REVISED VERSION (10/25/04)
Ecosystem Processes: Leaf Decomposition
Summary
Leaf breakdown studies allow ecologists to measure decomposition rates in different
environments. We will use the litterbag technique and a 2-way ANOVA study design to
compare leaf breakdown in aquatic and terrestrial ecosystems, each with forested and
urbanized land uses. We expect to see differences in breakdown rates between aquatic
and terrestrial ecosystems because they have different moisture and nutrient regimes, and
we expect to see differences in land use type because of the different influences of human
modifications on physical processes. We will also examine differences at these sites in
species richness and abundance of macroinvertebrates which play an important role in
decomposition.
Required reading:
Textbook: Chapter 9
Day, F. 1982. Litter decomposition rates in the seasonally flooded Great Dismal Swamp.
Ecology 63 (3):670-678.
Barajas-Guzman, G., and J. Alvarez-Sanchez. 2003. The relationships between litter
fauna and rates of litter decomposition in a tropical rainforest. Applied Soil Ecology
24:91-100.
Mathuriau, C., and E. Chauvet. 2002. Breakdown of leaf litter in a neotropical stream.
Journal of the North American Benthological Society 21(3): 384-396.
Introduction
What is leaf breakdown?
Leaf breakdown is the combined result of physical breakage of leaves, leaching of
dissolved components, microbial decomposition, and animal consumption. For example,
in a temperate (i.e., mid-latitude) stream setting, trees shed their leaves in the fall and
these leaves enter the stream. The breakdown process begins with the leaching of
dissolved nutrients from the leaves and colonization of the leaves by microbes and fungi.
Fungi physically penetrate cellulose with hyphae and secrete exoenzymes to degrade
organic matter while microbes metabolize simple monomers and polymers that leach
from the leaves. As microbes colonize and process leaves, they become “conditioned,”
and stream insects begin to consume them. Leaves conditioned with a film of microbes
and fungi have been likened to “peanut butter on a cracker” by a prominent stream
ecologist, an analogy which highlights the nutritional importance of the leaf colonists
rather than the leaves themselves. In terrestrial settings the process is the same, but due
to environmental differences, leaf breakdown occurs on longer time scales than in aquatic
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systems. Because breakdown occurs much more slowly in terrestrial ecosystems, the
breakdown and decomposition products are more easily grouped into categories. Dead
plant material is referred to as litter when its original identity can still be distinguished.
As further decomposition degrades litter into an unrecognizable form, it becomes soil
organic matter. Fungi and microbes further degrade the easily metabolized components
leaving behind humus, which is composed of chemically complex organic matter that
resists decomposition. Many factors affect leaf breakdown in terrestrial and aquatic
ecosystems, including temperature, moisture, nutrients, organism type and abundance,
and the nature of the material itself.
Why is leaf breakdown important?
In temperate ecosystems, leaves are part of a major pathway of energy flow and nutrient
cycling in forest and stream ecosystems. Nearly 100% of life on earth requires energy
from carbon fixed by photosynthesis, so leaf breakdown represents a key step in the
carbon cycle. Photosynthesis uses energy from sunlight to fix gaseous carbon (CO2) into
carbohydrates (C6H12O6). This process stores the sunlight’s energy and sets up a redox
(oxidation-reduction) gradient whereby organisms convert the fixed carbon back into
CO2, releasing the stored energy to sustain metabolic processes. In terrestrial
ecosystems, organic matter fixed by primary producers fuels the ecosystem by providing
food for decomposers and consumers, which in turn provide food for predators.
Ecologists differentiate between carbon fixed within an ecosystem (autochthonous) and
carbon fixed outside an ecosystem that enters the ecosystem (allochthonous). This
concept is particularly important in headwater streams, many of which receive nearly all
their energy input from allochthonous energy sources, highlighting the importance of
streamside vegetation to the energy budget of those streams.
Objectives
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You will learn techniques and formulae used in assessing leaf breakdown rates.
You will investigate leaf breakdown differences between terrestrial and aquatic
ecosystems.
You will investigate how land use affects leaf breakdown rates.
You will learn how to identify terrestrial and aquatic macroinvertebrates involved
in the decomposition and breakdown of leaves.
