The Northward Expansion of the Albertan Parkland-Boreal Forest Ecotone Boundary in Response to Mid-Holocene Climatic Warming

Abstract

A 7.09 m lacustrine sediment core was taken from Mariana Lake, Alberta (55º57'N, 112º01'W) to determine if the regional vegetational complex had been affected by the mid-Holocene Hypsithermal. Dating control was provided by 6 radiocarbon dates, with a second degree polynomial fitted to the dates to give an age to depth curve. The basal date of the core is 11 300 ± 110 yr BP. Fossil pollen analysis of the core revealed five distinct pollen zones. Between 11 856 and 10 434 yr BP the study site supported a sparse herb dominated vegetation. A spruce and shrub birch assemblage followed, from 10 434 to 9 100 yr BP, with increased vegetation density. The climate was likely similar to today. This was replaced by a paper birch and spruce complex from 9 100 to 7 638 yr BP likely as a result of warming climate. A mild Hypsithermal effect is recorded between 7 638 and 5 623 yr BP. A forest of decreased crown density was created, with spruce, paper birch and poplar being the major vegetational components. Modern conditions have existed at the site from 5 623 yr BP onwards, though it is hypothesised the regional water budget may have increased slightly around 2 228 yr BP; increased peat development appears to have occurred at this time. Through the use of difference diagrams the site is compared to three other published sites which, along with Mariana Lake, form a north-south transect from the southern Boreal Forest to the northern Alberta Boreal Forest. The hypsithermal vegetation changes varies with distance to the Parkland-Boreal Forest ecotone boundary. It is concluded the parkland did not reach Mariana Lake during the mid-Holocene, though conditions at the site became similar to those at the southern edge of the Boreal Forest that is proximal to the parkland.