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Erika Magnusson

15 October 2013

Plant Ecology Scientific Method Lab: Sumac Community Understory Vegetation

Introduction: Smith & Smith, 2012 say, “community structure varies in space and time.”

Succession is a process that changes community structure through time from grass to shrub to

forest. “Succession refers to changes in community structure at a given location on the landscape

through time” (Smith & Smith, 2012). The transition of community structure in space and time

can be observed by distinct community species composition. As noted on a sign at Rasmussen

Woods located at “the edge”, or where deciduous forest meets prairie, “appearance of Sumac is

an early sign that the forest is trying to expand its growing borders past the transitional zone

between forest and prairie (Minnesota, City of Mankato, 2009). Sumac (Rhus typhina) is an early

successional species or pioneering species and is characterized by high growth rates, smaller

size, high degree of dispersal and high rates of per capita population growth. According to

previous research done by Robert Whittaker of Cornell University, species richness (species

diversity was reported as species richness in 0.3-ha samples) increases into the late herbaceous

stages and then decreases into shrub and older forest ages with a slight increase in young forests.

The objective of my experiment is to reinforce Whittaker’s previous findings of succession

(Smith & Smith, 2012). The purpose of my Sumac community field experiment is to test whether

Sumac community age affects understory species richness. Species richness will increase with

age during the early phases of understory succession as understory vegetation first colonizes the

site below the Sumac community. Colonization by new understory species increases local

species richness. As time progresses and the Sumac community becomes established in one

location, some understory species become displaced and are replaced by slower-growing, more

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shade-tolerant species. Understory species replacement over time acts to decrease species

richness. Competition from dominant slower-growing, more shade-tolerant species and/or an

inability of some understory species to tolerate changing environmental conditions (e.g., more

canopy coverage from the overstory of an older Sumac community) usually results in species

replacement (Smith &Smith, 2012).

Observation: Some Sumac communities appear to have changes in groundcover under their

developing shrub canopy.

Question: Does the stand age of the Sumac community and its overstory composition affect

understory vegetation species richness?

Hypothesis: Sumac communities’ age determines understory species richness. Understory

species richness will increase with Sumac communities age and peak during the middle stages of

succession, after the arrival of later understory successional species and before the decline of

understory species richness by replacement of early understory successional species.

Colonization by new understory species increases local species richness. After the understory

species richness reaches a peak, species richness will decline with Sumac community age

because slower-growing, more shade-tolerant species will become dominant by replacing faster-

growing, less shade-tolerant species. Competition and/or the inability of some understory species

to tolerate the changing environmental conditions of an older Sumac community are the result of

replacement over time, which acts to decrease species richness (Smith &Smith, 2012).

Prediction: Intermediate aged sumac communities will have greater understory vegetation

species richness than younger and older Sumac communities.

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Test/Experiment: I selected 20 Sumac communities along an age gradient. The height of the

tallest tree in the community and the approximate area of the community will determine the age

of the sumac community. A greater sumac community height (m) and area (m2) indicates an

older and more established sumac community.

Purpose/Objective: The purpose of my Sumac community experiment was to determine if

Sumac community understory species richness changes with Sumac community age.

Materials and Methods:

Populations and Communities: Twenty Sumac populations were selected to sample based on

their stand age. Stand age was assumed to be directly related to Sumac population height (m) or

the tallest individual within the population and area (m2). The twenty Sumac populations are not

sampled randomly because the objective is to measure Sumac understory species richness along

a Sumac community age gradient. Each Sumac population with all the individuals of the same

species (Sumac) sampled at a given location at a specific time along with all ground cover

species under each Sumac population is designated as a Sumac community. I sampled seven

Sumac communities at Rasmussen Woods, Mankato, MN on September 24, 2013 during a

timeframe of 2:00 p.m. – 5:00 p.m. I sampled the remaining thirteen Sumac communities along a

1.5 mile stretch on Stoltzman Road, Mankato, MN on October 4, 2013 during the time frame of

9:00 a.m. and 1:00 p.m.

