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Woody Plant Encroachment: Causes, Dynamics, and Impacts on Rangelands

This study explores the causes and dynamics of woody plant encroachment in rangelands, focusing on the effects of climate, herbivory, fire regimes, atmospheric CO2 enrichment, and N-deposition. The study also examines the impacts of woody plant encroachment on carbon cycle, land cover, and management in different regions.

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Woody Plant Encroachment: Causes, Dynamics, and Impacts on Rangelands

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  1. Land Cover, Climate and Management in Rangelands Steve Archer University of Arizona Earth Systems Feedbacks: Vulnerability of Carbon Cycle to Drought and Fire Canberra, Australia 7 June 2006

  2. Archer et al. 2001 Drought Loss of Fire/Browsers Grazing Tree Clearing Wood Harvesting Browsing Drought Grazing/Fire CLIMATE: Arid Semi-Arid Humid Sub-Humid Grassland, Pasture or Savanna Desert Scrub Woodland Grassland or Shrub-Steppe Grassland or Savanna Forest or Woodland DESERTIFICATION THICKETIZATION DEFORESTATION

  3. 1903 In many drylands, woody plants have displaced grasses in recent history…. 1941 New Mexico Arizona

  4. South Dakota 1874 Kansas 1975

  5. Causes subject of active debate; include changes in : • Climatic regimes (amount and seasonality of rainfall) • Herbivory (increased grazing or decreased browsing) • Fire regimes (decline in frequency, intensity) • Atmospheric CO2 Enrichment • N-deposition These factors have likely interacted to produce the changes observed to date

  6. Acacia nilotica invasion of Mitchell Grasslands, QLD, Australia • Cannot invoke climate, CO2, N-deposition as proximate causes for this change • Climate does, however, affect rates and dynamics of spread and stand development Photo: S. Archer

  7. WOODY PLANT ENCROACHMENT INTO GRASSLANDS & SAVANNA (see biblography at:http://ag.arizona.edu/research/archer/) LOCATION GENUS REFERENCE ARIZONA Prosopis Bahre & Shelton (1993)Larrea Humphrey & Mehrhoff (1958) CALIFORNIA Baccharis Williams et al. (1987)Juniperus Young & Evans (1981) KANSAS Quercus, Juniperus Knight et al. (1994) NEBRASKA Quercus, Juniperus Steuter et al. (1990) NEW MEXICO Prosopis, Larrea Buffington & Herbel (1965) MONTANA Pseudotsuga Arno & Gruell (1986) NEVADA Pinus, Juniperus Blackburn & Tueller (1970) NORTH DAKOTA Pinus Potter & Green (1964) OKLAHOMA Juniperus Snook (1985) SOUTH DAKOTA Pinus Progulske (1974)Quercus, Celtis Tieszen & Archer (1990) TEXAS Juniperus McPherson & Wright (1990)Prosopis, Other Archer (1989)Larrea Wondzell & Ludwig (1995) UTAH Pinus, Juniperus Madany & West (1983) Phenomenon also widespread in rangelands of Africa, South America,and Australia

  8. RATES, DYNAMICS & PATTERNS OF WOODY ENCROACHMENT • Rapid • Non-linear; punctuated by climatic events • Spatially heterogeneous - soils - topography • Topoedaphic carrying capacity?

  9. Flint Hills, KS Climate X Grazing X Fire Interaction Mitchell Grasslands, QLD, Australia

  10. Archer 1989 Graminoid-driven succession Woody Plant-driven succession Perennial grasses Short / low Herbaceous retrogression A = Tall / mid-grasses B = Mid / short grasses C = Short grass / annuals = Transition threshold A B C Time or cultural energy increments required to drive system to new configuration CommunityComposition Shrubland or Woodland Domain Grassland Domain Woody plants Long / high Time high low low low low high Fire Frequency Grazing Pressure high Probability & rate of woody plant establishment

  11. Changes in woody plant cover, southern Great Plains (Archer et al . 1988) 40 30 20 10 Site 1 Site 2 Site 3 Woody Cover (%) 1941 1963 1983 Year

