1. 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) 194119631983 Year 40302010 Woody Cover (%) Site 1 Site 2 Site 3

2. 1950 (3.59 ha) Landscape 1 (1950) Changes in Woody Plant Cover (1950-1990) WoodlandsGrovesClustersHerbaceous La Copita Site, Texas 1976 (5.26 ha) 1990 (6.52 ha)

3. - 200 yearsPresent+ 200 years (1983) Woody PatchesHerbaceous Patches 8000600040002000 Area Weighted CO 2 Flux (kg C ha -1 y -1 ) 10%  4%  Annual measurements of soil respiration (McCulley 1999) coupled with a successional model of vegetation change (Scanlan & Archer 1991), indicate successional model of vegetation change (Scanlan & Archer 1991), indicate soil CO 2 flux may have increased ca. 10% subsequent to woody plant soil CO 2 flux may have increased ca. 10% subsequent to woody plantencroachment

4. (from Hibbard et al. 2000) Annual N min in southern Texas are highest in soils associated with woody vegetation known to have developed over the past century AnnualN-Mineralization Patch Type(µg g -1 y -1 ) Herbaceous 42 + 5 Shrub Cluster200 + 18 Grove137 + 16 Woodland127 + 29

5. NITRIC OXIDE FLUXES La Copita Site, Texas (from Cole et al. 1996) 0 5 10 15 20 mg NO cm -2 hr -1 Wet Soil Dry Soil Herbaceous Patch Grove Patch Woodland Patch Upland, Sandy Loam Low land, Clay Loam

6. Increases in woody plant abundance have increased NMHC emissions 3-fold at the La Copita site.

7. Soil Organic Carbon (kg m -2 ) SOC in Grazed Grasslands La Copita Site, Texas (from Hibbard 1995) 175018001850190019502000 Year Onset of Heavy Grazing Lowland (Clay Loam) Upland (Sandy Loam) 4.0 3.5 3.0 2.5

8. 0.5 1.5 2.5 3.5 4.5 1900191019201930194019501960197019801993 Year Plant Carbon (kg m -2 ) Historical Changes in Plant Carbon Pool La Copita Site, Texas (from Hibbard 1995) La Copita Site, Texas (from Hibbard 1995) Grass + Woody Patches "Pristine" Grassland Heavily Grazed Grassland

9. 1.5 2.5 3.5 1900191019201930194019501960197019801993 Year Grass + Woody Patches "Pristine" Grassland Heavily Grazed Grassland Soil Organic Carbon (kg m -2 ) Historical Changes in Soil Carbon Pool La Copita Site, Texas (from Hibbard 1995)

10. Estimates of C-sequestration by dryland woody plants must account for deep root systems plants must account for deep root systems (photo by S. Archer, La Copita site, Texas) (photo by S. Archer, La Copita site, Texas)

11. high Woody plant establishment Tree-shrub community development heavy, continuous grazing heavy, continuous grazing elimination of fire elimination of fire minimal browsing minimal browsing Time Time low Nutrient pools & fluxes ProductivityBiodiversity (from Archer et al. 2000) Degradation Phase Degradation Phase Tree/shrub-driven Tree/shrub-driven succession succession Aggradation Phase Aggradation Phase Herbaceousretrogression