Vertical Structure and Abiotics of Disturbed and Undisturbed Site

Community Ecology of Coastal Maine homepage

Home > Student Reports > A Comparison Between Vertical Structure and Abiotics of Disturbed versus Undisturbed Site

A Comparison Between Vertical Structure and Abiotics of Disturbed versus Undisturbed Site

Lab report by Justin Armetta

Introduction:
Disturbances are factors that alter the community or landscape of a given area. In particular, weather has a significant impact upon the landscape and can cause great devastation to communities. Wind, precipitation, and temperature are all examples of weather that can disturb a community. These disturbances can occur on a small, medium, or large scale. An example of a small-scale disturbance would be rain causing the soil to erode. An example of a medium-scale disturbance would be wind throw knocking a group of trees over. An example of a large-scale disturbance could be a tornado. This could move rocks and uproot huge areas of trees. Other disturbances include fire and flooding.

Disturbances can cause changes to the structure of a community. In an aquatic environment, light first hits the canopy, then the understory, and continues on to algal turf. The removal of trees from a terrestrial environment can cause changes to vertical structure by allowing increased light to penetrate the canopy. This sunlight allows more growth underneath. The vertical structure of a community is dependent upon light entering the habitat. It is dependent on the vertical gradient of this sunlight. Light first hits the canopy, followed by the lower canopy, the understory trees, shrubs, herbaceous vegetation, and finally the organic layer. The larger the disturbance to the terrestrial community, the more effect it will have on the vertical gradient of sunlight. The larger and more tightly packed trees are in a community, the more difficult it becomes for light to penetrate and reach the lower structure of the community.

The purpose of this data report was to compare the vertical structure of two quadrats, one a disturbed site and another an undisturbed site, at two different areas around Jordan Pond. Abiotic data was also gathered and a species list was compiled to determine under what conditions specific organisms functioned best under and how the vertical layers and the species within them compared.

Materials and Methods:
The materials utilized in this experiment included orange tape and four stakes to seal off each quadrat. Field guides were used to identify species within each quadrat and they were recorded on paper. In order to obtain abiotic data instruments such as a compass, portable weather station, thermometer, altimeter, and a global positioning system were utilized. Other devices utilized measured soil moisture, soil pH, and light levels. The first quadrant at the disturbed site had an area of 129m2 and second quadrant at the undisturbed site had an area of 150m2. These were the zones where species were identified. The abiotic factors determined using these devices were wind direction/speed, latitude, longitude, soil moisture, altitude, pH levels in the soil, soil temperature, air temperature, light levels, and humidity.

Results:
Abiotic measurements that were relatively similar among the disturbed and undisturbed sites were air temperature, wind speed, and pH levels (Table 1). However, soil moisture, slope, and light levels were radically different between the two sites (Table 1). Some notable species trends were the abundance of sphagnum moss, sundews, and the abundance of black spruce found in the shrub and understory zones in the disturbed site (Table 2). An abundance of Canada mayflowers were found in the herbaceous zone, as were many red spruces in the canopy, understory, shrub, and herbaceous zones of the undisturbed site (Table 2).

Discussion:
Disturbances caused by the man made creation of the path around Jordan Pond had an effect on the vertical structure of the community along the edge of the pond at the first site. It was clearly evident that the removal of canopy trees from the area caused increased light to reach the forest bottom. Also, the lack of canopy trees caused less water to be absorbed by the roots of the plants. Since there was no slope in the surrounding area of the first site anyway, the removal of the large canopy trees caused water to be trapped in large pockets where trees used to be. This resulted in the creation of a bog type environment with sphagnum moss and sundews. This was unexpected because the conditions in site one were not typical to that particular area. For instance, the soil was unusually acidic with an approximate acidity of 6.1. Soil moisture was at 80% and the soil temperature registered 57 degrees F, which was slightly cooler than the second site.

However, these conditions help to account for why there was such a dramatic difference in the plant life. Sphagnum moss and sundews both thrive on greater acidity, increased soil moisture, and cooler temperatures with plenty of access to sunlight for more development and growth. Incidentally, these conditions are also more favorable to black spruce, bunchberries, and arbor vetae. In particular, the bunchberries number on the forest floor was close to 1500. They definitely benefited from the bog type conditions.

When comparing the herbaceous layer of the disturbed site to the undisturbed site, it was evident that there was far more ground cover in the disturbed layer. There was not much of an upper canopy in the disturbed site because of the man made creation of the path around the pond. This let an extraordinary amount of light into the herbaceous zone and promoted plant growth for 774 star flowers, 1419 bunchberries, and 40 small black spruce. These numbers are huge when compared with the undisturbed site that had only 18 star flowers, 5 striped maples, and 8 red spruce trees.

At the second site there was a fairly large slope of 25 degrees. This caused a great nutrient run off down the slope and did not help to promote growth for the understory, shrub, or herbaceous layer. The only herbaceous vegetation to do well was the Canada mayflower. That was because the Canada mayflowers can tolerant shade and withstand a certain degree of acidity in the soil. Red spruce seemed to be the climax species in the second site, as they were present in the canopy, understory, shrub, and herbaceous layer. They handle poorer soil conditions better than most other trees.

It can be concluded that disturbances did have an impact on the vertical structure of the community at Jordan pond when comparing the disturbed first site with the undisturbed second site. The removal of large canopy trees in the first site promoted growth for new herbaceous vegetation. The second site had large level canopy trees that blocked a majority of sunlight and therefore stunted growth for herbaceous vegetation.

References Cited:
Kircher, J., and Morrison, G. 1998. Eastern Forests. Houghton Mifflin Company: New York.