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The Fairfax line, surveyed in 1746 starts at the southern tip of western Maryland and extends 76.5 miles (123km), bearing S 46° E, to the headwaters of the Conway River in Madison County, Virginia. The forests of the Fairfax line can be divided into five map units, based on the presence or absence of ‘key’ species, which have significantly different woody floras. The forest units show a pattern of alternations with topography and underlying rock type which can be best explained by differences m substrate moisture. Although the common woody species appear in recurring groups, each species responds more or less independently to variation in topographic form, rock type, and direction of exposure. The surveyor's journal documents a large blowdown in the presettlement forests of the Appalachian Plateau, and witness tree data suggest that distribution patterns of the common woody species have changed little m more than two hundred years. [ABSTRACT FROM AUTHOR]