Unit D: Changes in Living Systems

Lesson 2.2: Secondary Succession


Have you seen the charred landscape left after a forest fire? Even after such devastation, it is remarkable how ecosystems recover. Succession is responsible for this recovery.

When soil is present when succession begins, it is called secondary succession.  It occurs much more quickly than primary succession. Natural and man-made disturbances start secondary succession.
A natural disturbance is wildfire.  Prairie and forest organisms are adapted to survive the natural cycle of fire.  You know that most of the biomass of prairie grasses is below the surface, so secondary succession begins very quickly after wildfire.  In fact, some species of pine are adapted to take advantage of wildfire to germinate their seeds.


Figure 1:  Weeds between patio stones

Figure 2:  Harvesting timber by BLMOregon

Figure 3:  Prescribed burn

Examples of man made all around you.  Figure 1 shows weeds growing out from between patio stones. Harvesting forest is a disturbance that begins the cycle of succession.  These disturbed areas are unsightly, but important to the economy.  Underneath the mess lay millions of plant seeds ready to germinate in the sun.
Forest fires are both natural and man made disturbances.  Wildfire is a natural part of ecology in Canada.  Large fires can begin a secondary succession of plant species that produce a monoculture.  This lack of diversity is a problem.   Another issue is that conditions will be right for another large forest fire  if a large area of forest reaches maturity at the same time.
Logging forests (Figure 2) is one way to deal with this issue.  Cutting small blocks of forest stores carbon, and breaks an even aged stand of trees into a more diverse habitat for plants and animals.  Uneven aged stands of trees are less likely to all burn at the same time, so that wildfires will not be so big..
Small, controlled fires called prescribed burns (Figure 3) will have the same effect as logging (except the carbon is released).   First Nations people used their knowledge of secondary succession to better manage their environment.  Dene and Cree people used prescribed burns to encourage the growth of berries and medicinal plants.  Blackfoot used them to prevent prairie fires and to ensure good pasture for bison and horses.  



Figure 4:  First Nations Burning Practices by Todd Kirstensen  (Statistics and spacial data provided by Alberta Agriculture and Forestry Wildfire Website) 



  • Read pages 476 to 477 of the textbook, ending at the investigation. 

  • Read pages 478 and 479 of the textbook, ending at the “Forest Harvesting” activity.  You may want to spend a little time answering questions 2 - 3. 

     

    Science 20 Textbook.  Alberta Education


     

Many trees are destroyed during a forest fire, and it takes some time for the forest to return to maturity. When the forest industry removes trees from a forest, reforestation is legally required. Reforestation with seedlings from a nursery returns the forest to maturity in less time than natural processes. Yet the recovery of the forest depends very much on the method by which the forest was harvested.



  • Read “2.2 Summary” on page 481 of the textbook. Then, complete questions 1 - 4, 6. 

     

    Science 20 Textbook.  Alberta Education


     

Go to Assignment 2.2: Secondary Succession.