EXERCISE 2
Terrestrial Ecology: Old-Field
Succession
Vascular plants play an important role in many terrestrial and aquatic ecosystems, not only because they may supply
energy and materials for organisms at higher trophic levels, but they often characterize the physical structure, or physiognomy, of particular habitats. For example, the trees of forest ecosystems create a wide variety of habitats above
ground level for insects, birds, and many mammals that donÕt exist when trees are not present. These trees also act to
modify microhabitats below the canopy by providing shade and wind protection as well as supplying dead leaves to the
forest ßoor. Ground-level habitats are much different after trees have been removed.
Ecosystems rarely remain the same for long periods of time. When new landforms are created (e.g., volcanic islands,
reef islands, sand dunes, dry lake beds, river banks, etc.), they undergo countless gradual changes as plants colonize
them. Even after seemingly stable communities have been well established, they are still subject to disturbance events
that can set the colonization processes back to its beginning. For each basic ecosystem type, there is a recognizable, and
predictable, pattern to the vegetational changes that occur on newly created landforms and as well as in older ecosystems recovering from disturbance. This pattern of community development is called succession. Primary succession
refers to community development on new landforms and secondary succession to community recovery after disturbance. Secondary succession tends to be more rapid because there are usually lingering roots and seeds from the previous communities.
Communities can be disturbed by a variety of natural and man-made events. Floods, forest Þres, glaciation, and volcanos are some examples some of natural disturbances that can affect wide regions; but ecosystems have been altered
most in recent times by primarily man-made events such as agriculture, forestry, urbanization, and pollution. These
anthropogenic disturbances have greatly inßuenced most of MichiganÕs landscape and only small pockets of relatively
unaffected land remains (e.g., Isle Royale, some virgin White Pine stands near Grayling and in the Upper Peninsula, and
some secluded peatlands). The Greater Lansing region is no exception to man-made inßuence; itÕs been hard-hit by
lumbering in the mid-1800Õs, then by agriculture, and now by urban sprawl.
Old-Þeld Succession
In this lab, we will examine one form of secondary succession, that of old-Þeld succession, on a small scale. When cultivated land is abandoned there is often a fairly predictable pattern of succession that occurs as it gradually changes back
to a forested ecosystem. Depending on what residual effects remain from plowing and the application of fertilizers and
herbicides, and what seeds are available from nearby habitats, there is some variability in the actual composition and
speed of old-Þeld succession; but there are several general characteristics of this development that are expected. Some
of these are:
a change in plant species composition,
an increase in plant species richness and diversity,
12. an increase in vegetation biomass,
13. a decrease in light reaching the ground level,
14. a decrease in thermal variability at ground level,
10.
11.
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Terrestrial Ecology: Old-Field Succession
15.
16.
an increase in litter (e.g., dead leaves, stems, etc.) on the soil surface, and
an increase in soil organic matter.
Facilitation
Most of these characteristics are common all forms succession, whether it be an old-Þeld, a sand dune, or even stream
cobble being colonized by algae. A common process in many forms of succession is facilitation. With facilitation, the initial colonizers of new or disturbed habitats modify the physical conditions of the habitat by their occupancy, and this modiÞcation helps along the invasion by other species that could not have survived under the initial conditions. For example,
the effects of sun and wind are more extreme than they are under a forest canopy. Certain species, many of them common
weeds, that can tolerate extreme conditions and are Þrst to appear in a recently abandoned Þeld. As this pioneer community grows, it provides shelter from wind and sun for a whole new set of species now able to colonize, and a new community results. The original primary colonizers are less successful competing for resources with the secondary colonizers, so
they die out. However, tertiary colonizers become established as the secondary community growth adds to the soil richness, temperature stability, etc., eventually replacing them, and so on.
As succession progresses, community changes occur more and more gradually until a set of species persists, basically just
replacing themselves, with almost no detectable changes in the habitat. This set is often referred to as the climax community. However, many community ecologist today debate the usefulness of the climax community concept because it makes
unrealistic assumptions as to our knowledge of pre-settlement vegetation, long-term climate stability, and community
dynamics. A careful look at these climax communities shows there are many dynamics still at work.
