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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 (%). 1941 1963 1983. Year. Woodlands Groves Clusters Herbaceous.

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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 (%)

1941 1963 1983

Year


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)


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 Patches Herbaceous Patches

8000

6000

4000

2000

10%

4%

Area Weighted

CO2 Flux

(kg C ha-1 y-1)

- 200 years Present + 200 years

(1983)


Annual

N-Mineralization

Patch Type (µg g-1 y-1)

Herbaceous 42 + 5

Shrub Cluster 200 + 18

Grove 137 + 16

Woodland 127 + 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)


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)


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

at the La Copita site.


SOC in Grazed Grasslands emissions 3-fold

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

1750 1800 1850 1900 1950 2000

Year


Historical Changes in Plant Carbon Pool emissions 3-fold

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


Historical Changes in Soil Carbon Pool emissions 3-fold

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


Estimates of C-sequestration by dryland woody emissions 3-fold

plants must account for deep root systems

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


( emissions 3-fold 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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