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
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 fastergrowing, 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.
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
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 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
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.
Sumac area (m2) was not statistically different from Sumac species richness with a p- value of
Sumac height (m) was not statistically different from Sumac species richness with a p-value of
Table 1. Linear regression analysis of variance descriptive statistics of twenty non-randomly
selected Sumac communities area (m2) and species richness.
p = < 0.674
Sumac commmunity area (m2)
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.
Table 2. Linear regression analysis of variance descriptive statistics of twenty
non-randomly selected Sumac communities height of tallest Sumac individual height (m) and
p = < 0.361
Sumac community height (m)
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.
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.
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
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, 10year 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
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
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 m2
community area. I would change the size of the quadrat used dependent on the area of the Sumac
City of Mankato, Minnesota. (2009). The edge. Informational park sign, Rasmussen
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:188.8.131.52
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