ABSTRACT

Michigan’s glacial history played a key role in the formation of ecosystems and landforms. This study utilizes two glacial landforms formed during the Pleistocene Epoch, a moraine and outwash plain, of approximately equivalent age in Northern Michigan to observe differences in tree composition. Moraines are comprised of mesic soil that has a higher clay and moisture content than xeric soil found in outwash plains. As expected, the moraine had a larger average DBH for bigtooth aspen (Populus gradidentata) as well as a larger number of both adult and juvenile species. This study demonstrates the importance of understanding an area’s glacial history in order to develop a comprehensive understanding of the surrounding ecosystems.

INTRODUCTION

Some of the most diverse landforms and ecosystems in northern Michigan are found around the University of Michigan Biological Station (UMBS). The glacial history of the region played a major role in this diversity (Pearsall et al., 1995). About 2 million years ago, a series of glaciations, known as the Pleistocene Epoch began. During this time, soils and moderate climates developed considerably in Michigan (Farrand, 1988). These glacial events had long-term effects on geological formations and ecological communities.

Moraines and outwash plains are two major landforms that developed during the Pleistocene Epoch. Moraines formed along the margin of advancing or retreating glaciers and are comprised of mesic soil, poorly-sorted gravel, sand, silt and clay (Barnes & Wagner, 2004). Outwash plains formed downstream of glaciers and are characterized by xeric soils, flat, well-sorted sand (Leahy and Pregitzer, 2003). Because mesic soil contains more clay and is more basic than xeric soil, it is able to retain more moisture and provides a more suitable habitat for a wider variety of trees (Barnes & Wagner, 2004). A tree’s success in different environments is largely determined by its ability to acquire water and cations/nutrients to contribute to its growth, development and reproduction (Hirons & Thomas, 2018).

We focused on the following tree species that are found in northern Michigan: American beech (Fagus grandifolia), white pine (Pinus strobus), jack pine (Pinus banksiana), red pine (Pinus resinosa), bigtooth aspen (Populus grandidentata), red oak (Quercus rubra), trembling/quaking aspen (Populus tremuloides), white oak (Quercus alba), balsam fir (Abies balsamea), red maple (Acer rubrum), sugar maple (Acer saccharum) and moose/striped maple (Acer pensylvanicum) (Barnes & Wagner, 2004). During the late 19th and early 20th centuries, extensive harvesting and severe fires occurred near the UMBS, killing most of the trees in the area (Bergen & Dronova, 2007). Thus, the current trees in the forests near the UMBS are the same age. The two tree species that became dominant after the fires were early-successional aspen and birch (Hardiman et al., 2011).

This study compares the differences in soil and tree composition between an outwash plain and moraine located near the UMBS in northern Michigan. Moraines have soil that is comprised mostly of clay, whereas outwash plains are comprised mainly of sand. Since clay is better at retaining water and nutrients, we predict that moraines will have a greater number of trees than outwash plains. Additionally, we expect the mean diameter at breast height (DBH) of adult bigtooth aspen to be larger at the moraine site.

METHODS

This study was conducted near the UMBS in Pellston, Michigan (USA) at a moraine site and an outwash plain site. At each site, we sampled trees in five transects that were spaced 10m apart. We created three 50m sectors with a sample range that included 1m on each side of the transect. We recorded the species of all live adult trees and juvenile trees. We defined adult trees as having a DBH greater than 4cm and taller than 2m, and juvenile trees as having a DBH less than 4cm and shorter than 2m. We recorded the DBH of adult bigtooth aspen at both sites.

At both sites, we determined the type of soil that was present. Soil plots at each site allowed us to see the different soil horizons: O (organic), A (surface), E (eluvium), B (subsoil) and C (substratum). To analyze general soil type and moisture at each site, we used the ribbon test method. We also used a pH soil test kit in order to analyze the soil profile at each site.

