Ecosystem Processes: Leaf Decomposition



Ecosystem Processes: Leaf Decomposition

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.

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 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.

• 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.

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.

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 |Urban |

|pick-up |Terrestrial |Aquatic |Terrestrial |Aquatic |

|day 0 |1 |1 |1 |1 |

|day 28 |3 |3 |3 |3 |

|day 56 |3 |3 |3 |3 |

|day 84 |9 |9 |9 |9 |

|total |16 |16 |16 |16 |

| | |Grand total=64 | |

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 250μm 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.

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 p-value.

LAB REPORT

WHAT TO TURN IN

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.

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.

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.

Example graphs:

[pic]

Figure 1. Linearized regression of percent leaf mass remaining versus days spent in the stream.

[pic]

Figure 5: Line graph comparing percent mass remaining among treatments over time.

[pic]

Figure 6. Bar graph comparing mean species richness and mean species diversity at the four different sites.

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Summary

Introduction

Methods

Objectives

Discussion

Results

Data Analysis

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