Savanna
Introduction
Savannas
are ecosystems with a continuous grass layer and scattered trees or shrubs.
UNESCO classification, savanna is less
than 10% tree cover
(10% to 40% cover is open
woodland)
Daubenmire
said savanna has trees not touching.
McPherson considers up to 30% cover to
be savanna
Central
to concept: two distinct layers. Woody overstory, graminoid
understory.
I. Distribution
A. North American savannas range from xeric pinyon-juniper in
B. Communities having low-growing shrub
understory are not savannas
C. Major North American savannas
1.
Pinyon-juniper
subset
of p-j woodland
occurs
at warm, dry limits of coniferous vegetation
ten different juniper species dominate
eastern
red cedar dominates in
2.
Ponderosa pine
central
and S. rockies, northern Sierra Madre occidental,
PNW, Black Hills of S. Dakota
3.
Longleaf pine
natural
range is a 150-250 km wide belt
along
now
highly fragmented
4.
outliers
in B.C,
5.
Midwestern oak
mostly
plowed
6. Southwestern oak
much
in
some
in
7.
subtropical
large
areas in southwest
low rolling topography where
agriculture limited by rocky soil or drought
II. Savanna genesis and
maintenance
A.
Herbivory
1.
Grazing accelerates establishment of woody plants (except in some
cattle and sheep consume herbaceous
plants in preference to woody spp.
Deer and goats prefer browse
Bison
grazed intensively for short periods, then moved on (this disturbance favored
grasses)
2. Livestock disperse seed in manure
3. Release of moisture favors woody seedlings.
4. Reduce fuel biomass
B.
Fire
1. Major factor in savanna maintenance
2. Grasses adapted to periodic fires, woody
seedlings suppressed
3. Mature woody plants fire-adapted
thick
bark
root
sprouts
4. Parcel fragmentation and fire suppression
have eliminated wildfire
III. Historic changes
A. Pre-1930’s goal was to maximize livestock
production to feed a growing nation.
1.
Grazing is now closely regulated
2.
Non-consumptive goals such as eco-tourism are recognized.
B. Major factors changing historical savannas
1.
Urbanization and agricultural expansion
1700’s in east
1800’s in
only
very lately in some western savannas
2. Introduction of non-native plant
species
3. Altered abundance and distribution of woody
plants (have increased since settlement, encroached into grasslands)
tree
harvesting and planting
mining
atmospheric
and climatic changes
livestock
grazing
fire regimes
IV. Restoration
A. Land ownership: large parcels help preserve savanna
1. Government ownership
2. Large ranches
3. Public/private partnerships
B.
Management
1. Grazing
2. Native herbivores
3. Fire management
fire
suppression
prescribed
burns
4. Mechanical and chemical treatments
5. Seeding
Case histories
I. Kerr Wildlife Management Area, Texas Hill
Country
A.
Historical savanna, with bison until 1900.
B.
Settlement resulted in fencing and intensive grazing
C.
Deer populations boom and bust, exacerbating
problem
D.
Problem is that everything besides juniper is preferentially eaten.
E.
Fire was suppressed
F. Juniper had taken over entire 6500 acres by
1950’s when bought by state
G. Management actions:
1.
1964-66, large acreage of juniper cleared for posts
2.
500 acre plot left for Golden Cheek Warbler habitat
3.
Grazing and rotation were tried, did not suppress juniper regrowth
4.
1979, burning instituted
5. Cattle grazed immediately after burns, taking
65% of prickly pear cactus
6. Each year 800-1000 acres set aside for late
winter burn
7. Bare hotspots under burned brushpiles are baited with saltlicks; cattle deposit seeds
in manure
8. Deer population exploded (1 deer per 10 acres
is carrying capacity), lowered diversity of everything, juniper began to
spread.
9. Currently added to management is a both sex
deer hunt to lower density to carrying capacity.
10. 7 ½ foot tall game fence was installed
around perimeter of site to keep out neighboring deer and non-native game
animals
11. Dense cedar left in canyons and steep lands
for black-capped vireo and golden cheek warbler.
12. Birds are parasitized by brown-headed cowbird.
(Follows livestock; originally called the buffalo bird)
13. Cowbirds are trapped by grazing prior to
vireo nesting period; this attracts cowbirds; mobile cowbird trap moves with
the herd; they are fairly easy to trap.
II.
A. Develop threatened source of water and water
quality in Barton Springs, an
B. Proposition 2, a 65 million dollar bond
election was passed in 1998 to purchase land in the Barton Springs watershed
and recharge zone
C. Lands were purchased for protection,
currently up to 15,000 acres.
D. Lands to be managed for water quality and
recharge quantity protection
1.
Enhancing endangered species habitat secondary to managing for water
2.
Public access restricted. Not for
parkland
E. Best science indicated that <15% woody
plant cover provided maximum
recharge.
1.
Priority is to keep grasslands intact
2. Where woody cover can be reduced to 15%
without excessive cost or environmental damage, it can be done.
F.
Management available
1. Prescribed fire most cost-effective
2. Prescribe grazing more expensive, but viable
option
3. Prescribed mowing
4. Brush removal and invasive species control
hand
removal
fire
mechanical
chemical
biological
5. Seeding
6. Container plants.
G.
Example prescription for a parcel
1. All management should take place outside of
golden cheek warbler breeding season (Mar 15 to May 15), or when warblers are
not present.
2. If deer population is high, and site is
greater than 500 acres, authorize public hunt.
3. Maintain mature juniper stands as golden
cheek warbler habitat.
4. Maintain open grassland with initial
mechanical brush removal, followed by burning, mowing or haying every 2-4
years.
5. If any juniper removed, sow sites with rapid revegetation seed mix.