Community Diversity of Dorr Mountain

Community Ecology of Coastal Maine homepage

Home > Student Reports > Community Diversity of Dorr Mountain

Community Diversity of Dorr Mountain

Lab report by Raimon Cary

Introduction:
Environmental factors greatly affect the species diversity within a community. Such factors as slope, temperature, soil pH, and altitude can all influence the growth of species. When looking at a large area that encompasses several communities, it is clear that abiotic factors are significantly influential. Whether the transitions are abrupt like an edge, or gradual like an ecotone, it is evident that slight as well as great changes in the environment can change species diversity.

There are two general types of transitions between communities. These consist of ecotones and edges. Ecotones are gradual transitions between communities. They are a mixture of species from neighboring communities; however, they are unique by themselves. Overall, the species tend to be smaller because there is a higher amount of competition that exists. This competition limits vertical zonation but it increases the species richness. The plants from both communities are competing for space, light, soil nutrients, etc. and so the ecotone represents the dominance of one community over another the closer to either area one gets. An edge is simply an abrupt ecotone. Many ecotones can be extremely gradual spanning miles, whereas others can be a matter of feet. Edges are usually caused by a drastic change in abiotic factors such as slope and soil thickness. It is clear that a cliff would be an edge because the slope change is severe, whereas going from an open field to a small forest might be much more gradual (Kricher and Morrison, 1998).

The purpose of this study was to observe the community diversity of Dorr Mountain through measurements of abiotic factors and observations of species abundance. As one follows the transitions down the mountainside, it is clear that there are several distinct community zones that exist. These zones differ slightly and sometimes drastically from each other. That difference can be temperature, wind speed, altitude, slope, or various other abiotic factors. By looking at these environmental influences, trends could be made about the community diversity.

Methods:
Beginning at the top of Dorr Mountain, the altimeter and GPS were calibrated and/or started. Measurements of various abiotic factors were calculated, such as temperature, humidity, wind speed, etc. (table 1, fig 1). The dominant species of plants were also identified. Then following the path down the mountain, substantial changes in surroundings were observed, and then the presence of a transition was noted. The same abiotic data was calculated for each zone that was found from the top of the mountain to the path at the bottom. All soil samples were taken off the trail as well as all the other measurements so as to maximize the accuracy. The trail might have affected the data, so all measurements were done away from any possibly disturbances. With all the zones completed, the data was compiled to look for trends. Altitude and distance were plotted together and a graphical representation of the zones was obtained (fig 1). From this figure as well as the compiled data trends were then observed.

Results:
The results revealed numerous trends throughout the communities of Dorr Mountain. It was clear that the distances of the zones varied throughout the mountainside, and, in general, became smaller as altitude decreased. The Alpine zone was the largest spanning a distance of over 800 m and decreasing in altitude over 200 m. This large area included most of the top of the mountain. It contained dominate species like the Pitch Pine, Black Spruce, and Bear Berry. All of these were either stunted trees or low lying plants and shrubs. Also these plants only grew in small patches between the rocks, or near places where there was the presence of lichens. The light level was high and there was no existence of a canopy. The humidity was around 34% and the air temperature was 21.6º. The soil was fairly thin and contained no moisture. There was the presence of an O, A, and B layer in the soil. The soil temperature was 72º F. The average pH of the soil was 7.0 or neutral. In addition the average slope was 15.4º (table 1, table 2). The terrain was extremely rocky, containing few places with any form of soil or plant life. The Alpine Zone transitioned through an ecotone to the Birch Grove. The ecotone was only 45m long and consisted of species from both the Alpine Zone and the Birch Grove. The wind speed slowly decreased throughout the ecotone. The species of trees from the Alpine Zone became smaller, but they also became fewer in abundance. Likewise, the species from the Birch Zone became taller, but they became greater in abundance.

The second zone was the Birch Grove which consisted of Gray Birch, White Birch, Red Oak, and White Spruce. The zone was dominated by canopy trees and some under-story trees as well. The ground cover became dominated by only a few low lying plants such as ferns. The distance of the zone was only about 90m, and the altitude was a decrease of only about 10m. The slope was 18º on average. The light level was much lower than the Alpine Zone. The air temperature remained the same. The wind speed decreased to almost zero. Also, the humidity was greater, being 37%. Also, the soil moisture increased to 4%. The soil temperature remained close to that of the Alpine Zone, being 74.3º F. The pH dropped to about 6.8 on average. Soil thickness decreased in the O layer but greatly increased in the A and B layers, reaching unattainable lengths (table 1, table 2). The terrain was less rocky and contained greater areas of soil. There was a distinct existence of a canopy, and the density of the zone was much higher than the Alpine. This zone then transitioned through an edge to the Mixed Pine Zone. The edge only spanned a distance of about 1 foot. There was no apparent species change within the transition; however, there was a slope increase. Also the terrain became mostly rock, much like the Alpine Zone.

