Exploring the Coexistence of Acer saccharum and Fagus grandifolia in Warren Woods

Exploring the Coexistence of Acer saccharum and Fagus grandifolia in Warren Woods State Park

Adrian Raygoza (Undergraduate Senior BioS major)

The University of Illinois at Chicago

1200 W Harrison St, Chicago, IL 60607

Abstract: This study explored coexistence the species Acer saccharum (sugar maple) and Fagus grandifolia (American beech) in the Warren Woods, located in Michigan, US. We explored whether lottery competition is occurring, if coexistence between the sugar maple and the American beech tree exist by recruitment, and attempted to determine the mechanisms, if any, of coexistence. Our data suggest that lottery competition does seem to be taking place. This was determined by examining the total number of seedlings, saplings, subcanopies, and canopies within a given hectare. As hypothesized, the number of seedlings > saplings > subcanopies > canopies; suggesting lottery competition exists because the trees reproduce at a very high rate, but, only very few trees, of both species, make it to full maturity and eventually fill up the canopy. We also determined that the data suggests that there is coexistence occurring between the sugar maple and American beech tree in the Warren Woods by recruitment. We compared the number of subcanopies to canopies and found that the subcanopies vastly outnumber the canopies, which suggests that recruiting is occurring; as hypothesized. In order to determine the mechanism of coexistence, we measured the index of symmetry of both species for comparison of different ecological strategies. The p-value of our two-tailed T-test suggested that there was no difference in symmetry of the two trees. In this case, we failed to reject our null hypothesis that American beech trees are more symmetrical. Furthermore, we tested environmental heterogeneity by determining the nearest neighbor to 4 focal American beech trees and 4 focal sugar maple trees. Using our data, we ran a Chi-square test to compare if a certain species would appear more than expected, by chance. Because the P-value was high, we failed to accept the null hypothesis that the nearest neighbors are what we expected to appear by chance. No significant determination was made for the mechanism of coexistence in this study.

Introduction:

Warren Woods has been a long-studied area for forest ecology scientists due to its old-growth natural ecological history (Greenburg 2004). Warren Woods is a large state park is located in the southeastern part of the state of Michigan, US. A very unique aspect of this particular forest is the coexistence of two dominant species of trees, Acer saccharum (sugar maple) and Fagus grandifolia (American beech), within the canopy of the forest (Tatina 2015). 

In our study, we were interested in investigating the unusual occurrence of coexistence of two large canopy trees. In most forests found around the world, one specific type of tree species dominates a canopy in a given forest. Well studied in forest ecology, canopy space is very quick to fill up when an opening is present (Runkin 2013). Old-growth Beech-maple forests have been discovered in other areas as well, such as Ohio (Runkin 2013), which sparks the questions of how and why they occur. Runkin (2013) summarized that species that can fluctuate to occupy space by combining growth mechanisms that will be favored. Because Acer saccharum and Fagus grandifolia seem to have different strategies of growth it is possible that each species can fluctuate favorability in oscillations to create codominance. Fagus grandifolia, known for its slow and steady growth in shaded areas, contrasts the Acer saccharum, known for its fast growth to reach sunlight upon a canopy (Tatina 2015), may quite possibly be creating an equilibrium of oscillation.

Several scientists have studied this on-going phenomenon in Warren Woods. Poulson and Platt (1996) have suggested that this phenomenon may exist due to the two species differing in response to microenvironments and tolerance for levels change in light frequencies. Robert Tatina (2015) proposed that the reason for the coexistence of these two species may be due to fluctuating factors, over the past couple centuries, that favor a particular species over an amount of time but changes eventually and benefit the other species. An example of one of these fluctuating factors, occurring today, is the higher mortality rate for Fagus grandifolia caused by windthrow, which can be expected to increase as anthropogenic climate change continues to rise (Tatina 2015) allowing for Acer saccharum to possibly increase in abundance in the future.

We investigated whether reproductive competition is occurring in the Acer saccharum (sugar maple) and Fagus grandifolia (American beech) species, by measuring the class sizes of growth to see if lottery competition was occurring. We also investigated if there existed any evidence of coexistence between Acer saccharum (sugar maple) and Fagus grandifolia (American beech). This was tested by exploring if recruitment was occurring in both species. We further investigated whether mechanisms of coexistence are related to environmental heterogeneity and any trade-offs occurred. This was done by measuring the size of canopy sizes between the species and comparing them against each other. We then studied four focal trees of each species to determine if reciprocal replacement patterns are occurring.

We hypothesized that coexistence is occurring between the species Acer saccharum (sugar maple) and Fagus grandifolia (American beech). We expected to measure the number of seedlings to be greater than other size classes of growth; suggesting that lottery competition is occurring. We expected to find the number of subcanopies for both species to be greater than the number of canopies found; suggesting that recruitment is occurring between the species. We expected the measurements of Fagus grandifolia species to be less symmetrical than Acer saccharum; suggesting that Fagus grandifolia grows asymmetrically as a method of coexistence. We also expected to find a method of coexistence by reciprocal replacement between the Acer saccharum and Fagus grandifolia.

