The interaction
of three factors maintains the Prairie as a grassland:
Drought:
The periodic lack
of rainfall is a very important factor in preventing the
growth of trees in prairies. A drought is when the
prairie receives no more than 10 inches of rain per
year. In the picture below, you can see how a few trees
are tenuously rooted in slight depressions where what
little moisture there is gets concentrated.
photo
M. Noonan
Grazing:
Continuous grazing
by bison, prairie dogs, and numerous other species,
keeps the grass short. In a feedback cycle, the grazing
also stimulates new shoots to grow which the grazing
animals depend upon. As bison graze, their hoof-action
also sows new seeds into the earth which helps to
promote new vegetation.
photo
M. Noonan
Fire:
Every three to seven
years, any given area of tall grass prairie is likely to
burn due to fires started naturally by lightning. This
too prevents long-lived species like trees from becoming
established.
photo
M. Noonan