Methods: The twenty Sumac populations/communities were sampled by measuring species

richness in 0.25 m2

quadrats. A standardized method was used to place each 0.25 m2 quadrat in

the approximate center of every Sumac community. The centroid of each of the twenty Sumac

communities was different because every Sumac community was selected to sample based on

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different areas (m2) and heights (m) assumed to be representative of age. Greater Sumac

community area (m2) and height of the tallest Sumac individual indicated an older Sumac

community. I measured the length and width of each Sumac community using a 30-meter field

tape measurer. The centroid of each Sumac community was found by dividing the length and

width measured in half. Once the centroid of each Sumac community was found, I placed the

0.25 m2 quadrat there. The species richness was a measure of species number in 0.25 m

2 quadrats

located at the community centroid. This method is appropriate for the purpose of this Sumac

community study because the measure of species richness is dependent on the Sumac community

area, which indicates age. This method uses the community centroid as the most representative

site of Sumac community understory species richness. If the quadrat was placed more towards

the perimeter and away from the centroid of the Sumac community area, the understory species

number in the quadrat would not represent true Sumac community age. The Sumac individuals

located near the perimeter of the Sumac community can be either older or younger than the true

mean community age. Younger Sumac individuals found on the perimeter of the Sumac

community may be spreading towards the prairie. Older Sumac individuals found on the

perimeter of the Sumac community may be closer to the forest edge. The species richness

sampled within the quadrat will be determined by quadrat placement within the Sumac

community. The centroid quadrat method also avoids bias of quadrat placement where species

richness would support the study hypothesis. The centroid method did not allow me to place the

quadrat where I saw the greatest species richness under a Sumac community with a larger area

(older) or where the species richness was least in a smaller area Sumac community (younger).

My methods fit the mechanism described in the hypothesis because the quadrat sampling

location is standardized at the centroid of every Sumac community. The center of the

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community is assumed to be the area that best represents the median succession of the

community sampled at a specific location in an instant of time. My sampling methods provide a

measure of species richness for each community that is relative to one another based on the

standard of a quadrat placed at median succession. The median succession of an older

community will have less species richness than the median succession of an intermediate aged

community. The dependent variables area and height of the Sumac community were the best

available measures of Sumac community age given the constraints of available time of five

weeks and funding (lack of equipment or methods find the true age the Sumac community).

I will use a simple linear regression analysis to analyze my data. Simple linear regression

analysis is performed under the condition that there is a linear relationship between a dependent

y variable and an independent x variable (Fowler, Cohen, & Jarvis, 1998). The simple linear

regression analysis is applicable to my experiment because the response of the y or dependent

variable is hypothesized to be dependent upon the x variable or independent variable. The

regression analysis will be demonstrated using a scatter plot with a regression line of each

dependent variable (area and height) versus the independent variable, species richness. A p-value

for regression analysis that is less than 0.05 demonstrates a significantly significant difference

between the independent and dependent variable. The regression line defines the relationship

between species richness and ether area or height (Fowler, Cohen, & Jarvis 1998). In my

experiment the independent variables, sumac community height (m) and sumac community area

(m2) were not random. I controlled the independent variables to ensure a Sumac community age

gradient (young to old) was sampled. Sumac community age was assumed to be selected for

based on community area and height of tallest Sumac individual.

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Results:

Sumac area (m2) was not statistically different from Sumac species richness with a p- value of

0.674.

Sumac height (m) was not statistically different from Sumac species richness with a p-value of

0.361.

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Table 1. Linear regression analysis of variance descriptive statistics of twenty non-randomly

selected Sumac communities area (m2) and species richness.

Descriptive Statistics

DF SS MS F P

Regression 1 0.380 0.380 0.183 0.674

Residual 18 37.370 2.076

Total 19 37.750 1.987

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Sumac commmunity area (m2)

0 500 1000 1500 2000 2500 3000

Sp

ecie

s r

ichne

ss

0

1

2

3

4

5

6

7 p = < 0.674

Figure 1. Sumac community area (m2) and species richness of twenty non-randomly selected

Sumac communities. Species richness = 3.328 - (0.000212 * area (m²)). R = 0.100. Rsqr = 0.010.