Materials and Methods:

This research study took place in May of 2019 at the Warren Wood State Park in Michigan, US. The site was near the Galilean River and contained a vast density of canopy trees of both species, Acer saccharum, and Fagus grandifolia, along with a larger density of the other size classes of growth (subcanopy, saplings, seedlings).

For this specific study, we used tape measures, in feet and inches, and subsequently converted measurements to meters, as our measurement instrument for measuring distances for the quadrats, canopy lengths, and circumferences of different trees. Yard-length measuring sticks were used for measuring subcanopies and saplings. Small flags were used as markers in order to set the boundaries of quadrats. We counted hand-counted the number of seedlings in a given quadrat. 

Our first objective was to measure out the seedlings, saplings, and poles of the two species in a 10 x 10-meter quadrat. We selected a corner to begin by randomly tossing a stick into the woods. The spot where the stick landed became the first corner of the boundary of the quadrat. Using the tape measure and flags, we measured and marked out a 10 x 10-meter, square-shaped, quadrat. Within the quadrat, we recorded and measured the species of the canopy trees and their circumferences at breast height. We then counted and recorded the number of subcanopy trees and saplings within the 10 x 10-meter quadrat. Within the same quadrat, we measured off a smaller 2 x 2-meter quadrat and recorded the number of seedlings within the smaller quadrat. These steps were repeated 3 more times at other random spots in the forest.

Our second objective was to complete a focal sampling for both species, Acer saccharum, and Fagus grandifolia. We selected a focal canopy of Fagus grandifolia, away from the previous quadrats, and measured its circumference at breast height. We then measured the distance to the nearest canopy tree from our focal tree, calling the direction of the nearest canopy the north direction, and measured the circumference and recorded the species. Following this, we then measured the distance and recorded the species of the nearest canopy trees in all three of the other cardinal directions (south, east, west). After, we returned to the focal tree, that we started with, and recorded the canopy size by measuring the distance of the edge of the canopy cover in all four cardinal directions to the tree base. Subsequently, we measured out a 5 x 5-meter quadrat around the focal canopy tree. Within this quadrat, we counted the number of canopy trees, subcanopy trees, saplings, and seedlings. This was repeated until four focal trees of Acer saccharum and Fagus grandifolia were studied. Afterward, data collected from the nearest neighbor to the focal tree was shared by an entire class of students performing the same experiments. This data was compiled together for all four focal canopy trees of Acer saccharum and Fagus grandifolia. 

Statistical data analysis was conducted using Microsoft Excel spreadsheets and using the Vassar website for statistical testing. All data recorded in the field were placed into Microsoft Excel. Using the data collected, tables and graphs were created showing the findings from the data collection in the field. A counting and comparison of all size classes of growth were used to determine whether lottery completion was occurring. A counting and comparison of the size classes of growth of canopy and subcanopy were used to determine if recruitment existed amongst the two coexisting species. A two-tailed T-test was performed on the average index of symmetry between the two species to compare symmetries of each of two tree species as a mechanism of co-existence. A chi-square test was performed on the focal tree sampling to determine if a mechanism for coexistence was due to environmental heterogeneity that favors the other species. 

Results:

Seedlings of both species vastly outnumbered the other size classes of growth as seen in Fig. 1 and Table 1. The graph (Fig 1.) is highly skewed toward the right side, trending toward the smaller class sizes. Seedlings had an average density of 240,000 per hectare. This is relatively large in comparison to 175 canopies per hectare. Saplings were the second-largest at 2400 hectare, followed by subcanopies at 1350 per hectare. Saplings also showed the greatest standard error of all the size classes at 25145.4.

Subcanopies of both species, seen in Fig. 2 and Table 2, outnumbered the canopies. The graph (Fig. 2) is skewed toward the right, toward the subcanopies. Acer saccharum subcanopies had the highest average density with 850/hectare (Table 2). In comparison, Acer saccharum canopies had the lowest average density at 75/hectare. The standard error was largest at 64.5 for Acer saccharum subcanopies and lowest at 25.0 for Acer saccharum canopies.

Acer saccharums showed a larger variation in the average index of symmetry, as seen in Fig. 3. Acer saccharum in the data seems to trend larger in the east and west directions. Fagus grandifolia showed some variation in symmetry with a standard error of 7.62 (Table 3); however, it was much less than the Acer saccharum with a standard error of 33.0. Acer saccharum had an average index of symmetry at 78.6 (Table 3), whereas the Fagus grandifolia had an average index of symmetry at 15.7. The results of the two-tailed T-test revealed a P-Value of 0.1122 with 6 degrees of freedom. 

Closest neighbors to the focal canopy trees of both species (Fig. 4) showed a trend to the opposite species for Acer saccharum, as the focal Acer saccharum canopies had 19 Fagus grandifolia neighbors and only 3 Acer saccharum neighbors. Focal Fagus grandifolia neighbors, however, seemed to trend slightly more toward their own species with 15 Fagus grandifolia neighbors and only 8 Acer saccharum neighbors. This is also reflected in Table 4. The chi-square test that was run for this data contained a P-value of 0.1923, with one degree of freedom.

