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.
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.
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
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.
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
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
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 16year-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
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
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.
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:126.96.36.199
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