Transcript Conclude grasslands and savanna - Powerpoint for May 2.

Illinois Plant Communities –
Prairie Ecosystems
Influence of Fire
Hypsithermal or Holocene
Climatic Optimum
Native American Fires
Meadows Burning by George Catlin - 1832
Rabbit Browsing
Regrowth After Fire
Decline with spring fires
Kentucky Bluegrass
Bicknell’s Sedge
Increase with spring fires
Canada Wild Rye
Prairie Dropseed
Fire Effects
• If fire is followed by adequate precipitation, biomass
production will increase in the next 2 to 3 years following
the fire; if precipitation is less than adequate, biomass
production will decrease
• Species richness of plants usually increases in burned
compared to unburned areas - species richness also
increases when fire is combined with grazing - so fire and
grazing both act to limit growth by competitive dominants
and allow competitively inferior species to increase
Increase with fire Prairie Grasshoppers
Decrease with fire
Beetles
Leafhoppers
LeConte’s Sparrow
Return first year after fire
Bobolink
Savannah sparrow
Requires at least two years post fire
Savanna
Pembroke Savanna – Kankakee, IL
Definition
• Savanna is usually defined as being an area
with a continuous understorey of grass and
forbs with an overstorey of scattered trees,
in which the tree canopy typically covers
less than 60% of the ground
Types of Savanna
• The term savanna is applied to different
types of communities in different parts of
the world - being used to describe any
grassland in South America whether trees
are present or not; in Africa it is used to
describe grasslands and open deciduous
woodlands; in Australia and Asia it is
applied to park-like tree studded grasslands
as well as areas with tall very leafy trees
with a well defined grass layer
Savanna Worldwide – in Purple
Types of Savanna
• In fact many authors restrict the term
savanna to tropical communities - however
in recent years, the term has been expanded
to include any open woodland with a grassforb understorey - hence the use of the term
for plant communities once fairly common
in Illinois and the Midwest
Savanna in the Midwest
• Savanna in the Midwest basically occurs as
an ecotone region between eastern
deciduous forest and tallgrass prairie
• Ecotone - a region where two different
ecosystems grade into one another
• This ecotone extends in an eastward arc
from Minnesota to Texas and projects into
Illinois, Indiana and Kentucky
Oak Savanna in North America
Savanna vs. Prairie
More North American Savannas
Midwest Oak Savanna
• Oak savanna exists as an ecotone which is usually fairly
abrupt with deciduous forest giving way to tallgrass prairie
over a very short distance
• The ecotone is very complex - sometimes the transition
from closed oak forest to tallgrass prairie can occur over a
distance as short as 10 m
• In other places, the forest gradually thins to a woodland of
more open trees with a grassy understorey, then to a grassy
savanna of scattered trees (where trees make up less than
30% cover) and finally to a grassland over a distance of 50
km
Midwest Oak Savanna
• The ecotone boundary is often sinuous with
interdigitations of grassland and forest - in areas it is also a
mosaic with numerous island patches of prairie within the
forest and in other places, outliers of forest surrounded by
grassland
• The major tree species in the savanna ecosystem/ecotone
are bur oak and white oak (the most common savanna tree
in Illinois), post oak, black oak, and blackjack oak
Factors governing Savanna
formation in North America
1. Precipitation to evaporation ratio drops below 1
(remember that in tallgrass prairie P/E ratio is typically
between .6 and .8)
2. Annual precipitation varies between 60 and 100 cm
3. Yearly variability in precipitation increases relative to
forest ecosystems; the frequency of drought increases drought cannot be the whole reason for forest to give way
to grasslands, however, because grassland borders further
west support trees under even more arid conditions (Rocky
Mountain front)
4. Fire frequency increases, favoring grasses
Factors governing Savanna
formation in North America
5. Finer, deeper prairie soils favor fibrous roots of grasses
6. Tree mycorrhizal fungi are absent in prairie soils and tree
saplings are at a competitive disadvantage without
mycorrhizae
7. Heavy grazing by bison and elk until the late 19th century
may have eliminated tree saplings – along with browsing
by with rabbits
8. The tree line may have been pushed east by Xerothermic
period of 6,000 years ago, and tree line has not yet
regained its climatic limits
Wet-dry cycles and savanna
Fire in Savanna
Fire Effects
• In the absence of fire, some studies in Wisconsin have
shown that woody species invade prairie at an average
annual rate of expansion of 3 m per year
• Fire in savannas stimulates the growth of grasses,
increasing productivity (biomass) and flowering 2 to 3
times that in absence of fire - this is primarily in response
to removal of litter layer
• With litter removed, sun warms the soil surface to 5 C
more than with litter - this increased warmth in turn
stimulates plant growth and decomposer activity
Savanna in Wisconsin
Savanna Restoration
Fire in Savannas - Wisconsin
Fire in Savannas - Minnesota
Fire Effects
• The presence of oak species in midwestern savanna is due
to their having thick bark which resists fire and insulates
the growing cambial layer of the tree from damage by fire thus well established oaks could survive fire
• In the absence of fire, some studies in Wisconsin have
shown that woody species invade prairie at an average
annual rate of expansion of 3 m per year
• Fire in savannas stimulates the growth of grasses,
increasing productivity (biomass) and flowering 2 to 3
times that in absence of fire - this is primarily in response
to removal of litter layer
Oregon Oak Savanna