does not seem to lie in peculiarities of temperature or of precipitation; for trees thrive where they are properly planted on the prairies; every town and farm to-day has its avenues and groves of trees; but it should be noted that west of the Mississippi river increasing aridity becomes an important factor, and is the chief cause of the treelessness of the Great Plains (see below).
