Saturday, September 1, 2007

Oak Savanna

The region was, in one sense, wild,though it offered a picture that was not without some of the strongest and most pleasing features of civilization... Although wooded, it was not, as the American forest is wont to grow, with tall straight trees towering toward the light, but with intervals between the low oaks that were scattered profusely over the view, and with much of that air of negligence that one is apt to see in grounds where art is made to assume the character of nature... In places they [the oaks] stand with a regularity resembling that of an orchard; then, again, they are more scattered and less formal, while wide breadths of the land are occasionally seen in which they stand in copses, with vacant spaces, that bear no small affinity to artificial lawns, being covered with verdure. The grasses are supposed to be owing to the fires lighted periodically by the Indians in order to clear their hunting grounds.
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.

A sign posted near one of the oak savanna restorations in the Lower Arboretum. In the background is the scourge of the savanna: buckthorn (Rhamnus cathartica), the invasive species that filled the understory and is now nearly impossible to eradicate.


1 comment:

fabrile heart said...

If only trees could talk, what tales they would tell! Of course the Royal Oak has many a legend associated with it in England, and so too the Longfellow's 'Speading Chestnut Tree'. It's good to see a restoration project underway here in the US.

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