Introduction: “Community structure varies in space and time.” Succession is a process
that changes community structure thr...
Prediction: Intermediate aged sumac communities will have greater understory
vegetation species richness than younger and ...
area (older) or where the species richness was least in a smaller area Sumac community
(younger). My methods fit the mecha...
rate of community succession and value of species richness would be affected by soil
nutrient availability changing in spa...
community age classes, annual changes in dispersal of seeds and establishment of seeds
that were dispersed as understory v...
canopy coverage I would account for the other species canopy coverage Another method,
I would change would be the size of ...
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Scientific method sumac


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Scientific method sumac

  1. 1. 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. 1
  2. 2. 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. ???? more?? 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 2
  3. 3. 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 3
  4. 4. 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 4
  5. 5. 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 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 5
  6. 6. 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: 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 6