James Fenimore Cooper, The Oak Openings (1848)
When the first white settlers arrived in what would become Northfield in the early 1850s, they found the hills and bluffs above the river covered with oak savanna: irregularly- spaced groupings of oak trees with prairie grasses growing between and beneath them. The trees were bur oaks (Quercus macrocarpa): slow-growing, deep-rooted, wide-spreading, and fire-resistant. Their fire resistance was important, because it was periodic fires that kept the savannas clear of woody undergrowth and enabled the prairie grasses to flow in among the trees. John Muir, who grew up among the oak savanna land of Wisconsin, saw what happened when white farmers arrived and put an end to the prairie fires: without fire to purge the savannas, the understory filled up with invasive weeds and shrubs, especially buckthorn, blackberries, and honeysuckle. The "oak openings," as they were often called, were shut down.
Under the right conditions, a bur oak is a wide-spreading tree, like this bur oak in front of Olin Hall, on the Carleton campus. (Because these oaks are so slow-growing and long-lived, it's probable that this same tree was growing on this spot when the first white settlers arrived.) The deep roots help the oak resist drought, the thick bark helps it resist fire, and the aerodynamic shape helps it resist the limb-tearing winds on the prairie. Unfortunately, as the understory filled up with weeds, the oaks of the savanna became crowded and had to compete for sunlight. This affected the shape of the trees.
In the mid-1980s, members of the biology department at Carleton studied the land survey records from the 1850s and identified the areas of the Arboretum that were originally oak savanna. Then they began to clear the brush from the understory with a view to restoring the savanna to its original condition. Here you can see one of the more recent restoration projects, with piles of cleared brush nearby at the left of the photograph. As you can see, crowding and competition for sunlight has made the bur oaks tall and spindly and lopsided—very different from the spreading, nearly symmetrical bur oaks on campus.