The Battle for Iowa

The Battle for Iowa

The forest-prairie fight for ground over the last 10,000 years

Carissa Shoemaker

November 19, 2013

    Conard Environmental Research Area, July 2013. Photo taken by author.

What do you see in this photo? Prairie? Forest? Both? This is a picture of prairie-forest ecotone, where prairie and forest meet, two ecosystems blending and transitioning into each other to make a new community. Outside of photos, however, ecotones aren't static. Different forces such as fire, climate change, pathogens and pests, and humans invite one ecosystem to march into the other, thriving on whatever variables have been introduced. Consequently, ecotones have a lot of potential energy—it's like they're teetering at the top of a mountain just waiting to be tipped one way or the other: will it be grasses, or trees? You wouldn't expect forest to shift to prairie and back again all too quickly but, according to John Wilson and his colleagues (2009), that's just what's been happening for the last

10,000 years.

Wilson et. al. researched the rate and causes of prairie-forest ecotone (and thus, ecosystem) shift throughout the Holocene epoch, the 10,000 year span of time from the retreat of the glaciers to present- day. Previous studies had discussed the historical composition of Iowa's landscape, its shifts, and causes for these shifts, but they had been relatively small-scale and had utilized incomplete pollen and plant macrofossil samples. By comparing recent macrofossil and arboreal pollen distribution records across the continental interior, Wilson et. al. were able to geographically and temporally map the battle between prairie and forest during the Holocene.

The timing and extent of Wilson's ecotonal shift (see below) generally correspond to that of previous research. Wilson et. al. (2009) and the geologist Kent Van Zant (1979), who found and drew conclusions from arboreal pollen and macrofossils in the sediment of Lake West Okoboji, agree that Iowa was in  closed coniferous forest around 14,000 years before present, transitioning to deciduous forests of oak and elm by 11,000-9,000 years before present, and that the battle between forest and prairie started around 9,000 years before present. Both assert that that's when prairie and forest really started to duke it out, with prairie taking the upper hand from 7,700 to 3,200 years before present, and forest starting to creep back in 3,200 years before present (Van Zant 1979; Wilson 2009). In addition, Wilson et. al. (2009) and Baker (2001) both consider changes in climate to be the greatest natural catalyst for ecotonal shift.

Wilson and colleagues' study offered several new take-away points, however. They found that deforestation early in the Holocene was more abrupt than previously thought, largely due to the rapid drying of the continental interior. Relatedly, they concluded that the prairie-forest ecotone is particularly sensitive to environmental change, despite our perception of forests as impenetrable and slow-moving. Wilson's et. al. work and findings were important additions to the field for various reasons. Generally, they give more insight into the physical, biological, and cultural processes behind the position and structure of ecotone. Specifically, they lend information about the past, present, and future effects of land management, climate change, pests, and pathogens on Iowa's prairie-forest ecotone. Ecotone study is especially important in the face of climate change, as tree cover and variation therein determines the regulation of energy and water exchange from surface to atmosphere, contributing regionally to climate change.

When Iowa's land was less intensively managed, the increasing variability and intensity of precipitation, dry periods, and extreme weather would have tipped the balance, shifting the ecotone and selecting for a particular ecosystem. Now, however, everything's effects have been minimized. Fire's role as a landscape process and land management tool has been suppressed. Flooding and droughts are still impactful, but tiling and irrigation have been implemented, at least in an agricultural context. But even in a natural context, much of the prairie-forest ecotone's potential energy has dissipated. The land in the photo above is managed as part of a prairie, savanna, and Iowa woodland conservation effort; it's not likely they'd let the balance tip in favor of one ecosystem over another. Likewise, the few remaining natural ecosystems in Iowa are confined and fragmented by big agriculture. Compare  Iowa of the 1800s with its  current landscape; most of the native landscape and biodiversity has been plowed under and the prairie-forest ecotone has been replaced by prairie-highway or corn-forest ecotones. If the ecotone's going to shift now, it will be because of a pest or pathogen invasion (Dutch elm disease, beech bark disease, chestnut blight, butternut canker, and emerald ash borer are already transforming the composition of eastern forests) or human action.

 Sources

Baker, R.G., Rhodes, S., Schwert, D.P., Ashworth, A.C.,  Frest, T.J., Hallberg, G.R., Jansenns, J.A., A Full-Glacial Biota from Southeastern Iowa, USA. 2001.

Van Zant, K. Late Glacial and Postglacial Pollen and Plant Macrofossils from Lake West Okoboji, Northwestern Iowa. Quaternary Research 12, 358-380. 1979.

Williams, J.W., Shuman, B., Bartlein, P.J. Rapid Responses of the Prairie-Forest 

         Ecotone to Early Holocene Aridity in Mid-Continental North America. 2009.