Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

Forest Diversity Lab Report

Introduction:

The thickness of topsoil can have a large effect on tree density, tree diversity, and

lifespan of the trees in the area. Topsoil is the part of soil that contains the most nutrients and

therefore, is what most aids trees in their growth. It is composed of two different layers. The top

layer is called the O layer and is the thicker layer of the two. The bottom, A layer is the thinner

of the two. Biological soil crusts are helpful in increasing the fertility in desert sand (Gao, et. al.

2014). Given that information, assumptions can be made regarding biological activity also

helping the fertility of the topsoil. Trees mainly keep their roots in the topsoil layer because of

the nutrient abundance that can be found here. Given this information, it could be expected that

the more topsoil there is, the more trees there would be growing in that location.

Tree density is the amount of trees in a specific measured area. Many factors could

influence tree density including but not limited to soil quality, amount of space, amount of light,

competition between trees, and even the thickness of the topsoil. There have already been tests

on different varieties of trees and the varying soil composition that they grow in (Marcos; et. al.

2010). Studies have also been done in Georgia to test the relationship between soil physical

property changes and the growth of the loblolly pine (Lincoln; et. al. 2007). No mention was

found as to how topsoil thickness was related to tree density.

This study aims to research the relationship between tree density and topsoil thickness.

We believe that the greater the tree density the thicker the topsoil. This is the stance we have

taken because there must be more nutrients in the soil, potentially caused by there being an

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

abundance of topsoil to support a denser tree population. This information may be useful when

growing trees in a dense space. We will conduct a study to test this hypothesis.

Methods:

Our study took place on February 21, 2014 at the State Botanical Garden of Georgia,

located at 2450 South Milledge Avenue, Athens, GA 30606. The temperature was approximately

60˚F and the ground was a little damp. When we tested our hypothesis, the greater the tree

density the thicker the topsoil, the materials needed were, a soil probe, two different tape

measures (one 50 meter and one smaller ones), and a ruler. We started our study by measuring

out one 50 meter transect as straight as possible from the pathway into the forest. We then used a

random number table to select a number and proceeded to go that number of meters down the 50

meter transect. We used one of the smaller tape measure to mark the distance perpendicular to

the 50 meter transect and divided the area into four different quadrants. We then took a soil

sample and measured the depth of the topsoil using the ruler. These measurements were taken at

the intersection of the two tape measures marking boundaries, the measurements were then

recorded. With the intersection tape measure a 2x2 meter area was created. We counted the

number of trees in the area and this allowed us to measure the tree density. We repeated this

process a total of 29 more times, four on the same 50 meter measurement with different

randomized distances away from the last one and 25 more on five different new 50 meter

transects. We looked to see if there was a correlation between tree density and topsoil thickness.

These were both continuous variables and therefore, a linear regression was used to analyze the

data.

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

Results:

After performing the study the data collected was assembled and analyzed. Tree density

and topsoil thickness turned out to not be correlated to one another as can be seen in figures 1

and 2. The calculated R2 value for the two continuous variables is 0.2407, which means that only

24.07% of the variability in the data can be explained by the model represented in figure 1. This

is a low value because the higher the R2 value the more correlated the two data are. The P-value

calculated for the data is 0.005912. This means that the hypothesis that topsoil thickness would

affect tree density can be rejected because it is below the standard 5% significance level.

Figure 1: Linear Regression for Tree Density vs Topsoil Thickness

Figure 1 shows the linear regression of tree density vs topsoil thickness. The R2 = 0.2407 this means that about 24%

of the variability can be explained by the model, which is low.

It is hard to see any kind of pattern in the data when looking at figure 1, other than it is

clumped towards the bottom left side of the figure. However, figure 2 shows how the two

R² = 0.2407

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Tre

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Tree Density vs Topsoil Thickness

Series1

Linear (Series1)

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

variables relate to each other better. It shows tree density in an area and the thickness of the soil

in a way that is easy to compare the two to each other. Some patterns can be seen and for some

of the data points the two variables follow suit with one another, but in other cases they move in

opposite directions from each other.

Figure 2: Comparison of Tree Density and Topsoil Thickness in a Line Graph

Figure 2 shows another comparison of the tree density in a 2x2 meter area and the topsoil thickness in centimeters.

This figure shows how the tree density and topsoil thickness relate to each other clearer.

Thirty total measurements were taken for both tree density and topsoil thickness. The

mean of the tree density and the topsoil thickness were both measured for each transect and

compiled into table 1. This table makes it easy to look at the averages of both data sets and see

0

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Comparison of Tree Density and Topsoil Thickness

Soil Thickness (cm)

Tree Density (2x2 meterarea)

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

numerically if there are any noticeable patterns in the data. When looking at the data there is no

distinguishable pattern. The thickest topsoil mean does not match with the highest tree density

mean. Likewise, the thinnest topsoil mean does not match with the smallest tree density mean.

