Direct and Indirect Effects of Fire on Shrub Density and Aboveground Productivity in a Mesic Grassla

1
Direct and Indirect Effects of Fire on Shrub
Density and Aboveground Productivity in a Mesic
Grassland

• J. L. Heisler, J. M. Briggs,
• K. Knapp, J. M. Blair
• and A. Seery

2
The Study

• Cornus drummondii
• Konza Prairie Biological Station in Kansas
• A 3,487 hectare native C4 grassland located
  within the Flint Hills (largest continuous
  expanse of unplowed tall grass prairie in North
  America)

3
Objectives of the Study

• To quantify the importance of resource limitation
  and above ground survival/mortality as a
  consequence of fire that may affect the balance
  between grass and shrub dominance.
• They manipulated fire, light (using litter), and
  N availability and measured changes in stem
  density, above ground persistence/mortality and
  ANPP.

4
Methods

• Protected islands (control shrub plots)
• Protected nitrogen islands
• Burned islands
• Burned nitrogen islands
• Burned litter islands
• Grassland plots (burned, nitrogen, control)

5
Results

• Fire significantly increased stem density
• Post-fire resprouting resulted in 600 increase
  in stem density compared to a 200 increase in
  the protected shrub islands
• Pre-existing stems stemleaf ratio 1.1
• New stems stemleaf ratio 3.7
• In burn litter islands temperature and light
  penetration was reduced, but stem density
  increased by 400
• N addition had no effect on total ANPP, new stem
  density or productivity

6
Results

• Post-fire total productivity was greater in
  grassland (523.3 g/m²) than the shrub islands
  (449.5 g/m²)
• Grass ANPP in protected shrub islands was reduced
  by 50 compared to the burned grassland
• Pre-existing stems stem leaf ratio 1.1
• New stems stem leaf ratio 3.7

7
Results

• Effect of litter stems produced via basal
  resprouting grow rapidly in order to emerge from
  light constraints of litter layer
• Fire resets C. drummondiis growth course rather
  than reducing its abundance

8
Results

• Fire suppression restricts light availability to
  grasses (litter accumulation) emerging from soil
• Grass ANPP is significantly reduced as a
  consequence of fire exclusion and shrub
  establishment
• A condition that is not reversed when fire is
  restored to the system

9
Results

• Two annual fires are not effective in reducing
  the abundance of C. drummondii
• Immediate effects of fire reduce productivity of
  the shrub and stimulate grass production a
  dramatic post-fire increase in stem density
  allows the species to persist and take advantage
  of fire-free intervals.

10
Conclusion

• The study indicates that in just a few years C.
  drummondii can establish and become a persistent
  component of the plant community within mesic
  grasslands...and that periodic fire acts to
  facilitate increases in shrub stem density and
  the displacement of C4 grasses in the system.

11
(No Transcript)