We used a t-test to determine if the mean DBH of adult bigtooth aspens at each site were the same. We rejected the null hypothesis of no difference if p < 0.05. We used Pearson’s chi-square test to determine if the number of adult and juvenile trees at each site were the same, rejecting the null hypothesis of no difference if p<0.05. RESULTS We found statistically significant results for average DBH of adult bigtooth aspens, number of adult trees and number of juvenile trees between the two sites. Our first test looked at average DBH of adult bigtooth aspens at both sites and found that the moraine site had an overall average of 20.77cm, whereas the overall average at the outwash site was 38.39cm (Fig. 1, p<0.001). Statistical analysis showed that the two sites had significantly different proportions of adult tree species with the chi-squared test of independence having a critical value of 14.1 and p<0.05 at 7 degrees of freedom (Fig. 2). Both sites had adult bigtooth aspens (Populus grandidentata), red maples (Acer rubrum), sugar maples (Acer saccharum) and Beeches (Fagus grandifolia). Adult striped/moose maples (Acer pensylvanicum) were only found at the moraine site. Adult red oaks (Quercus rubra), red pines (Pinus resinosa) and white pines (Pinus strobus) were only found at the outwash site. Our comparison of juvenile trees at the two sites showed a significant difference with the moraine having a higher number of juvenile trees than the outwash (Fig. 3). The chi-squared test had a critical value of 9.5 and p<0.05 at 4 degrees of freedom. Juvenile red maples (Acer rubrum), sugar maples (Acer saccharum), striped/moose maples (Acer pensylvanicum) and beeches (Fagus grandifolia) were only found at the moraine site. Juvenile red pines (Pinus resinosa) and white pines (Pinus strobus) were only found at the outwash site. Our analysis of soil pH revealed that the moraine consisted of basic soil, whereas the outwash plain was comprised of acidic soil. The moraine site had more clay, while the outwash had more sand. DISCUSSION This study aimed to determine the effects of glaciation on tree composition in moraines and outwash plains by comparing adult bigtooth aspen DBH, number of adult species and number of juvenile species. Between the moraine and outwash plain, we observed significant differences in average DBH of adult bigtooth aspens and number of adult and juvenile species. Our results were consistent with our hypotheses and as expected the moraine had a larger average DBH for bigtooth aspens, as well as a larger number of both adult and juvenile species. The different soil types at the moraine versus the outwash plain could be the cause of the differences in size and number of tree species. Soil plots are comprised of the following different soil horizons: O (organic), A (surface), E (eluvium), B (subsoil) and C (substratum). At the moraine, the mesic soil has clay in the C horizon, whereas the outwash has xeric soil comprised of mainly sand. Cation exchange capacity (CEC) is a measure of the soil’s ability to absorb cations and supply them. Soils with large quantities of clay and organic matter have high CECs, whereas soils with high proportions of sand and low amounts of organic matter have low CECs (Hirons & Thomas, 2018). Thus, soils at the moraine would have higher CECs that could supply the trees with more nutrients than at the outwash plain. Our soil results showed that the outwash plain had more acidic soil than the moraine. This could be the reason for the higher number of red and white pines at the outwash site. Pine needles can enhance soil acidity and low-nutrient status making the surrounding environment more suitable for pine occupation (Marshall, 2009). Additionally, studies have shown that tree species drive the differences detected in soil and aspen stands have warmer and wetter soils than coniferous stands (Ayres et al., 2009; Zheng et al., 2017). Being knowledgeable about glaciation affecting soil and tree composition is important for understanding ecosystems. This study is relevant to those utilizing and researching land in northern Michigan. Our findings could affect the choices that entrepreneurs, farmers, researchers and scientists make related to land use. We suggest that further studies focus on the average DBH for all adult trees at moraines and outwash plains. In addition, future research should focus on more in-depth soil analysis. This analysis could compare the amount and types of anions and cations present in the soils at both sites. Figure 1: Comparison of mean bigtooth aspen DBH between the moraine and outwash sites. Aspens sampled at the moraine site had significantly larger average DBH than aspens at the outwash site (p<0.001). Figure 2: Comparison of number of adult tree species between the moraine and outwash sites. The moraine site had significantly more adult trees than the outwash site (p<0.05). Figure 3: Comparison of number of juvenile tree species between the moraine and outwash sites. The moraine site had significantly more juvenile trees than the outwash site (p<0.05). References Ayres, E., Steltzer, H., Berg, S., Wallenstein, M.D., Simmons, B.L., Wall, D.H. 2009. High Species Traits Influence Soil Physical, Chemical, and Biological Properties in High Elevation Forests. PLoS One 4(6): e5964. Barnes, B.V. & Wagner, W.H. 1981, 2004. Michigan Trees: A Guide to the Trees of the Great Lakes Region. Ann Arbor, Michigan: The University of Michigan. Bergen, K.M. & Dronova, I. 2007. Observing Succession on aspen-Dominated Landscapes Using a Remote Sensing-Ecosystem Approach. 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Landscape Ecosystems of the University of Michigan Biological Station. School of Natural Resources & Environment. University of Michigan, Ann Arbor, Michigan. Zheng, X., Wei, X., Zhang, S. 2017. Tree Species Diversity and Identity Effects on Soil Properties in the Huoditang Area of the Qinling Mountains, China. Ecosphere 8(3): e01732. Get Help With Your Essay If you need assistance with writing your essay, our professional essay writing service is here to help! Find out more