Land Cover, Climate and Management Land Cover, Climate and Management in Rangelandsin Rangelands

Steve ArcherSteve ArcherUniversity of ArizonaUniversity of Arizona

Earth Systems Feedbacks:Earth Systems Feedbacks:Vulnerability of Carbon Cycle Vulnerability of Carbon Cycle

to Drought and Fireto Drought and Fire

Canberra, AustraliaCanberra, Australia

7 June 2006 7 June 2006

Grassland orGrassland orShrub-SteppeShrub-Steppe

Desert ScrubDesert Scrub

DESERTIFICATIONDESERTIFICATION

Grassland Grassland or Savannaor Savanna

Woodland Woodland

THICKETIZATIONTHICKETIZATION

Forest or Forest or WoodlandWoodland

Grassland, Grassland, Pasture orPasture orSavannaSavanna

DEFORESTATIONDEFORESTATION

CLIMATE:CLIMATE: AridArid Semi-AridSemi-Arid Sub-HumidSub-Humid HumidHumid

DroughtDroughtGrazing/FireGrazing/Fire

DroughtDroughtLoss of Fire/BrowsersLoss of Fire/Browsers

GrazingGrazing

Tree ClearingTree ClearingWood HarvestingWood Harvesting

BrowsingBrowsing

Archer et al. 2001Archer et al. 2001

In many drylands, woody plants In many drylands, woody plants have displaced grasses in recent have displaced grasses in recent history….history….

1903

1941

ArizonaArizona New MexicoNew Mexico

18741874

19751975

South DakotaSouth Dakota

KansasKansas

Causes subject of active debate; Causes subject of active debate; include changes in :include changes in :

• • Climatic regimes Climatic regimes (amount and seasonality of rainfall)(amount and seasonality of rainfall)

• • HerbivoryHerbivory (increased grazing or decreased browsing)(increased grazing or decreased browsing)

• • Fire regimesFire regimes (decline in frequency, intensity)(decline in frequency, intensity)

• • Atmospheric COAtmospheric CO22 Enrichment Enrichment• • N-depositionN-deposition

These factors have likely These factors have likely interactedinteracted to produce the to produce the changes observed to datechanges observed to date

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. ArcherPhoto: S. Archer

LOCATIONLOCATION GENUS GENUS REFERENCEREFERENCE

ARIZONAARIZONA ProsopisProsopis Bahre & Shelton (1993)Bahre & Shelton (1993)LarreaLarrea Humphrey & Mehrhoff (1958) Humphrey & Mehrhoff (1958)

CALIFORNIACALIFORNIA BaccharisBaccharis Williams Williams et al.et al. (1987) (1987)JuniperusJuniperus Young & Evans (1981)Young & Evans (1981)

KANSASKANSAS Quercus, JuniperusQuercus, Juniperus Knight Knight et al.et al. (1994) (1994)NEBRASKANEBRASKA Quercus, JuniperusQuercus, Juniperus Steuter Steuter et al.et al. (1990) (1990)NEW MEXICONEW MEXICO Prosopis, LarreaProsopis, Larrea Buffington & Herbel (1965)Buffington & Herbel (1965)MONTANAMONTANA PseudotsugaPseudotsuga Arno & Gruell (1986)Arno & Gruell (1986)NEVADANEVADA Pinus, JuniperusPinus, Juniperus Blackburn & Tueller (1970)Blackburn & Tueller (1970)NORTH DAKOTANORTH DAKOTA PinusPinus Potter & Green (1964)Potter & Green (1964)OKLAHOMAOKLAHOMA JuniperusJuniperus Snook (1985)Snook (1985)SOUTH DAKOTASOUTH DAKOTA PinusPinus Progulske (1974)Progulske (1974)

Quercus, CeltisQuercus, Celtis Tieszen & Archer (1990)Tieszen & Archer (1990)TEXASTEXAS JuniperusJuniperus McPherson & Wright (1990)McPherson & Wright (1990)

Prosopis, OtherProsopis, Other Archer (1989)Archer (1989)LarreaLarrea Wondzell & Ludwig (1995)Wondzell & Ludwig (1995)

UTAHUTAH Pinus, JuniperusPinus, Juniperus Madany & West (1983)Madany & West (1983)

Phenomenon also widespread in rangelands of Africa, South America,and AustraliaPhenomenon also widespread in rangelands of Africa, South America,and Australia

WOODY PLANT ENCROACHMENTWOODY PLANT ENCROACHMENTINTO GRASSLANDS & SAVANNAINTO GRASSLANDS & SAVANNA

(see biblography at:(see biblography at: http://ag.arizona.edu/research/archer/)

• • RapidRapid

• • Non-linear; punctuated by Non-linear; punctuated by climatic eventsclimatic events

• • Spatially heterogeneousSpatially heterogeneous- soils- soils- topography- topography

• • Topoedaphic carrying capacity?Topoedaphic carrying capacity?