The third zone was the Mixed Pine Zone. It contained various species of pine, spruce, and birch. The trees were not evenly spaced, growing between large crevices in the rock or where there was an abundance of lichen. There was no apparent canopy and the trees in general were smaller than in the Birch Grove. There was an increase in the abundance of low lying plants. The density of the zone was much less than the previous zone, but the diversity was greater than the Alpine Zone. The distance of this zone was 77m, and the altitude decreased about 40m. The presence of low lying plants was near to zero and the density of the plants was much less than the Birch Grove. The wind speed was zero and the humidity was back down to 34%. The air temperature increased to 27º C. The light level increased up to that of the Alpine Zone. The soil became thinner, increasing the pH to 6.9 and decreasing the soil moisture to zero. The soil temperature was still around 72ºF. The slope also increased to 23.7º. The terrain was mostly steep sloping rock. The Mixed Pine Zone transitioned through an ecotone to the Hardwood Forest, over a distance of 46.5m (table 1, table 2). The transition contained no significant altitude change, but the slope did level out slightly. The presence of the pines and birches became fewer as the presence of maples increased. The heights of the trees became smaller for the Mixed Pine species and larger for the Hardwood species. Also the presence of low lying plants became less and less towards the Hardwood zone. In addition, the rocky terrain slowly turned into soil bearing ground.

The Hardwood Forest was clearly denser than the earlier zones. It was dominated by tall maple and oak trees creating a significant canopy. The distance of the zone was about 370m, and the altitude decreased by 60m. The light level was lower than the Mixed Pine zone, and even lower than the Birch Grove. The wind speed remained close to zero and the humidity remained at 34%. The air temperature went back down to 23.5ºC. The soil thickness increased, and the amount of noticeable rocky ground decreased. The O layer became 4 cm, and the A and B layers reached 7 cm and above. The soil temperature remained the same. However, the pH dropped to 5.6 and the soil moisture increased to 10%. The slope also decreased to only 15º. The species diversity deceased, but the density and evenness increased. There were much less low lying plants and much more canopy and under-story trees, such as Sugar Maples, Striped Maples, and Red Oak. There was a definite difference between the open area of the Alpine and the density of the Hardwood Zone. The Hardwood Forest Zone transitioned through an ecotone to the Hemlock Forest Zone. This was a gradual transitioned which slowly introduced the presence of Eastern Hemlock over a distance of 50m (table 1, table 2). The slope became much steeper and the terrain became rockier. The plant diversity increased and the same trend of decreasing and increasing abundances of species from the neighboring zones was seen.

The Hemlock Forest Zone was significantly different from all the other zones. There was an evident change in light level, being significantly lower than any previous zone. The humidity greatly increased to 51%. There was no wind at all and the air temperature only decreased by a couple degrees to 21ºC. The distance of the zone was 170m, and the altitude decrease was 30m. The slope increased to 23º, but the soil thickness remained about the same. The terrain did become rockier, but the density of the forest increased. The species that dominated were primarily Eastern Hemlock, with several maples, pines, and birches. The canopy was not as high as the Hardwood Zone, but there was a distinct presence of one. There was little to none low lying plants. The soil pH increased back up to 6.8, but the temperature of the soil decreased to 66ºF. Soil moisture decreased to zero (table 1, table 2). The diversity of the zone was greatly decreased despite the density and abundance of the tall canopy trees. This zone then transitioned through an edge to the Lowland Hardwood Forest Zone. There was an abrupt change from the dense Hemlock Zone to the less dense Lowland Zone. The abundance of Hemlock was significantly decreased, and the abundance of low lying growth was greatly increased. The slope significantly leveled out as well.

The last zone that was observed was the Lowland Hardwood Forest. This zone was dominated by Maples, Oaks, and mostly low lying plant life. The path of the park ran right between the Hemlock and the Lowland zones. The density of this zone was much less that the Hemlock and even the Hardwood Zone. The light level increased, as well as the air temperature, which was about 27ºC. The humidity dropped to 23%. The soil remained thick and also remained cooler being only 65ºF. The soil moisture remained zero and the soil pH stayed at 6.8.