Methods
Your TA has created leaf bags and placed them in two different ecosystems, each of
which has two different land use types. At two points during the semester, some of the
bags were retrieved and weighed to calculate mass lost over time. You will participate in
the third (and last) leaf bag retrieval. After collection, we will weigh the decomposed
material and pick macroinvertebrates from the leaf packs so that we can assess
differences in invertebrate species richness and abundance between the ecosystems and
land use types.
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Making leaf packs and placement (FOR YOUR INFORMATION; PACKS WERE
MADE AND PLACED BY THE TA)
1. Fill 64 mesh bags with 10g (±0.1g) each of dry tulip poplar leaves. To prevent
leaf breakage, mist the leaves with de-ionized water before placing in the mesh
bags but after weighing the 10g.
2. Your TA will place the 4 groups of bags at the beginning of the semester: 2
groups in aquatic habitats of Juday Creek and 2 groups in terrestrial habitats near
the creek. Of the 2 groups place in the creek, one will be in a forested reach and
one in an urbanized reach of Juday Creek. Of the 2 groups placed in a terrestrial
setting, one will be in the riparian zone of the forested reach of Juday Creek and
one will be in the urbanized reach of Juday Creek (see figure below). Fifteen
bags will be placed at each site and one bag from each site will be brought back to
account for loss of leaf mass due to handling and not decomposition. We will use
the values from these four bags as the initial mass for the experiment.
Experimental design
(#'s represent leaf pack bags retrieved)
Forest
pick-up
Terrestrial
day 0
day 28
day 56
day 84
total
1
3
3
9
16
Urban
Aquatic
Terrestrial
1
1
3
3
3
3
9
9
16
16
Grand total=64
Aquatic
1
3
3
9
16
Retrieval
1. After 4 and 8 weeks your TA retrieved 3 of the leaf packs from each site and
weighed the material. After 12 weeks, each lab will retrieve the 3 remaining leaf
packs at each site, placing each into a labeled Ziploc bag and returning them to
the lab for processing.
2. While at the different sites, be sure to note differences you observe about
the two ecosystems (terrestrial vs. aquatic) and the two land use types
(forested and urban).
Processing
1. In the lab, process the aquatic and terrestrial leaf packs. Follow the directions
below:
a. Aquatic: gently rinse each aquatic leaf pack of silt and debris over a
250m sieve. Find any macroinvertebrates rinsed onto the sieve as well
as attached to the leaves and put them into a labeled jar of ethanol to save
for next week. The label should have the lab day, the ecosystem type,
the land use type, and the replicate number. Place the processed leaf
packs in a paper bag labeled the same way and hang on the outstretched
line in the lab to air dry.
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b. Terrestrial: Place each leaf pack in a Berlese funnel for 24 hours. The
heat from the lamp will to force the insects out of the leaves and into a
labeled jar of ethanol. The next lab day will cap the ethanol jars and
finish processing the leaves by placing the processed leaf packs in a
labeled paper bag and hanging on the outstretched line in the lab to air
dry. They will do this prior to adding the leaf packs they retrieved to the
now empty Berlese funnels. The TA’s will finish the processing for
Thursday’s lab. The jars of insects will be saved for next week.
2. On the following week:
a. Identify any macroinvertebrates found on the leaf packs using a
dichotomous key provided by the TA. Record the data on data sheets
provided in class.
b. Weigh all the dried leaves using the top-loading balance to get their dry
mass (DM). Record all data on the data sheet, and turn in these data to
the TA.
Calculations
1. For this lab you will need to do 4 calculations for each of the collection periods:
mean % dry mass (DM) remaining, leaf breakdown rate (k), taxonomic richness,
and taxonomic abundance.
a. %Mr: 1- [(M0-Mt)/(M0)]*100
 %Mr= percent mass remaining
 M0= initial DM, the mean DM from the handling loss leaf packs.
 Mt= final DM, the mean DM from each collection date.
