Winona State University
BIOL 415/527 – ECOLOGY OF LARGE RIVERS

LAB EXERCISE #1

LEAF DECOMPOSITION

OBJECTIVE

Organic matter from different sources often decomposes at different rates, and those rates also can differ among river systems. This exercise will compare rates of decomposition of leaves from different plans, and in different river/stream systems.

HYPOTHESIS

Leaves from native species will decompose faster than those from non-native plants, and decomposition will be more rapid in the Mississippi River than in a tributary coldwater stream.

METHODOLOGY

Leaf decomposition will be compared using four species and two lotic systems. The following describes the general procedure we will follow to examine the hypothesis listed above.

Experimental set-up:

Leaves will be collected from native silver maple, eastern cottonwood, and American lotus, and from non-native European buckthorn.

1) Dry leaves for each species, weigh out 4 g (dry weight) of one species, and place into small mesh bags.

2) Prepare three such bags for each of the four species, and place all 12 bags into a large mesh bag to create a sampling unit.

3) Create eight of these identical large mesh bags (eight sampling units) containing the 12 small mesh bags with leaves.

4) Suspend four of these leaf bags (4 sampling units) in a Mississippi River secondary channel and the remaining four in a pool in Pleasant Valley Creek.

Collections and laboratory processing:

1) One week after placing leaf bags into the river and stream, retrieve one large mesh bag (one sampling unit) from each location. Remove small mesh bags (DO NOT REMOVE LEAVES FROM THE SMALL MESH BAGS!) and place into pans, then place in the drying ovens under low heat for 24 hours.

2) After 24 hours, remove leaves from small mesh bags, weigh the leaves from each bag separately, and record weights on the data sheet. You will have triplicate weights for each of four species (12 total weights) from each location (river and stream).

3) Repeat bag retrievals, drying, and weighing after 2 weeks, 4 weeks, and 8 weeks.

ANALYSIS

1) Use data from all weeks and both sites for your analyses.

2) Calculate means and SDs for the triplicate samples for each species/date/site combination (4 species/4 dates/2 sites = 32 means/SDs).

3) Plot on the same graph the change in mean/SD dry leaf weights through time for the four species. Be sure to include the starting weights (4 g) for all species on week 0. Create one graph for the Mississippi River samples, and a separate graph for the Pleasant Valley Creek samples.

4) Use a two-factor ANOVA (species and time) with replication to determine if rates of decomposition differed among the species examined. Do separate tests for each site (perform two ANOVAs).

5) Use a two-factor ANOVA (site and time) with replication to determine if rates of decomposition differed between the sites examined. Do separate tests for each species (perform four ANOVAs).

6) Did the native species decompose faster than the non-native species? Did leaves in the river decompose faster than those in the coldwater stream?

EQUIPMENT

Large and small mesh bags
Zip-ties
Top-loading balances
Drying ovens

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Neal D. Mundahl
Winona State University
Winona, MN 55987-5838