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Changes in woody plant cover may be rapid, non-linear, and triggered by extreme environmental events (e.g., drought of 1950s) (from Archer et al. 1988). 40 30 20 10. Site 1 Site 2 Site 3. Woody Cover (%). 194119631983. Year. Woodlands Groves Clusters Herbaceous.

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194119631983

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1941 1963 1983

Changes in woody plant cover may be rapid, non-linear, and triggered by extreme environmental events (e.g., drought of 1950s) (from Archer et al. 1988)

40

30

20

10

Site 1

Site 2

Site 3

Woody Cover (%)

194119631983

Year


1941 1963 1983

Woodlands

Groves

Clusters

Herbaceous

Changes in Woody Plant Cover (1950-1990)

La Copita Site, Texas

Landscape 1

(1950)

1976

(5.26 ha)

1990

(6.52 ha)

1950

(3.59 ha)


1941 1963 1983

Annual measurements of soil respiration (McCulley 1999) coupled with a

successional model of vegetation change (Scanlan & Archer 1991), indicate

soil CO2 flux may have increased ca. 10% subsequent to woody plant

encroachment

Woody PatchesHerbaceous Patches

8000

6000

4000

2000

10%

4%

Area Weighted

CO2 Flux

(kg C ha-1 y-1)

- 200 yearsPresent+ 200 years

(1983)


1941 1963 1983

Annual

N-Mineralization

Patch Type(µg g-1 y-1)

Herbaceous 42 + 5

Shrub Cluster200 + 18

Grove137 + 16

Woodland127 + 29

Annual Nmin in southern Texas are highest in soils associated with woody vegetation known to have developed over the past century

(from Hibbard et al. 2000)


1941 1963 1983

20

Dry Soil

Wet Soil

15

mg NO cm-2 hr-1

10

5

0

Herbaceous

Grove

Woodland

Patch

Patch

Patch

Upland, Sandy Loam

Low

land, Clay Loam

NITRIC OXIDE FLUXES

La Copita Site, Texas (from Cole et al. 1996)


1941 1963 1983

Increases in woody plant abundance have increased NMHC emissions 3-fold

at the La Copita site.


1941 1963 1983

SOC in Grazed Grasslands

La Copita Site, Texas(from Hibbard 1995)

4.0

Onset of Heavy Grazing

3.5

Lowland

(Clay Loam)

Soil Organic Carbon (kg m-2)

3.0

Upland

(Sandy Loam)

2.5

175018001850190019502000

Year


1941 1963 1983

Historical Changes in Plant Carbon Pool

La Copita Site, Texas (from Hibbard 1995)

"Pristine" Grassland

4.5

Heavily Grazed Grassland

Grass + Woody Patches

3.5

2.5

Plant Carbon (kg m-2)

1.5

0.5

1900

1910

1920

1930

1940

1950

1960

1970

1980

1993

Year


1941 1963 1983

Historical Changes in Soil Carbon Pool

La Copita Site, Texas (from Hibbard 1995)

"Pristine" Grassland

3.5

Heavily Grazed Grassland

Grass + Woody Patches

2.5

Soil Organic Carbon (kg m-2)

1.5

1900

1910

1920

1930

1940

1950

1960

1970

1980

1993

Year


1941 1963 1983

Estimates of C-sequestration by dryland woody

plants must account for deep root systems

(photo by S. Archer, La Copita site, Texas)


1941 1963 1983

(from Archer et al. 2000)

Aggradation Phase

Degradation Phase

Tree/shrub-driven

Herbaceous

retrogression

succession

high

Nutrient pools & fluxes

Productivity

Biodiversity

Tree-shrub community

Woody plant

development

establishment

low

Time

• heavy, continuous grazing

• elimination of fire

• minimal browsing


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