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Detailed case histories of long-term successional changes in vegetation are crucial for assessing ecological integrity and developing restoration objectives on prairie preserves in North America's northern Great Plains. Such documentation generally is lacking, however. We used aerial photo measurements plus records from land surveyors and other sources to document change in extent of woodland across four National Wildlife Refuges in northern North Dakota during the 1800s and 1900s. Woodlands were rare when Europeans settled the region in the early 1900s, except green ash (Fraxinus pennsylvanica Marsh.)-American elm (Ulmus americana L.) woodland occurred within the floodplain of the Souris River, and stunted copses of quaking aspen (Populus tremuloides Michx.)-bur oak (Quercus macrocarpa Michx.) occurred along fire-protected scarps of sandhills prairie in north central North Dakota. Ash-elm woodland expanded in the Des Lacs and Souris River valleys especially along adjoining, intermittent tributaries (coulees) of the Souris River, mainly during the early- to mid-1900s. During the mid- to late-1900s, aspen woodland expanded in sandhills of the Souris Lake Plain and in the Missouri Coteau, changing much of the mixed-grass prairie to parkland. With settlement by people of European descent, large herbivores were extirpated from the region and natural and anthropogenic fires were suppressed. These changes are implicated in expansion of woodland into native prairies. [ABSTRACT FROM AUTHOR]