 You will need the mean %DM remaining for each collection date!
b. To calculate the breakdown rate, regress the natural log (ln) of percentage
of DM remaining (y-axis) on days of exposure (x-axis) using the DM of
the handling-loss leaf packs as 100% remaining for Day 0. The negative
slope of the regression line is equal to the processing coefficient (k).
c. Taxonomic abundance= # of each macroinvertebrate found in ea. leaf bag
d. Taxonomic richness= # of macroinvertebrate species found in ea. leaf bag
Statistics
This lab was designed to determine if land use and ecosystem type affects leaf
breakdown rates. To statistically analyze the data, we will use a two-way ANOVA. This
statistic tests for differences between two different levels (land use) of two different
treatments (ecosystem type) on some measured variable, in this case mass loss. For this
lab, calculate a two-way ANOVA on mass loss of the leaf packs from the last collection
day. Your TA will calculate the degrees of freedom (df) and the sum of squares (SS).
You will be required to calculate the mean square (MS), the F-ratio, and the pvalue.
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LAB REPORT
WHAT TO TURN IN
Data Analysis
Tables:
 Table 1: Raw Leaf Breakdown, dry weights, percentage mass remaining,
average percent mass remaining.
 Table 2: Grams leaf mass lost: data arranged for Two-Way ANOVA.
 Table 3: Taxon names and abundance of invertebrates per bag;
Average taxonomic richness and average abundance.
Graphs (see examples on pg. 7):
 Linear Regressions:
Figure 1: Ln % mass remaining in Urban Terrestrial vs. days in stream
Figure 2: Ln % mass remaining in Urban Aquatic vs. days in stream
Figure 3: Ln % mass remaining in Forested Terrestrial vs. days in stream
Figure 4: Ln % mass remaining in Forested Aquatic vs. days in stream
 Figure 5: Line graph comparing mean percent mass remaining among
treatments over time. Include SE bars.
 Figure 6: Bar graph comparing average invertebrate species richness and
average abundance at each site.
Results
Answer the questions in one or two sentences referencing figures when appropriate.
Just state the facts. Inferences will be made in the discussion section.
1. Which habitat type had the fastest breakdown rates? What were all breakdown
rates for each habitat? Was there a significant difference between the habitats?
2. Did percent mass loss vary over time or were leaves losing a consistent amount of
mass over time?
3. Did invertebrate abundance and species richness differ among sites? Which sites
had the highest and which sites had the lowest?
4. Concisely describe the differences you observed between the forested and urban
ecosystems.
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Discussion
Use the following five questions to guide your discussion of the results of our
experiment.
1. Why do breakdown rates in terrestrial and aquatic habitats differ from one
another? In terms of the data you collected, what were the major differences
between the two habitats?
2. How might differences in breakdown rates in urban and forested streams affect
the energetics of the ecosystems?
3. How might differences in leaf species diversity and macroinvertebrate species
diversity influence breakdown rates?
4. Predict the consequence of planting one fast breakdown leaf species in a
terrestrial forest and aquatic forested site. Give an example where this may have
been done.
5. Compare the breakdown rates and macroinvertebrate species abundance/richness
found in our experiment to those found in either Day et al. (1982) or Mathuria
and Chauvet (2002) and Barajas-Guzman and Alvarez-Sanchez (2003).
Discuss the reasons why there are differences between these ecosystems.
Speculate on the limiting factors for decomposition in these systems.
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Example graphs:
Forested Stream
Ln % Mass Remaining
5.00
y = -0.0248x + 3.9729
R2 = 0.8594
4.00
3.00
2.00
1.00
0.00
0
10
20
30
40
50
60
Days in Stream
Figure 1. Linearized regression of percent leaf mass remaining versus days spent in the
stream.
% mass remaining +/- SE
Leaf mass loss over time
120
100
Aquatic forrested
80
terrestrial forrested
60
Aquatic urban
40
Terrestrial urban
20
0
0
50
100
Days in stream
Figure 5: Line graph comparing percent mass remaining among treatments over time.
Species richness and abundance at different sites
Mean number +/- SE
140
120
100
80
Species Richness
60
Species Abundance
40
20
0
Aquatic
forested
Terrestrial
forested
Aquatic
urban
Terrestrial
urban
Site
Figure 6. Bar graph comparing mean species richness and mean species diversity at the
four different sites.
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