Measure twice, cut once: Vegetation history-style

A key component to the scientific method is replication, done through both increased sample sizes and studies with similar research questions. While it may not be the sexiest part of science, replication is critical to preventing poorly done research from gaining societal acceptance (think the infamous, destructive, and thoroughly debunked study that argued a causal relationship existed between the MMR vaccine and autism). Today, as our world battles COVID-19, research published from Israel (1) (2), Qatar, the United States, and other nations independently confirm vaccine efficacy rates advertised by Pfizer-BioNTech and Moderna. In this case, the data replication could increase individual and societal confidence in the vaccines and encourage more individuals to get them.

While on a different scale, Commerford et al.’s 2016 replication of the famed Baker et al. (1996) study on the vegetation history of the upper Midwestern prairie region increases confidence in Baker et al.’s results while also pushing the boundaries of what was previously known. Baker et al.’s article, titled “Holocene paleoenvironments of northeast Iowa,” used the Roberts Creek basin (Fig 1) as their study site because much of Iowa lacks the ideal lakes and bogs. There, they sought to establish a timeline ­­for plant and animal activity from the advent of the Holocene 12,500 years ago to today by examining the fossil record of pollen, plants, and insects and comparing it to the carbon isotopic records from Coldwater Cave. They called upon research that was done by Dorale et al. in 1992 at Coldwater Cave (Fig 1) to establish a comparison, as Coldwater Cave is in the prairie biome and Roberts Creek is mixed deciduous forest and prairie. Baker et al. sketched out a (mostly) concise timeline of what lived at Roberts Creek and when which Commerford et al. sought to replicate.

Figure 1. Map of study locations from Baker et al. (1996), Commerford et al. (2016), and Dorale et al. (1992).

Commerford et al. pushed northwest with their study site: Fox Lake (Fig. 1). At Fox Lake, Commerford et al. focused their research on magnetic susceptibility (to measure erosion), pollen influx, charcoal concentration, carbon, and silica. Based on peaks and falls in the magnetic susceptibility, they divided the Holocene into five “Zones”:

Zone 1: 9,300 to 7,500 yr BP

Zone 2: 7,500 to 5,500 yr BP

Zone 3: 5,500 to 3,850 yr BP

Zone 4: 3,850 to 1,400 yr BP

Zone 5: 1,400 yr BP to present

During Zone 1, there was a major shift from the dominant pollen type being arboreal to nonarboreal. Also seen at Roberts Creek and other nearby sites, this shift occurred around 8,200 years BP. At Fox Lake, Ambrosia (ragweed) pollen peaked around 8,200 yr BP, which Commerford et al. paired with a spike in charcoal concentration to conclude that there had been a period of drought followed by a fire. Zone 2 showed little change, aside from a spike in carbon concentration towards the end of the period. In Zones 3 and 4, there were consistently low values of magnetic susceptibility, showing that there were few ferrimagnetic minerals in the soil around Fox Lake and therefore lower rates of erosion. Arboreal pollen sources gradually increased, particularly Pinus (pine) and Quercus (oak). At 3,850 yr BP, there was a sudden peak then a fall in Ambrosia pollen. During Zone 5, the effects of European colonization started to be seen 300 yr BP as the pollen and charcoal records reflected an increased rate of “disturbances” like drought, fire, flood, and human use.

Figure 2. Adapted from Figure 3 of Commerford et al. (2016): Summary of main proxies for Fox Lake, Minnesota (this article), all plotted against age. From top to bottom: Fox Lake % Si, Fox Lake % C, Fox Lake charcoal concentrations (particles cm−3), Fox Lake % nonarboreal pollen, Fox Lake pollen influx (grains cm−2 yr−1), Fox Lake magnetic susceptibility (SI × 10−5).

After grouping data by time, Commerford et al. observed that the pollen composition did not drastically change over time as they had suspected it would. While charcoal increased steadily over the last 10,000 years and charcoal from arboreal sources was dominant, Commerford et al. found it difficult to conclude that all the charcoal found in Fox Lake originated in the immediate area as opposed to being deposited as part of the water cycle. They also determined that the observed magnetic susceptibility trends were unconvincing due to the possible confounding variable of additional ways that ferrimagnetic minerals could have ended up in Fox Lake.

The point of research is rarely to produce “new” knowledge but rather to build on and give more detail to pre-existing knowledge. In this case, Commerford et al. defined the western boundary of the 8,200 yr BP shift from deciduous woodland to grassland to be at or around Fox Lake. By replicating Baker et al.’s work 100 km away, they added validity to it and increased knowledge of how the vegetation of southern Minnesota may react to ongoing climate change. 

Citations:

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, 66 (2), 203-34.

Commerford, J. L., Leys, B., Mueller, J. R., & McLauchlan, K. K. (2016). ­­Great Plains vegetation dynamics in response to fire and climatic fluctuations during the Holocene at Fox Lake, Minnesota (USA). The Holocene, 26 (2), 302-13. DOI: 10.1177/0959683615608691

Dorale, J. A., Gonzalez, L. A., Reagan, M. K., Pickett, D. A., Murell, M. T., & Baker, R. G. (1992). A high-resolution record of Holocene climate change in speleothem calcite from Coldwater Cave, northeast Iowa. Science, 258, 1626-30.