Rates of vegetation change in the Peace-Athabasca delta

Landscape changes were examined in the Peace-Athabasca Delta, northern Alberta, Canada for the period 1945 to 2001 in 24 randomly chosen study areas. Proportions of the landscape covered by four community types (open water, marshes and meadows, shrub communities, and forests) were determined on air photos from five periods. Progressive, retrogressive, and oscillatory changes were observed. Variation in cover was about 4-10 times greater than net change. Median rates of both increase and decrease were about 0.2% to 0.6% of the landscape per year. There was evidence of temporal stability in some areas, while in other areas, a dynamic mosaic existed. Local or short-term changes were often large (> 1% per year), but averaged over the entire delta, or over time, net changes were small. Net 17 changes were the lowest in forests and the highest in marshes. Succession between open water and marshes and between marshes and willows were common. Forest showed little evidence of transition to other types. Oscillatory changes in open water and marshes were typical. Open water, marshes, and willows changed at similar rates and more rapidly than did forest. Succession from open water and marsh communities toward willow communities and forests is to be expected at centennial scales. In a naturally oscillating system, trends appear or disappear as temporal and spatial scales change. Short duration studies, or those of limited spatial extent, may provide inaccurate estimates of vegetation change in a large dynamic delta. Interactions between drivers such as isostatic rebound, sedimentation, and avulsions mean that the Peace-Athabasca Delta will remain dynamic.