From Prairie to Forest and (Hopefully) Back Again

A person that lived hundreds of years ago in the land that is now Iowa would not be able to recognize the land today. In their time, tallgrass prairie stretched as far as the eye could see, only slightly broken up in patches by lightly forested oak savannas. With few barriers to stop it, fire blazed over this land very frequently, ignited by natural causes or the Native American people who lived there. These fires were not a destructive force, but a rejuvenating touch to the landscape, allowing the fire-adapted prairies and savannas to grow free of obstructions by plants more sensitive to fire. Changes were coming, however, and with the arrival of European settlers, this land was greatly altered (Figure 1). Not only did settlers destroy large chunks of the prairies and savannas with their cities and farmland, they actively suppressed the fires that maintained these environments, leading to wetter conditions that drastically changed the habitats supported by allowing closed canopy forests to grow on what was once open land. This process of change is termed “mesophication” in a 2008 paper written by Nowacki and Abrams that discusses the landscape changes due to decrease of fire since European settlement in the eastern United States.

Figure 1. Past vegetation (a) and current vegetation (b) of the eastern United States, taken from paper by Nowacki and Abrams (2008).

            The word mesophication means the development of cool, moist understory conditions, coming from the term mesic, which means a habitat with a well-balanced amount of moisture. This process happens very quickly without frequent fire to suppress the shade-tolerant plants and keep them small. These plants were unable to survive strong fires, but in their absence can grow large and overtop neighboring plants, giving other shade-tolerant species a competitive advantage over the prairie and savanna plants. With even more shade, more water is kept in the soil and these types of plants continue to grow larger, quickly spiraling into a cycle that changes the landscape from open prairie or savanna to a closed canopy forest very quickly (Figure 2). Even a short period of fire suppression can start this shift towards mesophication.

 Figure 2. Changes in fire importance and mesophication over time for oak-pine ecosystems in the eastern United States. Olive and dark green trees represent oaks and pines; aquamarine trees represent mesophytic species. Taken from paper by Nowacki and Abrams (2008).

            Nowacki and Abrams discuss the difficulties for a community to return back to the old fire-adapted ecosystems that once existed after becoming mesophytic. Many species are lost, non-native invasives have been established, and performing prescribed burns at the necessary frequency can be difficult. These new mesophytic habitats result in decreases in species diversity, and will continue to do so unless fire is reintroduced into the landscape to avoid these losses. However, there is historical evidence that this change from mesophytic forests to a drier prairie is possible. A study done in 1996 by Baker and colleagues on paleoenvironments in northeast Iowa details the history of the region since 12,500 years before the present. Work from this paper shows that between the geological periods of the middle Holocene, which occurred 5,500 to 3,500 years ago, and the late Holocene, around 3,500 to 380 years ago, the vegetation shifted from a mesic forest to a prairie similar to the ones of today. They concluded that this shift was aided by an increase in fire frequency, but was primarily driven by a change in climate caused by drier Pacific air. Looking at their conclusions with knowledge of the work of Nowacki and Abrams, it is possible that Baker and their team may have undervalued the ability of fire to change environments. Instead of climate changes being the most important factor in converting mesic forests to prairies, the fires that were able to flourish under these dry conditions may have been the driving force of the change.

This conclusion may give hope to those trying to restore prairie and savanna from mesophytic forests using fire, but there are still many challenges. Without active efforts to preserve and reclaim areas as prairie and savanna, the mesophication process will continue, making it harder for people to conserve and restore the past fire-based communities to their former glory. With the expanses of agricultural land that cover Iowa, the range of these prairies and savannas may never be restored, but with some care and regular fires, the small patches that still exist may continue to provide a window to the days of yesteryear. 

References:

Baker, R. G., Bettis III, E. A., Schwert, D. P., Horton, D. G., Chumbley, C. A., Gonzalez, L. A., & Reagan, M. K. (1996). Holocene paleoenvironments of northeast Iowa. Ecological Monographs, 203-234.

Nowacki, G. J., & Abrams, M. D. (2008). The demise of fire and “mesophication” of forests in the eastern United States. BioScience, 58(2), 123-138.