Objectives:
After completing this exercise, you should be able to:
1.
2.
3.
4.
5.
DeÞne the terms in bold type.
Describe the physical/chemical changes that occur during old-Þeld succession.
Describe the effects and process of facilitation at work in old-Þeld succession.
Calculate density, relative density, dominance, relative dominance, frequency, relative frequency, and importance values of plant species from data obtained using quadrat, line transect, and point-quarter sampling methods.
Use the above values and other qualitative observations to describe changes in vegetation during old-Þeld succession.
Comparing 3 Communities Along a Successional Continuum
We will examine an area south of the Life Sciences Building and adjacent to Baker Woodlot. Here we can Þnd an excellent example of old-Þeld succession. By selecting habitats along successional continuum running from the open Þeld to
the woods, we can demonstrate many of the physical and vegetational changes that occur as this habitat recovers from
clearing and cultivation. The Þeld east of Baker Woodlot was last cultivated more than a decade ago, and has since overgrown with grasses, goldenrod, and many other herbaceous plants. Closer to the woodlot, youÕll Þnd a region of dense
brush and young aspenÑan intermediate zone between the open Þeld and the woods. Baker Woodlot has regrown from a
stand cut, in the 1800Õs, then trees selectively removed periodically until about 20 years ago, when it was declared a natural area and protected. We must be careful to make observations only, no collecting, and inßict no damage to the
woodlot; it is under the UniversityÕs highest level of protection.
22 BS/LBS 158H
Comparing 3 Communities Along a Successional Continuum
Physical and Chemical Measurements
Earlier we made the prediction that as succession progresses, we expect less light to reach the ground level, temperature to
be less extreme at the ground (i.e., generally cooler mid-day temperatures), leaf litter to increase, and an increase in soil
organic matter. At each of the three locations (old-Þeld, intermediate, and woods), we will take temperature and light measurements at ground level and 1.0 m above the ground to test the hypothesis concerning these parameters. We will also
make qualitative assessments of changes in leaf litter and organic matter.
Vegetation Measurements
Quantifying vegetation is a common part of ecological studies, sometimes even when plants arenÕt the primary focus of
the investigation. There are many methods to quantitatively describe the vegetation of habitats, but most of them are variations on three basic sampling types: the quadrat, the line transect, and the point quarter. The method of choice depends
primarily on where and how it is to be used; methods that are well-suited for one habitat may not be a good choice for
another habitat.
Our objective for this exercise is to quantify and compare vegetation at three points along a successional gradient. To do
this, we will use the quadrat method to quantify plant composition in the open Þeld, the line transect in the intermediate
zone, and the point quarter method for trees and shrubs in the woodlot. There are problems inherent in making good comparisons among habitats when different methods are used in each. However, each technique can be used to estimate a few
general parameters and the difference in composition among these zones is great enough that I think the primary trends
will be obvious.
Here are the vegetation parameters we will assess for each habitat. Density is the number of individuals per unit area (e.g.,
the density of golden rod might be 150/m2). It is perhaps the most common parameter estimated for living organisms.To
Number of individuals
Density = -----------------------------------------------------------Area or distance sampled
make density comparisons within and among sites easier, often the relative density is also calculated. Relative density is
Density for a species
Relative Density = ------------------------------------------------------------------ ´ 100
Total density for all species
a unitless measure of the density of each species relative to the densities for all species in a given area; values range from
0Ð100. Because one large oak tree may have a greater inßuence on the character of a habitat than one small grass plant,
dominance is another parameter usually measured. For each species the total coverage or basal area per unit area is its
Total basal area or coverage for a species
Dominance = --------------------------------------------------------------------------------------------------Area or distance sampled
dominance. Relative dominance is simply the coverage or basal area of a particular species relative to that of all species
in that sample; again, its range is 0Ð100. Because some plants are very dense and dominant in one area of a habitat but
Dominance of a species
Relative Dominance = --------------------------------------------------------------------------- ´ 100
Total dominance for all species
completely missing from other areas, frequency is also a common parameter measured. Frequency is simply the number
of plots or transects a in which a particular species is found per number of plots sampled, and a particular speciesÕ relative
Number of plots or transects in which the species occurrs
Frequency = ----------------------------------------------------------------------------------------------------------------------------------------Total number of plots or transects samples
frequency is the ratio its frequency to that of all species in the plots sampled, and ranges from 0Ð100. Finally, the three
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Terrestrial Ecology: Old-Field Succession
relative parameters can be summed and averaged to give the average importance value, which takes all aspects of the
density, coverage, and distribution into account, it can also range from 0Ð100.
