Aspen Invasion in a Portion of the Alberta Parklands

Brush areas on ranges in the parkland region of southcentral Alberta have increased from 4.8 to 8.0% of the land area between 1907 and 1966. The invasion was not constant each year, but was concentrated in two major periods between 1937 and 1970. Annual herbage production under aspen and willow was reduced by 80 to 90% when compared with the production of adjacent rough fescue grassland. The invasion of aspen into grasslands was correlated with high temperatures, particularly during the month of June, 1 and 2 years before tree establishment and with low precipitation 2 years prior to establishment.