1. 4/19/06
Historical Range of Variability, Part 2.
The role of fires in grasslands.

2. disturbances
Climate,
topography,
parent material
Biotic
interactions
with abiotic
conditions (changes in physical and
chemical environment for biota)
Old
community
“new”
biotic
community
regional pool of biota
including native &
non-native species

3. Fire return intervals The most fire- prone ecosystem in North America
Aber and Melillo 1991)
The most fire-
prone ecosystem
in North America
was in the semi-
humid zone!!!

4. The grasslands
WERE the
largest biome
type in
the lower 48.

5. (determines fire return interval (see next graph)
Probability of Fire:
Fuel load * Moisture
(determines fire return interval
(see next graph)
Fuel Load
fuel
Zero
fuel
Low precip high precip.

6. wet
Wet, low fuel
(a lake!)
wet, high fuel…
The asbestos forests
Moisture
index
Highest
Fire return
intervals
Any ecosystems
Over here don’t
Last long!
Dry, low fuel
(desert)
dry
Zero lots
biomass Fuel Index
(Note that very dry systems rarely have large biomass!)

9. Frequent fire:
1. Loss of N.
2. Increase
photosynth.
3. Increase C
storage in soil.
4. Immobilize
N in soil

10. Burning removes
canopy, volatilizes
N, warms soil

11. Fire
1) attracts grazers to area (a fact known to indigenous peoples)
2) Sends large amounts of N up in smoke…makes N less available.
3) In spite of reduced N availability, stimulation of Ps
Grasses generally results in higher NPP.
Fire only, or grazer only systems “aren’t natural”

12. Grazing:
1) lowers probability of fire (lower fuel load)
2) lowers intensity, amount of fuel loss of fire
3) Removes the canopy to enhance light on the surface
4) deposits N onto soil rather than into atmosphere (makes
N more available to plants)
5)neutral/reduces C input to soil!

13. In the absence of fire, all North American grasslands
would likely become shrublands or forest.
The rate at which these become “woody” systems is
a function of precipitation AND the availability of
woody seeds.

14. Long-term Dynamics Keeping the tallgrass prairie within its historical
range of variability maintains native cover.
Long-term Dynamics