Development of the cultural landscape in the forest-grassland transition in southern Alberta controlled by topographic variables.

摘要

Question: How has the terrain, namely elevation, slope and aspect, controlled the cultural modification of the forest-grassland transition in southern Alberta? Location: The aspen parkland region of the Rocky Mountains foothills southwest of Calgary, Alberta, Canada. Methods: Dominion Land Survey data, historical maps and a DEM (digital elevation model) were used to determine which decision rules settlers used to plough native grassland and parkland at the turn of the last century. Comparative measurements between historical and current vegetation patterns and GIS techniques were used to explore relationships between terrain, land use and the modification of the landscape. Results: Grassland at lower elevations, aspen parkland at mid elevations and closed forest at higher elevations dominated the pre-settlement forest-grassland transition. Elevation, hillslope angle and aspect affected which parts of the landscape were cleared and ploughed for agriculture or left in their natural state. Clearing and ploughing occurred mostly on gentle hillslopes (<=6 degrees ). At higher elevations with fewer gentle hillslopes clearing and ploughing were limited. Ca. 90% of the original vegetation at lower elevations has been cleared or ploughed, compared to ca. 30% at higher elevations. Almost all of the grassland at lower elevations has been converted to cropland. In the mid-elevation parkland 48% of the tree cover and >72% of the native grass have been converted to domesticated grass and cropland. Tree cover has expanded at the expense of grass on steep hillslopes and in ravines that presumably were burned frequently by wildfires in pre-settlement times. At higher elevations, the closed forest has remained more or less intact. Conclusions: Terrain structure has controlled the arrangement of suitable agricultural land and consequently determined the current vegetation patterns in the forest-grassland transition zone in southern Alberta. Tree invasion into grass has been minimal, and has been restricted to hilly topography or wet areas, even though the parkland has expanded somewhat in its distribution. Any effect the cessation of wildfires may have had on aspen expansion has been overshadowed by agriculture, which has converted vast areas of native grassland and aspen parkland to farmland. Thus, along this forest-grassland transition, the strong elevation, slope and aspect have maintained some of the regional variation in vegetation..