Discussion:
Weather, slope, and soil profile were three general factors that shaped each of the zones. Where there were high winds there was less vertical zonation, such as in the Alpine Zone. The plants were all stunted in growth because of the constant wind that they receive. In places where the wind is less of a factor, like in the Hardwood Forest, the plants are able to grow to larger sizes and develop vertical zonation. The daily and seasonal temperature changes can also affect the species that live within a zone. Since the Alpine Zone receives the most wind, it probably also receives the harshest weather, which would also account for the small growth that existed. In such areas as the Hemlock Forest where the wind was much less, the forest was much taller and the density was much higher. The trees were able to use supportive soil, gather nutrients, and grow tall. This effect by wind explains why most of the top of the mountain was Alpine forest. The majority of the upper part of the mountain experiences high winds, whereas the lower parts experience much less.

Slope plays an extremely significant role in the development of a community. The slightest changes in elevation can alter the species diversity drastically. This was seen in the transition between the Mixed Pine and the Hardwood Forest zones, as well as the Hemlock and the Lowland. In each case, the slope changed enough that the species diversity was greatly affected. From the Mixed Pine to the Hardwood, the slope leveled off and resulted in an increased ability for other types of trees to grow. The same was seen in the other instance. The natural contour of the mountain created numerous dips and peaks that have a significant influence on the species that can grow. This is why on the steep slopes there were more pine trees, since they are better able to grow in dryer, rockier places. The steep slope causes more water movement, and so in general the soil is thinner. Therefore, a firm substrate is not available for the tall canopy trees to grow in, like in the Hardwood Forest. When the slope was very gradual or level then the soil was able to be thicker because of the fewer amounts of large rocks and the slower flow of water. The soil is then thick enough to support the larger trees, both in support and nutrients.

Lastly, the soil profile also plays an important role in the development of a community. The weather and the slope could be just right, but if the terrain does not support soil then no plants will grow. The leveling of a slope allows for ground cover to be captured which allows for the beginning growth of pioneer plants. Once these plants establish themselves they begin to develop soil and nutrients which allow the growth of other species. This continues until there is a viable habitat in which the community can thrive. The soil is a by product as well as a determining factor in the presence of certain species. It is greatly affected by the climate, but it by itself influences the zonation. For instance, both the Birch Grove and the Hardwood Forest were similar in slope, and other abiotic factors, except the soil profile. The Hardwood forest contained 6% more moisture, and contained the presence of an O layer. Therefore, it was able to hold more nutrients in the O layer, as well as provide water for the trees. Also, the presence of this larger amount of moisture probably accounts for the lower pH in the Hardwood Zone. In order for there to be ions present there must be water; therefore, the other zones may have different pH levels, but the lack of water did not allow the measurement to be taken accurately. The color of the soil was an indicator of the soil’s nutrient level. In the species abundant forests, especially those with large canopy trees, the soil was much darker, almost black. In the areas with mostly ground cover, or small pine trees the soil was lighter in color, sometimes almost orange.

Combinations of these three factors are what influence the variability between the zones. In certain zones the factor that made them different from each other may have been simply the weather. In other zones the differences were a matter of altitude decrease or slope change. Once plants begin to grow, then the factors become more biotically focused. Two zones may be suitable for numerous species of trees, but the ones that out compete the others will flourish and define the community that will exist. The Hemlock Forest was dominated by Eastern Hemlock. As a result the canopy was extremely thick and the light level was greatly decreased, resulting in near to zero ground cover. However, in the Hardwood forest there was a defined canopy, but the trees were not as densely spaced as the Hemlock Zone. This resulted in a higher light level which allowed under-story and low lying plants to grow. Plants that are shade tolerant can survive in moderate amounts of shade, but not in extreme amounts like in the Hemlock forest. The soil also affects the types of plants that can grow. Depending upon what layers are available certain trees may not be able to grow. Without a sufficient O layer and/or A layer, many tall trees would not be able to grow because of a lack of nutrients. With all other factors being the same, it is the soil profile that will determine what types of forest will thrive. The different combinations of weather, slope, and soil profile are what provide the various zones that exist on Dorr Mountain (Kricher and Morrison, 1998).

References Cited:
Kricher, J. &. Morrison G. (1998). Eastern Forests. Boston: Houghton Mifflin
Company.