Discussion:

The evidence gathered during the initial phase of the study shows that seedlings far outnumber any other size class for growth (Fig. 1). The data supports our hypothesis that lottery competition seems to be occurring as the number of seedlings > saplings > subcanopies > canopies (Table 1). This is not very surprising as trees, in general, tend to show a Type III survivorship curve, where young have a high mortality rate, but the survivors that reach maturity face better survivorship odds. Lottery competition is a common form of Type III survivorship curve organisms.

The data for the second set phase of the study supports our hypothesis that recruitment is occurring (Table 2) as there are a larger number subcanopies of both species than canopies. We do recognize an anomaly in our data, however (Fig. 2), as there are far more Acer saccharum subcanopies than beech subcanopies. We expected to see more Fagus grandifolia subcanopies. The reason for this expectation is due to Fagus grandifolias higher survival and growth rate under low light conditions (Beudet et al. 2007). We do believe that a possibility for this anomaly is that some of our quadrats bordered a ridge that led down to a water source. It is possible that more light reached through the canopy in that area, assisting the conditions for maples (Poulson and Platt 1996).

Our hypothesis that a mechanism of coexistence due to Fagus grandifolia being less symmetrical was not supported by our data. In terms of our statistical analysis for data set, we failed to reject the null hypothesis that the mean of symmetry index of Fagus grandifolia equals the mean of symmetry index of maple., as the P-value of the T-test was >0.05. An anomaly that was seen with our data supported the opposite of our hypothesis. The Acer saccharum showed a far greater average index of symmetry, suggesting it was less symmetrical of the two species in this study.

Poulson and Platt (1996) and Runkle (2013) have suggested an equilibrium exists within the two species that allows them to coexist. As we hypothesized, another mechanism of coexistence was for the reciprocal replacement to be occurring. Our data did not support this hypothesis. Fagus grandifolia seemed to grow next to Fagus grandifolia nearly as much as it grew next to Acer saccharum (Table 4), though Acer saccharum seemed to favor growing next to beech. Our statistical analysis, a chi-square test, did not find any significant difference in the data, as the P-value was >0.05. Because of this, we fail to reject the null hypothesis that Acer saccharum and Fagus grandifolia occur independently. Our data did not support an equilibrium.

In further studies, it may be possible to gain better-supporting data for our mechanisms of coexistence hypothesis by possibly forming quadrats that are deeper into the forest and away from any edges. We believe that the edges of the forest did serve to play a role in our data as direct sunlight may appear at different angles than in the rest of the canopy area. The rate of seedlings for Fagus grandifolia may have been affected greatly by this. Focal tree sampling may have been skewed by this occurrence as well. For other possible studies, we would like to also gather samplings of the nutrient cycle taking place in the Warren Woods and compare them to favorability by each species. We expect that the nitrogen levels of the soils will differ from the middle of the forest and the edge of the forest near the river banks. Whether this directly affects the coexistence by creating favorability for a particular species will be the main focus for the study as we compare the species density from each area and with the nutrients examined from the soils of each, as well.

Literature Cited

Beaudet, M., J. Brisson, D. Gravel, and C. Messier. 2007. Effect of a major canopy disturbance on the coexistence of Acer saccharum and Fagus grandifolia in the understory of an old-growth forest. Journal of Ecology 95:458–467.

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Greenberg, J. 2004. A natural history of the Chicago region. University of Chicago Press. Chicago, IL.

Legault, S., D. Houle, A. Plouffe, A. Ameztegui, D. Kuehn, L. Chase, A. Blondlot, and T. D. Perkins. 2019. Perceptions of U.S. and Canadian maple syrup producers toward climate change, its impacts, and potential adaptation measures. Plos One 14.

Poulson, T. L., and W. J. Platt. 1996. Replacement Patterns of Beech and Sugar Maple in Warren Woods, Michigan. Ecology 77:1234–1253.

Runkle, J. R. 2013. Thirty-two years of change in an old-growth Ohio beech-maple forest. Ecology 94:1165–1175.

Tatina, R. 2015. Changes in Fagus grandifolia and Acer saccharum Abundance in an Old-Growth, Beech-Maple Forest at Warren Woods State Park, Berrien County, Michigan, USA. Castanea 80:95–102.

Appendix:

Figure 1 Density vs Size Class; A count of each level of growth for the canopy trees Hypothesis 1

Table 1 Growth Size classes by Avg Density and Standard Error, Hypothesis 1

Figure 2 Avg Density/Hectare vs Class & Species. This shows that subcanopies outnumber canopies. Hypothesis 2

Table 2 Avg Density per hectare and standard error for canopies and subcanopies. Hypothesis 2

Figure 3 Index of symmetry vs Focal tree species. Hypothesis 3a

Table 3 Avg index of symmetry for both species and standard error. Hypothesis 3a

Figure 4 Nearest neighbor counts vs Canopy tree species. Focal trees measured. Hypothesis 3b

Table 4 Count of neighboring canopies. Hypothesis 3b