John Burth
Forest Ecology
Lab #4
October 21, 2004
Abstract
Bulk density can tell us about soil characteristics such as amount of organic matter and pore space. If a soil has greater organic matter and pore space it will have a lower bulk density. Using basal area as an indicator of accumulated organic matter this lab attempts to establish a relationship between soil bulk density and basal area. The total basal area per acre for four plots was measured and soil cores were extracted from each of these plots. The bulk density of these samples was calculated. Points in the study area with the greatest bulk densities contained the least basal area and visa versa. Several points with similar basal areas did not have similar bulk density measurements. This could be attributed to a lack of samples or other factors such as different species composition and age.
Introduction
Primary succession is ecosystem change that occurs on previously unvegetated terrain and proceeds without any catastrophic occurrence (Barnes, 1998). The abandoned Jamesville Quarry demonstrates primary succession at several different stages. The owner’s of the quarry ceased using sections of the property at different points in time which resulted in each of these sections entering primary succession at a different point in time.
The sections of the quarry that are in different stages of primary succession contain varying amounts of vegetation and therefore varying basal area. Areas that have been in primary succession longer would tend to have a greater basal area and would accumulate more organic matter within the upper layers of the soil over time. Areas in primary succession longer would also experience more movement within the soil by living organisms such as plant roots and small invertebrates. These two factors would alter the structure of the soil and result in greater pore space in the upper layers. Pore space is the volume of soil that is occupied by either air or water (Briggs & Craul, 2004).
Soil bulk density is defined as the dry weight per unit volume of undisturbed soil including its pore space. It is influenced a great deal by organic matter content and soil structure (Briggs & Craul, 2004). In soils that are less compacted and contain more organic matter the bulk density will be lower. The objective of this lab was to measure the bulk density of soils and examine how these measurements varied between points where there were different amounts of total basal area due to different stages of primary succession. My hypothesis was that in areas with greater basal area the bulk density would be lower and visa versa.
Methods
Measurements for this lab were taken at the abandoned Jamesville Quarry in Syracuse, NY. Points were marked within the quarry that appeared to be in different stages of primary succession. I used a bulk density corer to extract a known volume of soil at each point. The first point was unvegetated, points 2 and 3 were both characterized by moderate amounts of vegetation as compared to the rest of the site, and point 4 was in a more heavily forested section of the site.
The extracted soil samples were dried and subsequently weighed. The weight of the soil in grams was divided by the volume in cubic centimeters to calculate the bulk density of each sample. Total basal area was measured at each point by Russ and Andy. I compared these measurements to my bulk density calculations to attempt to establish a relationship between the two.
Results
Table 1 shows the bulk density and total basal area measurements for each point in the study area. The relationship between these measurements is shown graphically in figure 1. At point 1, which was unvegetated, the total basal area was 0. The bulk density at this point was the largest in the study area at 1.54 grams per cubic centimeter. Point 4 had the greatest basal area as well as the lowest bulk density. Points 2 and 3 had very similar total basal areas at 30 and 29.38 square feet per acre. The bulk density values at these two points were not very similar, but they were in the middle of the other two measurements.
Total Basal Area (sq. ft./acre) / Bulk Density (g/cm^3)Point 1 / 0 / 1.54
Point 2 / 30 / 1.15
Point 3 / 29.38 / 1.43
Point 4 / 110.74 / 1.08
Table 1. Basal Area and Bulk Density Measurements for Each Point in the Study Area
Figure 1. Relationship between Bulk Density and Total Basal Area
Discussion
The results for points 1 and 4 support the idea that areas with a greater basal area have lower bulk densities. Point 1 was unvegetated and therefore had little activity from living organisms and little organic matter accumulated. Point 4 was forested for some time so it follows that there would be more organic matter and organism activity resulting in more pore space.
My hypothesis is somewhat supported by the fact that both the basal area and bulk density measurements for points 2 and 3 fall between those of points 1 and 4. These points appeared to be in stages of primary succession that were in between those of points 1 and 4. However, the data would have to show similar basal area as well as bulk densities for points 2 and 3 in order to fully support the hypothesis. The basal areas for these two points was nearly the same but the bulk density for point 2 was 1.15 grams per cubic centimeter and 1.43 grams per cubic centimeter for point 3.
One possibility for explaining this discrepancy is sampling error. It may have been helpful to take additional bulk density measurements within the plots to get a better idea of soil characteristics for the overall area. The sample for point 3 could have been taken at a point that was more compacted at the beginning of succession because it was part of a trail or road. The species of plants in the immediate vicinity could produce less biomass and therefore less organic matter. For some reason the microclimate at this point could result in less activity by small invertebrates. All of these factors could result in less pore space and therefore greater bulk density at the sample point while the basal area was nearly the same as in point 2. More core samples could help take differences like these into account in describing the entire study area.
Another way to help explain the difference in bulk densities could be to examine the species composition and actual ages of the trees. Different species could have different root structures and therefore create different amounts of pore space within the soil. It would be helpful to age the trees because points 2 and 3 could actually be at different stages of primary succession even though the basal areas are very similar. Because of environmental factors such as nutrient availability and available light the vegetation at point 2 could be older but not growing as quickly as the vegetation at point 3. If point 2 is in a later stage of primary succession that would explain the lower bulk density because organic matter has been accumulating and living organisms have been active for a longer period of time.
Conclusion
The results of the lab show a relationship between basal area and soil bulk density in that the point with the greatest basal area had the lowest bulk density and visa versa. There are discrepancies at other points because similar basal areas do not correlate with similar bulk density measurements. These differences could be due to sampling error or could be explained by further studying additional factors. This reminds us of the importance of proper sampling frequency and studying all possible factors that may help to describe the characteristics of a site.
By studying the characteristics of a site such as soil bulk density and learning about the development of primary succession we can better predict what the future holds for that area. This is helpful in making management decisions because we can predict what types of vegetation will occur naturally as well as what species would be suited to the conditions of a site if planted.
Literature Cited
Barnes, B.V., et. al. 1998. Forest Ecology. 4th ed. John Wiley & Sons, New York. pp. 774.
Briggs, R.D., P. J. Craul. 2004. Introduction to Soil FOR 545 Laboratory Manual. pp 13.