A restored savanna
generally has an extensive groundlayer of grasses. In fact,
some older definitions of savanna required prairie grasses
as a major part of the groundlayer. However, because of the
high heterogeneity of the savanna environment, presence of
prairie grasses will depend considerably on available light,
which will be influenced by how open the forest canopy is.
A savanna with widely spaced open-grown oaks (perhaps 10%
cover) can be considered a prairie with occasional scattered
trees. More closed canopies, even those with large open-grown
oaks, may have prairie grasses only on the edges.
Grasses can be
divided into two classes depending on whether they grow best
in the cooler or warmer part of the growing season. These
are called "cool season" and "warm season"
grasses. They differ in how they handle the photosynthesis
process during hot periods. The cool season grasses essentially
"shut down" during the summer heat, whereas the
warm season grasses are able to store carbon dioxide during
hot days and then utilize it for photosynthesis during the
cooler night periods.
In the Midwest
the principal prairie grasses are the warm season grasses
listed in the table below. On the other hand, the cool season
grasses shown in the table begin growth early and are often
finished flowering and beginning to set seed just when the
warm season grasses are beginning to thrive. Most of the cool
season grasses can best be called "savanna" grasses.
savannas, those that are more prairie in character, the warm
season grasses may predominate. In more closed savannas, the
cool season (savanna) grasses thrive. These cool season grasses
begin growing early in the season. In addition to growing
in savannas, they are often also present in more mesic habitats,
sometimes even wet mesic, and may even be found in nonsavanna
habitats such as lowland (riparian) forests.