THE PALAEOECOLOGY OF WHITE PINE (PINUS STROBUS) IN MINNESOTA.

(1) White pine (Pinus strobus L.), the last important tree species to reach Minnesota in the Holocene (Flandrian) period, entered from the east c. 7000 years ago, it was restricted to the north-eastern corner of the state for c. 2500 years, during the mid-postglacial period of warm dry climate, and then expanded its range to the west and south. (2) Witness-tree records from the U.S. Land Office Survey of the mid-1800s were examined to determine the nature of the natural vegetation in the study areas before logging. At the time of logging a century ago white pine was abundant in north central Minnesota, and was the major species cut for timber. (3) The immigration of white pine was studied in detail by pollen analysis of lake sediments at two paired sites in east-central and west-central Minnesota Small basins were selected, to differentiate local from regional components of the pollen rain. (4) In each case one member of the parr was on a jack-pine (Pinus banksiana) sand plain and the other on a white-pine till moraine, so that differences in the local vegetation related to soil and topography could be used to help determine how the pollen assemblage changed as white pine became important in the general area. (5) The matched pollen diagrams from each pair of sites were compared by subtracting the pollen influx for a taxon at one site from the corresponding influx at the other, and by calculating log-ratios of the pollen percentages of a taxon at the two sites. (6) White pine forests replaced oak-dominated hardwood forests in both study areas, despite the fact that the transition in north-east Minnesota occurred just prior to the maximum of the warm dry conditions of the mid-postglacial period about 7000 years ago, whereas in west-central Minnesota the change occurred only a few centuries ago. These conclusions are supported by pollen diagrams available for other areas in north-central Minnesota. The percentage profiles of oak pollen from a number of sites in... [ABSTRACT FROM AUTHOR]