Fire!
This was the chaotic scene in Ordway's prairie two Tuesdays ago: dark smoke rising from blackened earth, wind-driven waves of orange flame, urgent sounds of straining engines and shouting voices. If you haven't witnessed a controlled prairie burn before, it can be alarming.
And if you have any love for grasslands, the scariest part might be the destruction left behind. When the flames die and the smoke clears, the prairie has been completely erased. Any evidence of life that, moments ago, stood above ground is now reduced to a thin layer of soot evenly coating the landscape.
But things aren't always what they seem. As it turns out, the fire that seems to have destroyed the prairie is, in fact, helping ensure its continuation. To see why, it helps to understand a little bit about plant growth strategies.
The grasses and wildflowers that make up a tallgrass prairie are mostly perennial herbs. The herb part means the plants do not contain any woody tissue. The perennial part means they can live for many years. It's not the whole plant that does so, however, only its perennating organ. In most prairie herbs, this organ is buried in the soil, safe from winter storms and the flames the other three seasons can bring. Each spring it produces new stems and leaves that grow above ground, harvesting energy from sunlight to create the food that sustains its survival and growth. Each autumn, before the onset of winter, the perennating organ kills its leaves and stems in a controlled process called senescence that allows it to scavenge much of the material they contain for reuse in next year's leaves and stems. For such plants, a fire that occurs before they begin to resprout in the spring is no more harmful than a haircut is for a human. Less, in fact: it is often beneficial. Burning away last winter's litter clears space above ground for the plant's next 'head of hair', and can even speed its growth, by returning any nutrients that remain in last year's leaves and stems to the soil quickly enough to support fresh sprouts.
Fires are not helpful for every plant that lives in a prairie. Notable exceptions are many species of trees and shrubs. Unlike the herbs that can hide their perennating organs in the soil, woody plants don't have that option. This is because wood, while not actually alive itself, requires the activity of living tissue to produce it. The live portion of a woody plant is arranged as a thin, hollow layer in the trunk that extends into living branches, twigs and leaves. The living plant grows in breadth and height by adding new cells at the tips of stems and branches, using specialize organs called apical meristems. It grows in girth by adding layers of cells around the outside of the sheath-like live portions of the trunk: the cambium tissues. As the outer portion of this sheath expands, older living cells on the inside of the sheath become a new layer of non-living wood added to the plant's core. A fire that damages too much of these live above-ground tissues can kill the whole woody plant.
In the absence of fire, however, this vulnerability of woody plants becomes a strength. Building a living body around a core of rigid wood allows trees and shrubs to accumulate growth over multiple years, adding this year's gains to those made over its entire prior lifespan. This strategy plays a 'long game' - it takes time and resources to lay down new layers of wood each year, but the long-term payoff is an ability to grow to sizes not attainable in a single year by even the fastest-growing herb, and not supportable without the physical strength of wood. Given enough time, woody plants can overtop even their tallest herbaceous neighbors. For this reason, a prairie ecosystem will tend to change to a forest over time, unless it is burned regularly.
Prior to European settlement, fires happened often enough that prairies occupied about a third of the landscape in Minnesota. A great deal of evidence suggests that the majority of such fires were set deliberately by Native Americans. Tribes across the continent valued grasslands for the resources they provided and the ease of human travel they allowed. Indigenous peoples around the globe practice similar fire-based landscape maintenance to this day.
In Minnesota, only about 1% of the original prairie remains. While there is undoubtedly more than one reason for any decline that large, fire is likely to have played a central role. Or, more precisely, lack of fire. European colonists and their descendants have a more complicated relationship with fire than the Native tribes they displaced. In some areas of the U.S., early colonists continued with regular burning - mostly to clear land for farming, and mostly at the scale of family-sized plots. Then two things happened at the national scale. First, between ~1820 and ~1920, from New England to the Great Lakes, about 96% of the old growth forest was logged, in what has been dubbed 'The Great Cutover'. The growing U.S. population needed land to feed it, room to expand geographically, and fuel for the fires of its growing industries.
Second, the vast expanses of stumps and slash left by the cutover caught fire. They burned so often, for so long, and with such a fearful intensity that they changed cultural attitudes about wildfire in the United States. Two national organizations - the Forest Service, and the Park Service - were created during this period, and adopted reactionary policies of complete fire suppression within the territories they administered. In 1944, a charismatic spokesperson began to champion this view to the American public. The Smokey the Bear campaign was tremendously successful, helping to create a cultural aversion to fire that persists to this day.
From a conservation point of view this attitude is ironic, in more than one way. First, as we have seen, fire can help conserve prairies, maintaining a diversity of species and ecosystems on the landscape.
Second, a prairie fire is a much different creature than a clearcut fire or a forest fire. The grass and wildflower litter that fuels a prairie fire usually burns completely in a matter of minutes, and at a relatively low temperature. Trees and piles of woody debris burn about 50% hotter, and can smolder for months - or, in extreme cases, years. Avoiding grassland fires can, ironically, increase the chances of more catastrophic fires, by favoring the replacement of prairie herbs by hotter-burning woody species.
Finally, fire suppression may also contribute to the loss of some woody species. In Minnesota, an especially important group is the oaks (Quercus spp.). Many oak species native to the Midwest are adapted to conditions of frequent fire, thanks to fire's long history here. Bur oak (Q. macrocarpa), for example, survives fire by sheathing itself in a thick layer of durable bark: dead woody tissue that surrounds the exterior of the live portion of a tree or shrub. In bur oak, the bark is thick enough to protect the delicate live tissue from intense and frequent grass fires. 'Red oaks' such as Q. rubra and Q. ellipsoidalis also have thick bark, though thinner and less fire-resistant than bur oak. They supplement this shortcoming by forming buds belowground that can resprout rapidly if the main trunk is killed.
Surviving fires is only half of the oaks' evolutionary strategy, however. The other half is reproducing, and spreading offspring to new places. When fires were frequent prior to European colonization, adult oaks existed mostly as single trees or small clusters of a few trees scattered across the large grasslands, creating a savanna ecosystem. Under these conditions, the oak seedlings that survived best were probably those that were best able to use the ample sunlight to grow rapidly and reach a fire-resistant size before the next burn.
When those burns no longer happen, fire-susceptible woody species can replace prairie herbs, eventually converting grasslands to a closed-canopy forest of elm (Ulmus spp.), ash (Fraxinus spp.) and cherry (Prunus spp.) beneath the big, old oaks. Fast-growing oak seedlings survive poorly under this shade, especially compared with forest-adapted trees like basswood (Tilia americana) or maple (Acer spp.). Thus, the oak-dominated forests that, in many places across Minnesota (including Ordway) have developed from the old savannas are probably just a temporary state of the ecosystem. They are not likely to last beyond the lifespans of the old oaks.
What these ecosystems will become in the long-term absence of fire is an open question. At Ordway, we have about 74 acres of this king of oak forest that we monitor every year: both the adult trees and the ground layer vegetation that supports oak seedlings. Stay tuned for future updates!
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