Forest Succession

Forest ecosystems are always changing. Plants grow using soil nutrients and eventually die, returning nutrients to the soil. Animals feed on plants and leave waste. Bacteria, fungi and insects thrive on decaying plants, animals and animal waste, breaking down these materials and replacing soil nutrients. These interactions of plants, animals, bacteria, fungi and insects constantly occur in ecosystems.

Succession is a change in plants and animals which occurs periodically in all communities. An open space or meadow will eventually be overgrown by a forest which in turn will grow to a climax forest. The length of time and kinds of plants involved in each successional change depends on many factors. The successional progression can be changed at any stage by many different factors and disturbances. These factors and disturbances have negative and positive effects on succession. Disturbances in the forest can be human made or natural.

Succession Stages A common natural disturbance which affects succession is fire started by lightning. This has occurred throughout time and is a natural part of many forest ecosystems. A common human disturbance which affects succession is farming. Whether plowing fields or grazing livestock, farms provide an on-going disturbance, preventing forest succession. Whether natural or human-made, removal of the disturbance allows succession to proceed. Regardless of the existing successional stage, forests provide essential elements of habitat: food, water, shelter and space. In any ecosystem the diversity of plants and their spatial structures influence the diversity of animals utilizing available habitat. The plants and animals in an ecosystem change with each successional stage. Distinct stages of succession are:

1. Primary Succession: 0-5 years (Forbs stage) Certain "pioneer" plants germinate quickly on exposed soil including mosses, lichens and small herbaceous plants. Shrub and tree seedlings begin to establish in the soil. Insects and small rodents feed on grasses, herbaceous plants and seeds. Songbirds feed on insects and seeds. Predatory birds feed on small rodents.

2. Primary to Secondary Succession: 6-25 years (Shrub stage) Tree seedlings are established and larger shrubs shade out many of the herbaceous plants. Shrubs and fallen trees provide abundant nesting cover for birds, rodents and small mammals. The smaller animals attract larger predators, such as coyotes, weasels, and bobcats. Deer feed on shrubs and saplings. Larger predators are also attracted, such as cougars and bears.

3. Secondary Succession: 26-50 years (Young forest) The forest canopy begins to form with deciduous trees being the dominant species. The diversity and quantity of shrubs and herbaceous plants decreases. Young evergreens grow up among the shaded forest floor. A reduction of the kinds of food available to some wildlife species [especially large mammals] reduces the number of wildlife species. New species enter the area and take advantage of the resources. Beavers use young trees for food and lodges. 4. Secondary Succession to Climax: 51-150 years (Mature forest) A few large evergreen species dominate the ecosystem. Large deciduous trees die and fall leaving openings in the forest canopy. Shrubs and herbaceous plants take advantage of the sunlight and attract small birds and mammals, creating a diverse ecosystem.

5. Climax forest 150-300 years Large evergreens, which are able to grow beneath them selves in their own shade, grow in height and diameter. Fewer big trees take up more area so there are fewer trees per acre. Dead trees, called snags, provide nesting sites for woodpeckers and other animals. Particular kinds of animals move in utilizing available resources. Fallen trees add to the diversity of habitats and provide additional resources while adding soil nutrients. A climax forest could also be made up of large deciduous trees such as maple or oak.

Examples of disturbances which effect succession:

Bio-physical Factors Natural Disturbances Human-Made Disturbances

climate volcanic activity agriculture

water sources insects tree harvesting

weather fire road building

topography wind storms soil erosion

soil composition flood introduced species

wildlife soil erosion prescribed fire Questions: Please answer the following questions on a separate sheet of paper.

1. Refer back to the sketches you did of the three landscapes we visited this morning. Using the above reading as a guide, identify the stage of succession for landscape #1 and landscape # 2.

2. Identify the most probable disturbance that occurred in the past 20 years for landscape #1 and landscape #2.

3. Sketch a possible food chain for landscape # 1 and landscape # 2. Make sure you include a producer and at least 2 consumers.

4. Based on what you have learned in this reading, make some predictions about what would happen to New York City over a 100 year time period if humans suddenly disappeared from the planet.