Standard Error of Estimate = 1.441.

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Table 2. Linear regression analysis of variance descriptive statistics of twenty

non-randomly selected Sumac communities height of tallest Sumac individual height (m) and

species richness.

Descriptive Statistics

DF SS MS F P

Regression 1 4.206 4.206 0.879 0.361

Residual 18 86.082 4.782

Total 19 90.288 4.752

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Sumac community height (m)

2 4 6 8 10 12

Sp

ecie

s r

ichne

ss

0

1

2

3

4

5

6

7p = < 0.361

Figure 2. Sumac community height (m) of tallest Sumac individual and species richness of

twenty non-randomly selected sumac communities. Height (m) = 6.925 - (0.334 * species

richness) R = 0.216. Rsqr = 0.0466. Standard Error of Estimate = 2.187.

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Discussion:

The results of this study represent how community ecology can complex with multiple

dependent variables affecting one independent variable. Although the regression analysis proved

there was no significant relationship between Sumac community height versus species richness

and Sumac community area versus species richness, Sumac community age may still influence

species richness. Area and height alone may not significantly demonstrate species richness

without determining the influence of soil type or understory light availability from overstory

canopy coverage. For example, if primary succession occurred on newly deposited glacial

sediments the rate of community succession and value of species richness would be affected by

soil nutrient availability changing in space and time since the deposit of the glacial sediment.

Initially, the retreating glacier would create a soil profile that was underdeveloped and had little

nitrogen for the survival, reproduction and growth of plant colonization. However, in the case of

understory vegetation of a Sumac community, those plants that have a mutualistic association

with nitrogen-fixing Rhizobium bacteria are able to grow and dominate the site with their access

to atmospheric nitrogen. As the community ages and more time accumulates after the initial

glacial sediment deposit, places will shed their leaves and die releasing nitrogen to the soil as

plant litter decomposes. Soil organic matter accumulation and the increase of soil nitrogen levels;

allow other plant species to colonize the site. Plant species that do not have a mutualistic

relationship with Rhizobium bacteria cannot dominate the site with faster rates of growth and

recruitment (Smith & Smith, 2012). Soil texture is just one of many dependent variables that

may influence the Sumac community understory species richness during succession.

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My results were not what I expected based on my hypothesis that species richness would change

with the age of the community (defined by the area of the Sumac community and height of the

tallest sumac individual) and my prediction that intermediate aged communities would have the

greatest species richness.

I found that Sumac community area species richness in the 0.25 m2 quadrat samples was

unpredictable. I observed that species richness possibly changed with differences in the

abundance and distribution of the Sumac population within the Sumac community. Sumac

population density within a community was variable at every sampling location. The area of the

community was not a good measure of the establishment of the Sumac community at that

location at a given time, which directly relates to the age of the community.

Height of the tallest Sumac individual within the Sumac population of a community as a

dependent variable provided to be not a good measure of community age. Height of the Sumac is

not infinite and once the shrubs reach a certain height they are no longer representatives of the

age of the community (The University of Texas at Austin, 2013). Several communities or

varying area had the same measure of height.

It is hard to determine the observed pattern of understory species establishment when this study

lacks extensive information on the sumac populations and their respective understory species,

life histories and interactions with other plants and the abiotic environment. The mechanism by

which the Sumac population establishes in a given location at a specific time is important to the

succession of understory vegetation. In a previous study titled, “The Pattern of Tree Seedling

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Establishment Relative to Staghorn Sumac Cover in Michigan Old Fields,” higher turnover rates

at 5- 7 year Sumac community age classes and year- year fluctuations in seed input or early

seedling establishment may have resulted in a difference in age distribution of understory tree

seedling establishment (Werner & Harbeck, 1982). This previous study by Werner and Harbeck,

demonstrate that the results of my study may have been influenced by properties of specific

Sumac community age classes, annual changes in dispersal of seeds and establishment of seeds

that were dispersed as understory vegetation.