Relative density + Relative dominance + Relative frequency
Average Importance = ------------------------------------------------------------------------------------------------------------------------------------------------3
Old-Þeld Species Composition
In order to determine the species composition of the old-Þeld, we need to quantify the plant species present. Rather than
count all of the plants in the old-Þeld, we will sample the population using random quadrat plots, from which we will
estimate the entire composition while only counting a small percentage of the plants in the old-Þeld. We will also measure
percent cover rather than count number of individuals to increase our measuring efÞciency.
Procedure
The following materials will be provided:
1.
2.
3.
4.
5.
6.
7.
8.
a meter stick
a length of string, about 4.5 meters long
four stakes
a random number chart
species identiÞcation materials
a 5% cardboard square
a 25% cardboard square
a 15-m tape measure
Setting up your Þrst quadrat:
Your T.A. will help you Þnd your base point. Once there, choose a random number from the sheet (best done by closing
your eyes and putting your Þnger on the sheet). Each number on the sheet is a three digit number. Each digit tells you how
many meters to move from the starting point. To determine which direction to walk, use the following chart:
TABLE 3. Determining
a random quadrat plot
location from a 3-digit random number.
if the digit is
between:
So, if your random number is 803:
1.
2.
move 8 meters west of where you are
do not move
24 BS/LBS 158H
then move:
1-2
north
3-5
south
6-7
east
8-9
west
Intermediate Area Species Composition
3.
move 3 meters south
Once you are at your randomly determined point in the old-Þeld, measure a square with one meter sides. At each corner,
push a stake into the ground. When all four stakes are set, wrap the string around the four stakes, forming a square.
Next identify the plants within the plot. Use the identiÞcation materials and your instructor to accomplish this. Grasses do
not need to be identiÞed to species, but if you can recognize them as different species, then identify them as Grass Sp. 1,
Grass Sp 2, etc.
Use the cardboard percent squares to estimate the percent cover of each type of plant in the plot. Calculate percent cover
only for plants with their roots inside the plot. Since the plants may overlap each other, the total cover for a plot will often
be more than 100%. Estimate cover by looking at approximately how much of the plot is covered by the foliage of that
plant species. Record on Data Sheet 1 the cover class for each plant in your plot. Cover classes for this study will be
assigned using the following categories:
TABLE 4. Cover
classiÞcation based on percent
cover.
cover class:
range of
percent cover:
1
0-1%
2
1-5
3
5-25
4
25-50
5
50-75
6
75-100
Repeat these steps, beginning with Þnding a new plot with the random number technique, for 20 plots. Be efÞcient! Each
plot should take no more than Þve minutes to quantify. This process will get faster and easier as you proceed.
Calculation of Average Importance Value
For each species found, your group needs to calculate its average importance value. Average importance value we will
use is the average of the relative dominance and relative frequency for each species. Record these values on Data Sheet
the data sheet.
Intermediate Area Species Composition
There is an intermediate vegetation zone between the old-Þeld and the woodlot. In order to determine species composition
of the intermediate zone, we will use a transect line sampling method. And, since counting individual plants would be
time-consuming and difÞcult with species such as grasses, we will use a measurement of dominance rather than number
of individuals. As we quantify the species, we will identify them. We will look at species present along transect lines that
we run through the area.
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Terrestrial Ecology: Old-Field Succession
Procedure
The following materials will be provided:
1.
2.
