Climate Change or Settlers: Who’s Responsible for the Structure of Iowa’s Forests?

It’s all but certain that 2015 will end up as the hottest year on record, with global temperatures on track to finish the year a full 1 degree C above pre-industrial levels. This will undoubtedly impact vegetation worldwide. However climate change is not the only disturbance that influences vegetation. Global flora has historically been (and continues to be) influenced by a variety of human disturbances. It begs the question: between changing climates and human development, which is the more dominant actor? Gregory Nowacki and Marc D. Abrams (2015) have begun to untangle this complex relationship in their recent paper, Is climate an important driver of post-European vegetation change in the Eastern United States?  Though they studied the entire Eastern United States, I am most interested in their results that pertain specifically to Iowa. Iowa flora is particularly interesting to examine because it has undergone some of the most extreme modifications due to changing climates and human development.

Nowacki and Abrams (2015) explain to their readers, that between the 15^th to 19^th centuries the entire continent of North America was in the midst of the icy grips of the Little Ice Age (LIA). As you can imagine, the cold weather was not an ideal environment for fire. Thus, when the early European settlers arrived one could reasonably envision the flora to be an untouched wilderness. However, the Native Americans had an understanding of fire and commonly practiced burning the landscape, which resulted in the prevalence of pyrogenic (fire tolerant) flora. In Iowa the development of the fire-maintained prairie prevented the rise of forests that would have otherwise flourished and dominated the landscape during the climate of the LIA. The climate abruptly shifted and warmed, which clearly denotes the end of the LIA and the start of the Anthropocene—the current geological epoch marked by the period where humans have become a significant environmental disturbance (Fig 1).  This climate shift did not occur in isolation, but instead in conjunction with increased human activity including land use change, intensive resource utilization, and fire suppression.

To analyze the interaction between climate and disturbance on the tree composition of Iowa, Nowacki & Abrams used historic tree-census data from early settlers (remember that this is still in the middle of LIA). They categorized the trees into different categories including temperature, shade tolerance, and pyrogenicity. This data was compared to present tree censes, and then related changes in climate and disturbance.

After sifting through data on over 100 species of trees, this is what they learned about the development of the contemporary Iowa flora. Nowacki and Abrams (2015) determined that Iowa’s forests had historically been dominated by by Quercus, is now comprised of a variety of trees including an increase in composition of Celtis (6%), Gleditsia (5%), Maclura (5%), Juniperus (4%), Acer (4%), and Fraxinus (3%), with a decrease of overall Quercus (-39%).  This reflects a loss of pyrophilic, fire-loving trees (-43%), matched with an increase of fire-phobic, flamable trees (36%). Furthermore decreases in shade-intermediate and warm genera trees were observed, with gains of replacement species that were more shade-tolerant and classified as “cool” genera. During this time, temperature in Iowa temperature increased by 1.4% and precipitation increased by 5.6% during the Anthropocene (again meaning the period in time where humans became the major geologic disturbance).

Nowacki & Abrams results are supported by Baker et. al. (1996) research on the historical pollen data of Iowa that also support that a mesophication of the Iowan forests occurred during the period of warming. In combination, both studies show evidence of a change in tree composition from open prairie, savanna, and woodland to the presently dominant closed mesic forests. This occurred during human-induced fire suppression, which allowed for closed-canopy forests to develop. This process became a positive feedback loop because as a result of increased shading, shade-tolerant and fire-sensitive plants began to grow, which then provided microclimates that allowed this pattern to continue. Nowacki and Abrams (2015) also noted that the mesophication that has been documented in Iowa and the central U.S shows the larges decrease in oak trees (-39%) and in fire-tolerant trees (-43%). This pattern demonstrates the strong influence that humans have on the environment, since the number of shade-tolerant trees rose despite the warming of the climate. Thus, it’s safe to say that in Iowa human disturbances (specifically fire suppression) have surpassed the effects of a warming climate on the community composition of contemporary Iowan forests.

References:

Baker, R.G, Bettis III, E.A, Schwert, D.P, Horton, D.G., et. al. 1996. Halocene

paleoenvironments of Northeast Iowa. Ecological Monographs. 66, 203-234.

Nowacki, G.J. & Abrams, M.D. 2015. Is climate an important driver of post-European

vegetation change in the Eastern United States? Global Change Biology. 21, 314-334.