In Werner and Harbeck’s study, 1982, it was found that within the two different aged fields, 10-

year and 16-year-old abandoned fields in Michigan, density of hardwood seedlings was

significantly higher under the sumac canopy compared to lacking sumac area (Werner &

Harbeck, 1982). The findings of this study relate to the results of my study because community

age may not be the determining factor for understory species richness or density in a Sumac

community. Based on Werner’s and Harbeck’s, 1982 previous findings, it was not reasonable to

assume that Sumac canopy increases with community age. Canopy coverage may provide a

better gradient than community age for measuring differences in understory species richness. In

addition, the Werner’s and Harbeck’s, 1982 study found that density of trees in old abandoned

Michigan fields increased with increased sumac cover. The finding suggests that overall Sumac

cover may affect certain species density, such as the trees in this study, more than others. Werner

and Harbeck’s, 1982 study states, “the most important effect of sumac is the change in ground

cover which occurs as sumac becomes established.” The study summarizes that rhizomatous

herbaceous perennials and plant species that are considered allelopathic became early dominant

species in the abandoned Michigan fields. Once the Sumac plants became established and spread

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through clonal root growth the vegetation under the developing Shrub canopy changed greatly.

“The authors reported that both allelopathy and shade significantly reduced the number of prairie

plant seedlings, allowing invasion by tree seedlings” (Werner & Harbeck, 1982). The study’s

finding suggests that Sumac understory species richness relative to canopy coverage is species

dependent. My study methods did not classify species within the 0.25 m2 sampling quadrat. The

results of my study may have not been significant due to variation of species fitness under the

Sumac canopy. My study did not take into account the affect of the Sumac canopy or overstory

of the productivity of Sumac community understory. In another study

Smith, 2011 in his “Ecological Relationships between Overstory and Understory Vegetation in

Ponderosa Pine Forests of the Southwest” study suggests that mechanisms of overstory control

of understory vegetation include changes in sunlight quantity reaching the understory plant layer

surface, reduced below-ground resource availability and interactions of litter depth. It would

have been beneficial in my study to measure the impact of the Sumac overstory influencing

ecological niches and environmental constraints for understory species richness. I could have

measured soil texture, soil moisture, light availability, and litter depth at each centroid to better

understand understory productivity in relation to overstory coverage.

Other methods I would use to improve the study:

For my study to provide methods that give a better estimate of the true total understory species

richness of a Sumac community in a given location at a given time, I would measure Sumac

canopy coverage instead of community area and height of the tallest individual. Although Sumac

canopy coverage may not be representative of the true age of the community, it may provide for

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twenty selected Sumac communities of variable canopy coverage that give greater differences in

understory species richness. In my measure of canopy coverage, I would account for the other

species canopy coverage with another method. For future research, I would change the size of

the quadrat used in my experiment. I found it difficult to use a 0.25 m2 quadrat for a 2000 m

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community area. I would change the size of the quadrat used dependent on the area of the Sumac

community sampled.

Literature Cited:

City of Mankato, Minnesota. (2009). The edge. Informational park sign, Rasmussen Woods.

Mankato, Minnesota; City of Mankato, Minnesota.

Fowler, J., Cohen, L., & Jarvis, P. (1998). Practical statistics for field biology (second ed.,

pp. 141-147). Chichester, England: John Wiley & Sons.

Smith, E. (2011, May 2). Ecological relationship between overstory and understory

vegetation in ponderosa pine forest of the southwest. The Nature Conservancy, 4.

Retrieved October 10, 2013

Smith, T. M., & Smith, R. L. (2012). Elements of ecology (eighth ed., pp. 354-363).

Glenview, IL: Pearson Benjamin Cummings.

Werner, P. A., & Harbeck, A. L. (1982, July). The pattern of tree seedling establishment relative

to staghorn sumac cover in Michigan old fields. The American Midland Naturalist,

108(1), 124-132. Doi:134.29.12.206

The University of Texas at Austin. (2013). Rhus typhina (staghorn sumac). In Lady Bird

Johnson Wildflower Center, The University of Texas at Austin. Retrieved October 15,

2013, from http://www.wildflower.org/plants/result.php?id_plant=RHTY