Succession

Introduction

Succession is the replacement of one kind of community by another. It is a progression of community change that leads to a more stable (climax) state.

Characteristics of early colonizers

Species that produce many seeds and have high dispersal capabilities are more likely to become established in an area where a disturbance has removed part of an already-established community.  Examples of such areas would be newly formed sandbars, disturbance by tornado or fire, and abandoned agricultural fields.

Species with high dispersal rates and many seeds produce small seeds and small plants. These plants are small because they put a relatively large amount of their energy into seeds and a small amount into plant size (biomass). The seeds produced by these plants should have a long life expectancy because it may be years before conditions are adequate for their germination.

In order to produce large amounts of seeds quickly, early colonizing species must have rapid growth capability. A low root:shoot ratio enables them to take advantage of the sunlight while it is available. Plants that take advantage of bright sunlight normally have a low degree of shade tolerance.

As these shade-intolerant species grow, it becomes shady underneath them.  It is also more moist; there is less wind etc. More shade tolerant species will begin to grow in the area due to the shade. To be better competitors, shade-tolerant species need to have larger seeds so that they have sufficient food reserves for growth while they are trying to reach the light.

The Climax

Succession continues until change of species no longer modifies the environment significantly. Conditions change more slowly as the size of the organisms increase. The final size (biomass) of a climax community is limited by the climate.

Example: old-field succession (east, Midwest, and southern U.S.)

Initial Condition	bare fields
1st year	annual plants (crabgrass, horseweed, ragweed, seed-eating animals
2nd year	annuals decline, more perennials
3rd year	perennials and shrubs dominate; woody species appear during the first few years
10 years	woody species appear during the first few years but become more important; aspen and elm in north, pine in the south
20 - 40 years	young forest

Primary succession

Primary succession refers to the establishment and development of plant communities in newly-created habitats or habitats that were previously without plants.

Secondary succession

Secondary succession occurs in habitats where the community has been disturbed or removed.

Time required to produce a climax community (secondary succession)

North Carolina Old-field ® mature oak-hickory forest - 150 years

New England old field ® climax - 150 to 300 years but major disturbances (fire or hurricanes) occur every 70 years

Western grasslands succession - 20 to 40 years

Michigan sand dunes ® Beech-Maple forest - up to 1000 years

Humid tropics - regain most climax species within 100 years if soil is not severely altered; truly mature forest requires many centuries

Laboratory Exercise

Objectives

Students will develop an understanding of the dynamic nature of communities. Communities change.

Students will develop an understanding of the characteristics of early Vs late-successional communities.  

Materials Needed

Thermometer

Several balloons - These will be used to measure wind speed.

Tape measure

Procedure

In this exercise, you will need to observe two or three communities in different stages of succession. Choose one of the options described below for your observations.

Choose one option:

Option 1: Grassland to Forest Succession

You will need to find two areas for observation: a forested area and a nearby area that is not forested. An abandoned field is ideal for the nonforested area. Try to find areas that have not been used for agriculture or disturbed by humans for several years. If possible, try to find third area that is a transition (intermediate) between the field and the forest. These three areas will be at different stages of succession; the forest is the latest stage of the three and the field is the earliest.

Option 2: Lake Shore to Forest (or Grassland) Succession

Select an area along a lake where there is little vegetation near the shore but the vegetation becomes forest a short distance (<200m) from the shore. The area immediately adjacent to the water will have little vegetation due to disturbance by waves and fluctuating water levels.  Try to find an area that has not been disturbed by humans for several years. You will sample the shore area and the forest area for the exercises below. If possible, try to find third area that is a transition (intermediate) between the shore and the forest. The shore area is at a different stage of succession than the forest. Early successional species will dominate the shore and later successional species will be found in the forest.

Data Collection

For each of the two or three areas that you select, record the following in table 1.

1.   Measure the air temperature 1 m above the ground.

2.   Measure the soil temperature at the surface of the ground and also 25 cm below the surface.

3.   Measure the depth of the litter layer, humus layer, and mineral layer. Litter consists of leaves, needles, dead grass, twigs, etc. The humus layer is underneath the litter. It consists of organic material (leaves etc) which has decomposed so that the material is no longer identifiable. The mineral layer is the remainder of the soil underneath the humus.

4.   Measure the wind speed. If you do not have a device for measuring wind speed, a balloon, stopwatch, and tape measure can be used. Hold a balloon at shoulder height above the ground and drop it. At the moment that you release it, start your stopwatch. Stop your watch when the balloon touches the ground. Measure the horizontal distance (along the ground) from the point directly below where it was released to the point where it touched the ground as shown below. 


Wind speed is equal to this distance divided by the time. Convert this measurement to kilometers per hour. 

Example: Suppose that you drop the balloon and it strikes the ground 2.5 seconds later. During that time it moved 1.7 meters. First, divide the distance by the time. 

1.7 meters / 2.5 seconds = 0.68 meters per second

The above calculation is wind speed in meters per second. The next step is to convert this to kilometers per hour.  There are 1000 meters in a kilometer and there are 3600 seconds in one hour. To convert this answer to km/hr, multiply your answer above by 3600 and then divide it by 1000.  

0.68 X 3600 = 2448 meters per hour

2448 / 1000 = 2.448 kilometers per hour

5.   Estimate the number of different plant species in a 2 m X 2 m area.

6.   Count the number of different kinds of insects and other arthropods that you find in the 2 m X 2 m area.

7.   Record the average height of the dominant vegetation.  

Lab Report

Turn in table 1 and your answers the questions below.

Questions

1.   Describe the vegetation in each of the two or three areas that you sampled. (moss, herb, shrub, tree, etc.)  

2.   What characteristics would you expect to see in species typical of each area?  

3..      Did the vegetation affect the depth of humus and litter? Explain.

4.      Did the vegetation affect soil temperature, air temperature, or wind speed? If so, which area had the greatest influence?  

Table 1

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