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Introduction: “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.” 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 prarie, “appearance of Sumac is an early

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

between forest and prarie. Sumac are early successional species or pioneering species and

are 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 Wittaker’s previous thinking of succession.

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

conolizes the site below the Sumac community. Colonization by new understoy 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 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.

Observation: Some Sumac communities appear to have changes in ground cover 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 succesional 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.

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Prediction: Intermediate aged sumac communities will have greater understory

vegetation species richness than younger and older Sumac communities.

Test/Experiment: I will select 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 is to determine if

Sumac community understory species richness changes with Sumac community age.

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Materials and Methods: PAST TENSE????

Populations and Communities: Twenty Sumac populations were selected to sample

based on their stand age. Stand age is 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 population is

designated as a Sumac community. ….DESCRIBE WHEN AND WHERE!!

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 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 m2 quadrats located at the

community centroid. This method is appropiriate 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 more towards the perimeter of

the total community area would be representative of either a younger or older community

than the true mean community age dependent on what point of the perimeter the quadrat

was placed. This 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

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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 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 5 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. 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.

The regression line defines the relationship between species richnesss and ether area or

height. In my experiment the independent variables, sumac community height (m) and

sumac community area (m2) are under my control and are not random. I controlled the

independent variables to ensure a sumac community age gradient (young to old) is

sampled. Sumac community age was assumed to be selected for based on

community area and height of tallest Sumac individual.

Results:

Discussion: This results of this study represent how community ecology can complex with multiple

dependent variables affecting one independent variable. Although the regression analysis

for Sumac community for both height vs. species richness and area vs. species

richness were not significant, this does not rule out that Sumac community age does

not influence species richness. …How wwell the study fulfills the definie purpose!

Area and height alone may not significantly demonstrate species richness without 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

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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 assocaiton with nitrogen-fixing Rhizobium bacttera are able to grow

and dominate the site with their access to atomposheric 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 can not dominate the site with faster rates of growth and recruitment. (Smith,

362-363). Soil texture is just one of many dependent variables that may influence the

Sumac community understory species richness during succession.

Height and area… why not significant?

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 with 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 commnity as a

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

Sumac is not infinite and once the trees reach a certain height they are no longer

representative of the age of the community. 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 The Pattern of Tree Seedling 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 distrubution of understory tree seedling establishment. The

results of my study may have been influenced by properties of specific Sumac

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

that were dispersed as understory vegetation.

In this same study they 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. This

findings of thits 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 this studies previoius findings, it is 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 previous study finds that density of trees in old abandoned Michigan fields

increased with increased sumac cover. This finding suggests that certain species density,

such as the trees in this study, may be affected by overall Sumac cover more than others.

The same study states that “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 through clonal roog growth the vegtation 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.” 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

significat 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

Ecological Relationships between Overstory and Understory Vegetation

in Ponderosa Pine Forests of the Southwest 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 moistur, 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 20 selected Sumac communities of variable canopy

coverage that give greater differences in understory species richness. In my measure of

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canopy coverage I would account for the other species canopy coverage Another method,

I would change would be the size of the quadrat used in my experieminent. I found it

difficult to use a 0.25 m2 quadrat for a 2000 m2 community area. Due to the difference in

area of each community selected, I could not multiply the species richness within the

0.25m2 quadrat samples to represent a larger sample size.

Literature Cited:

Fowler, J., Cohen, L., & Jarvis, P. (1998). Practical statistics for field biology (second ed., pp. 141-147). Chichester, England: John Wiley & Sons. 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 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