This shows even more data that the hypothesis can be rejected.

Table 1: Mean Tree Density and Mean Topsoil Thickness in each Transect

Transect # Mean Tree Density (2x2 meter area) Mean Topsoil Thickness (cm)

1 6.00 2.14

2 4.20 1.98

3 3.80 2.46

4 5.40 4.80

5 5.20 3.06

6 5.60 1.98

Table 1 shows the mean tree density and mean topsoil thickness in each transect measured. This allows for a

numerical observation of the means for the two variables.

Discussion:

The results from our study did not support our hypothesis that the thicker the topsoil the

more dense the trees would be. An R2 value of 0.2407 was calculated and a P-value of 0.005912

was also calculated. Neither of these results could help support our hypothesis. One biological

mechanism that could have impacted our results could be the amount of decomposers in the soil

areas we measured. The more decomposers there are in an area the faster leaf litter and other

organic material gets turned into soil. This could explain the variety of different topsoil

thicknesses measured throughout the study. Many other factors could have been holding back

trees growth as well. Not getting enough sunlight to the lower level of the forest could result in

smaller trees dying off despite there possible being a good layer of fertile topsoil. A tree needs

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

many things to grow and just because there is a thick topsoil layer does not mean there will be a

large tree density.

A hypothesis that could be tested may be canopy cover vs tree density. Tree canopy also

has an impact on soil conditions and when soil is covered by a tree canopy it is more fertile than

when it is in the open (Isichei, 1992). Canopy cover vs tree density would be a good study to

conduct because the thicker the canopy the less light getting to the lower level of the forest.

Therefore, a hypothesis that could be made about this study would be that the less canopy cover

the denser the trees because more light will reach the lower level of the forest. A study compared

tree size to planting density and the result that was found was that the bigger trees were found to

be in the lower planting density area (Forrester, 2013). This is another biological factor that

could have impacted the results of the study conducted. Another study showed that tilling soil

had a small positive impact on the growth of the loblolly pine in Georgia (Lincoln, 2007). The

soil the trees were growing in for the study conducted in class was natural and had not been tilled

at all. Maybe if this soil had been tilled it would have had a higher tree density. This could be an

experiment done in the future to try and distinguish whether or not tilling an area of land would

result in a higher tree density.

The hypothesis that topsoil thickness will lead to a denser tree population could not be

proven with the data that was collected. There was only a 0.2407 R2 value and a 0.005912 P-

value. Looking at the graphs that were produced it is also evident that there is not much pattern

to the two variables in relation to each other. This is just one variable that could have been a

factor on tree density but the data showed that it didn’t have an impact. In the end, there are

many factors that go into the density of trees and because of that it is hard to distinguish between

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

them which ones are actually affecting the density of the trees. Experiments or studies may need

to be conducted in order to determine which variables impact the density of trees more than the

others. This study showed that trees do not need a thick layer of topsoil to grow in dense clusters.

Nicholas Wilson

ECOL 3500L

Kruger

2/28/14

Works Cited

Forrester, David I., John C. Wiedemann, Robert I. Forrester, and Thomas G. Baker. Effects of

Planting Density and Site Quality on Mean Tree Size and Total Stand Growth of

Eucalyptus Globulus Plantations (2013): n. pag. Web. 27 Feb. 2014.

Isichei, Augustine Onwuegbukiwe, and Joseph Ikechukwu Muoghalu. "The Effects of Tree

Canopy Cover on Soil Fertility in a Nigerian Savanna." Journal of Tropical Ecology8.03

(1992): 329. Print.

Lincoln, M. Chad, Rodney E. Will, Lawrence A. Morris, Emily A. Carter, Daniel Markewitz,

John R. Britt, Ben Cazell, and Vic Ford. "Soil Change and Loblolly Pine (Pinus Taeda)

Seedling Growth following Site Preparation Tillage in the Upper Coastal Plain of the

Southeastern United States." Forest Ecology and Management 242.2-3 (2007): 558-68.

Print.

Marcos, Elena, Leonor Calvo, José Antonio Marcos, Ángela Taboada, and Reyes Tárrega. "Tree

Effects on the Chemical Topsoil Features of Oak, Beech and Pine Forests." European

Journal of Forest Research 129.1 (2010): 25-30. Print.

Gao, G., Ding, G., Wu, B., Zhang, Y., Qin, S., Zhao, Y., . . . Deng, J. (2014). Fractal scaling of

particle size distribution and relationships with topsoil properties affected by biological

soil crusts. PLoS One, 9(2) doi:http://dx.doi.org/10.1371/journal.pone.0088559