  12. Traditional Perspectives on Woody Plants in Drylands • Livestock production • • forage production • • animal handling • • animal health • Wildlife habitat management (game species) • Watershed management • • stream flow • • ground water recharge Herbaceous Biomass Woody Plant Abundance

  13. Emerging perspectives • Biodiversity • Biogeochemistry and land surface-atmosphere feedbacks owing to changes in N- cycle C- cycle Water-cycle Non-methane hydrocarbon fluxes Biophysical properties

  14. BIOGEOCHEMICAL PERSPECTIVE • Land-based based inventory (Houghton et al. 1999, • 2003) and tracer-transport inversion (Pacala et al. • 2001) methods suggest woody plant proliferation • in non-forest systems is significant component • of ‘missing carbon sink’ in North America • Accurate estimates of extent of woody plant • encroachment lacking. Poor understanding of • effects on C pools/fluxes. Hence, estimates highly • uncertain.

  15. Woodlands Groves Clusters Herbaceous Changes in Woody Plant Cover (1950-1990) La Copita Site, Texas (Archer et al. 2001) Landscape 1 (1950) 1976 1990 1950

  16. Historical Changes in Plant & Soil Carbon Pool Southern Great Plains Site, Texas ( Hibbard et al. 2003) "Pristine" Grassland Heavily Grazed Grassland Grass + Woody Patches 4.5 3.5 Plant Carbon (kg m-2) 2.5 1.5 0.5 3.5 Soil Organic Carbon (kg m-2) 2.5 1.5 1900 1910 1920 1930 1940 1950 1960 1970 1980 1993 Year

  17. Change in Soil Carbon with Woody Plant Proliferation: Broad Survey Jackson et al. 2002 NM & Argentina TX & MN Venezuela & Africa Relative Change (%) Precipitation (mm)

  18. Why the variable effects on SOC? Species Effects Tarbush (Jornada site) Creosote bush Temperature Effect Temperate site 9% (Prosopis communities) Subtropical site 27-103% Soils Effect Sandy loam soils 27-37% (subtropical site) Clay loam soils 103% Land Use History Grazing, Fire, Brush Management, Erosion

  19. Asner et al. 2003 400 km2 1937 1999 Net 30% <10% >90%

  20. (Asner, et al. 2003) 1999 1937

  21. ARC Ongoing intercomparison of sites undergoing woody plant proliferation VCR WYO KNZ SEV VER JOR LAC

  22. ARC VCR WYO KNZ SEV VER JOR LAC

  23. ARC WYO VCR KNZ SEV VER JOR LAC

  24. ARC VCR WYO KNZ SEV VER JOR LAC

  25. ARC Compare/contrast changes accompanying WP increases in: ANPP Biomass LAI SOC VCR WYO KNZ SEV VER JOR LAC

  26. Archer, S. 1989. Have southern Texas savannas been converted to woodlands in recent history? American Naturalist 134:545-561. Archer, S. 2006. Bibliograpy of woody plant encroachment in drylands. http://ag.arizona.edu/research/archer/) Archer, S., T. W. Boutton, and K. A. Hibbard. 2001. Trees in grasslands: biogeochemical consequences of woody plant expansion, pp. 115-138. In: E.-D. Schulze, M. Heimann, S. Harrison, E. Holland, J. Lloyd, I. Prentice, and D. Schimel, eds. Global biogeochemical cycles in the climate system. Academic Press, San Diego. Archer, S., C. J. Scifres, C. R. Bassham, and R. Maggio. 1988. Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland. Ecological Monographs 58:111-127. Asner, G. P., S. Archer, R. F. Hughes, R. J. Ansley, and C. A. Wessman. 2003. Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937-1999. Global Change Biology 9:316-335. Hibbard, K., D. Schimel, S. Archer, D. Ojima, and W. Parton. 2003. Grassland to woodland transitions: integrating changes in landscape structure and biogeochemistry. Ecological Applications 13:911–926.

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