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. Woodland has trees 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 Great Basin to sub-tropical longleaf pine along Atlantic coast

 

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 Midwest

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 Atlantic and Gulf coasts

now highly fragmented

4. California oak

outliers in B.C, Washington, Oregon, Mexico

5. Midwestern oak

Minnesota to S.E. Texas

mostly plowed

6. Southwestern oak

much in Sierra Madre occidental

some in Arizona and New Mexico

7. Mesquite

subtropical

large areas in southwest U.S. and northern Mexico

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 California oak savanna because of phenology)

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-1930s 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

1700s in east

1800s in Midwest

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 1950s 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 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. Water Quality Protection Lands, Barton Creek Aquifer Recharge Zone, Austin, Texas

A. Develop threatened source of water and water quality in Barton Springs, an Austin sacred site.

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.