3.
a 100-m tape measure
a 15-m tape measure
species identiÞcation materials
Setting up the transect line:
Your instructor will bring you to the starting point. From this point, start the Þrst 100-m transect line running east-west
beginning just inside the old-Þeld and ending just inside the woodlot. Measure 10 meters down the line and begin your
Þrst interval at this point. Each interval will consist of a 3-meter section of the transect line. The second transect interval
will begin 10 meters from the end of the Þrst; again, it will extend for 3 meters of the transect line. Data will be recorded
from 7 intervals on each of 3, parallel transect lines. Thus, a total of 21 transect intervals will be sampled.
Within each interval, identify each plant type present and measure its intercept length as you progress along the interval.
The intercept length is simply the length of the transect line, in centimeters, that the plant covers. So, if a patch of goldenrod is under the tape measure for 20 centimeters, its intercept length is 20 centimeters. Plants should be tallied as present
if they are touched by the transect line, underlie it, or overlie it.
From the relative density, relative dominance, and relative frequency, an average importance value needs to be calculated
for each species found. This will give a picture of the species composition of the intermediate area.
Woodlot Species Composition
In order to determine the tree species composition of the woodlot, we will use point-quarter sampling, which will allow us
to generalize to the entire composition while only counting a small percentage of the trees. We will also use the quadrat
method for estimating composition of the understory. As we quantify the species, we will identify them.
Quadrats for Estimating Forest Floor Plants:
See procedure for setting up quadrats in the old-Þeld.
Point-Quarter Methods:
Procedure
The following materials will be provided:
1.
2.
3.
4.
5.
6.
a 50-m tape measure
a 15-m tape measure
two 5-m lengths of string
a wooden stake
a random number chart
species identiÞcation materials
26 BS/LBS 158H
Woodlot Species Composition
Finding a random point-quarter sampling point:
Your instructor will show you the starting point. From this point, you need to randomly Þnd the Þrst sampling point. To do
so, you will use a random number sheet. The woodlot and trees are much larger than plants at the other two sites so the
woodlot group will double the distance traveled between points. Once at the starting point, choose a random number from
the sheet (best done by closing your eyes and putting your Þnger on the sheet). Each 3-digit tells how many meters to
move from the starting point. To determine which direction to walk, use the following chart:
TABLE 5. Determining
a random point-quarter
plot location from a 3-digit random number.
if the digit is
between:
then move:
1-2
north
3-5
south
6-7
east
8-9
west
So, if your random number is 803:
1.
2.
3.
move 16 (8 X 2) meters west of where you are
do not move
move 6 (3 X 2) meters south
Once youÕve determined the point-quarter sampling point, mark it with a stake. Next create four quarters by dividing the
area around the point into four equal-sized quadrants. Do this by stretching out two lengths of string, perpendicular to
each other, with the stake point as their center. The orientation of the strings should be chosen randomly at each point (i.e.,
they should not always lie along the same directions). In each quadrat, the closest tree to the center stake should be identiÞed and its circumference measured. Record these results.
FIGURE 3.
This diagram illustrates the point-quarter technique:
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Terrestrial Ecology: Old-Field Succession
Dotted lines represent the strings creating the quadrat grid. Species, circumference (in cm) at 1.5 m above ground level,
distance between the center stake and the closest plant in each quadrat should be recorded, thus a total of four trees will be
measured for each sampling point. The arrows point to the trees that would be identiÞed and measured in this example.
Data need to be recorded at 20 sampling points; you will end up with information on a total of 80 trees. Use the random
number table and above procedure to Þnd the second sampling point.
There is a round-about way of calculating density from point-quarter data. First, one must calculated the total density for
all species:
2
1
Total density for all species (per m ) = ------------------------------------------------------------------------------------2
( mean point-plant distance (in m) )
The relative density of a particular species is simply:
Number individuals counted of a particular species
Relative density = -------------------------------------------------------------------------------------------------------------------------- ´ 100
Total number of individuals counted
The density of a particular species can then be calculated from total density and relative density values as:
Relative Density
2
Density (per m ) = ---------------------------------------- ´ Total Density
100
Dominance values for trees are based on their basal areas. The basal area for an individual tree can be calculated from its
circumference using the following equation:
2
2
Circumference (in m)
Individual basal area (per m ) = -----------------------------------------------------4P
And dominance for a particular species can be calculated as:
Dominance = Density ´ Average basal area for that species
Now determine the average importance value for each species based on its relative density, relative dominance, and relative frequency.
Acknowledgments
Parts of this exercise were written by Alyssa Shearer. Some sections relied heavily on inputs from Dr. Howard Hagerman
and Dr. Donald Beaver.
28 BS/LBS 158H
References
References
Brower, J. E. and J. H. Zar. 1977. Field and laboratory methods for general ecology. W.C. Brown Co. Pub., Dubuque, IA,
194 pp.
Cox, G. W. 1976. Laboratory manual of general ecology. Wm. C. Brown Co. Pub., Dubuque, IA, 232 pp.
Exercises:
1.
2.
3.
4.
5.
Groups will be assigned the following tasks: measuring light at ground level and 1.5 m high for each area, measuring
temperature at -0.05, 0, and 1.5 m for each area, making qualitative assessments of leaf litter and soil organic matter in
each area, old-Þeld vegetation measurements, intermediate zone vegetation measurements, and woodlot vegetation
measurements.
Follow the procedures outlined in this manual for completing these tasks.
Collect data and record them in the tables provided.
Calculate average importance values for the plant species found in each successional zone.
Answer the following questions:
¥ What is the pattern observed for changes temperature and light with respect to succession?
¥ What evidence, if any, was observed that shows facilitation might be an important process in this example of old-Þeld
succession?
¥ How does the vegetation change with respect to successional stage (e.g, number of vertical layers, species composition, life histories, etc.)?
¥ A list of the ten plant types, in each zone, with the highest importance values. How does average importance change as
succession progresses?
¥ Which species seem to be conÞned to a particular successional zone and which seem to be more generalists?
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Terrestrial Ecology: Old-Field Succession
Old-Þeld Data
Light:
Temperature:
-0.05 m____
Soil Litter:
0.0 m_____
1.5 m____
0.0 m_____
1.5 m____
Soil Organic Matter:
TABLE 6. Dominance
values for species encountered in old-Þeld quadrats.
Quadrat Number
Species
30 BS/LBS 158H
Old-Þeld Data
Summary Sheet for Old-Field
TABLE 7. Summary
Species
worksheet for old-Þeld species.
Density (
-2)
Relative
Density
Dominance
(
)
Relative
Dominance
Frequency
Relative
Frequency
Average
Importance
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Terrestrial Ecology: Old-Field Succession
Intermediate Data
Light:
Temperature:
-0.05 m____
0.0 m_____
1.5 m____
0.0 m_____
1.5 m____
Soil Litter:
Soil Organic Matter:
Total transect length:
Total number of transect intervals:
TABLE 8. Intercept
lengths (cm) for species encountered along transect intervals in the
intermediate zone.
Intercept inverval
Species
32 BS/LBS 158H
1
2
3
4
5
6
7
Total for
species
Intermediate Data
Summary Sheet for Intermediate Zone
TABLE 9. Summary
Species
worksheet for intermediate zone species.
Density (
-2)
Relative
Density
Dominance
(
)
Relative
Dominance
Frequency
Relative
Frequency
Average
Importance
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Terrestrial Ecology: Old-Field Succession
Woodlot Data
Light:
Temperature:
-0.05 m____
0.0 m_____
1.5 m____
0.0 m_____
1.5 m____
Soil Litter:
Soil Organic Matter:
Number of points sampled:
TABLE 10. Circumference,
basal area, and point-to-plant distances for tree species
encountered in woodlot point-quarter plots.
Point
Number
Quadrant
Number
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
34 BS/LBS 158H
Species
Circumference
(m)
Basal Area
(m2)
Point-to-plant
Distance (m)
Woodlot Data
Summary Sheet for Woodlot
TABLE 11. Summary
Species
worksheet for woodlot species.
Density (
-2)
Relative
Density
Dominance
(
)
Relative
Dominance
Frequency
Relative
Frequency
Average
Importance
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Terrestrial Ecology: Old-Field Succession
36 BS/LBS 158H