RATES, DYNAMICS & PATTERNS OF RATES, DYNAMICS & PATTERNS OF WOODY ENCROACHMENTWOODY ENCROACHMENT

Flint Hills, KSFlint Hills, KS

Mitchell Grasslands, QLD, AustraliaMitchell Grasslands, QLD, Australia

ClimateClimate XX GrazingGrazing XX Fire Fire InteractionInteraction

TimeTime

Fire FrequencyFire Frequency

Grazing PressureGrazing Pressure

Probability & rate of woody plant establishmentProbability & rate of woody plant establishment

AA = Tall / mid-grasses= Tall / mid-grassesBB = Mid / short grasses= Mid / short grassesCC = Short grass / annuals= Short grass / annuals

= Transition threshold= Transition threshold

AA BB CC

HerbaceousHerbaceousretrogressionretrogression

PerennialPerennialgrassesgrasses

WoodyWoodyplantsplants

Com

mun

ity C

ompo

sitio

nC

omm

unity

Com

posi

tion

Tim

e or

cul

tura

l ene

rgy

incr

emen

ts re

quire

dTi

me

or c

ultu

ral e

nerg

y in

crem

ents

requ

ired

to d

rive

syst

em to

new

con

figur

atio

nto

driv

e sy

stem

to n

ew c

onfig

urat

ion Short /Short /

lowlow

Long /Long /highhigh

Woody Plant-driven successionWoody Plant-driven successionGraminoid-drivenGraminoid-driven

successionsuccession

high high

low low

low low

low low

low low

high high

highhigh

GrasslandGrasslandDomainDomain

ShrublandShrublandor Woodlandor WoodlandDomainDomain

Archer 1989Archer 1989

Changes in woody plant cover, Changes in woody plant cover, southern Great Plains southern Great Plains (Archer et al . 1988)(Archer et al . 1988)

19411941 19631963 19831983YearYear

4040

3030

2020

1010

Woo

dy C

over

(%)

Woo

dy C

over

(%) Site 1Site 1

Site 2Site 2

Site 3Site 3

Traditional Perspectives on Traditional Perspectives on Woody Plants in DrylandsWoody Plants in Drylands

• • Livestock productionLivestock production• • • • forage productionforage production• • • • animal handlinganimal handling• • • • animal healthanimal health

• • Wildlife habitat management (game species)Wildlife habitat management (game species)• • Watershed managementWatershed management

• • • • stream flowstream flow• • • • ground water rechargeground water recharge

Her

bace

ous B

iom

ass

Her

bace

ous B

iom

ass

Woody Plant AbundanceWoody Plant Abundance

Emerging perspectivesEmerging perspectives• • BiodiversityBiodiversity

• • BiogeochemistryBiogeochemistry and and land surface-atmosphere land surface-atmosphere feedbacks owing to changes infeedbacks owing to changes in

N- cycleN- cycleC- cycleC- cycleWater-cycleWater-cycleNon-methane hydrocarbon fluxesNon-methane hydrocarbon fluxesBiophysical propertiesBiophysical properties

BIOGEOCHEMICAL PERSPECTIVEBIOGEOCHEMICAL PERSPECTIVE

• Land-based based inventory (Land-based based inventory (Houghton et al. 1999, Houghton et al. 1999, 20032003) and tracer-transport inversion () and tracer-transport inversion (Pacala et al. Pacala et al. 20012001) methods suggest woody plant proliferation ) methods suggest woody plant proliferation in non-forest systems is significant component in non-forest systems is significant component of ‘missing carbon sink’ in North Americaof ‘missing carbon sink’ in North America

• Accurate estimates of extent of woody plant Accurate estimates of extent of woody plant encroachment lacking. Poor understanding ofencroachment lacking. Poor understanding of effects on C pools/fluxes. Hence, estimates highly effects on C pools/fluxes. Hence, estimates highly uncertain.uncertain.

19501950

Landscape 1Landscape 1(1950)(1950)

Changes in Woody Plant Cover (1950-1990)Changes in Woody Plant Cover (1950-1990)

WoodlandsWoodlandsGrovesGrovesClustersClustersHerbaceousHerbaceous

La Copita Site, TexasLa Copita Site, Texas

19761976 19901990

(Archer et al. 2001)

19001900 19101910 19201920 19301930 19401940 19501950 19601960 19701970 19801980 19931993

YearYear

1.51.5

2.52.5

3.53.5

Soi l

Org

a nic

Soi l

Org

a nic

Car

bon

(kg

mC

arbo

n (k

g m

-2-2))

0.50.5

1.51.5

2.52.5

3.53.5

4.54.5

Pla n

t Car

bon

Pla n

t Car

bon

(kg

m(k

g m

-2-2))

Historical Changes in Plant & Soil Carbon PoolHistorical Changes in Plant & Soil Carbon Pool Southern Great Plains Site, Texas Southern Great Plains Site, Texas (( Hibbard et al. 2003 Hibbard et al. 2003))

Grass + Woody PatchesGrass + Woody Patches

"Pristine" Grassland"Pristine" GrasslandHeavily Grazed GrasslandHeavily Grazed Grassland

-100

-50

0

50

100

150

200

0 250 500 750 1000 1250

Change in Soil Carbon with Woody Change in Soil Carbon with Woody Plant Proliferation: Broad SurveyPlant Proliferation: Broad Survey

Precipitation (mm)Precipitation (mm)

Rel

ativ

e C

hang

e (%

)R

elat

ive

Cha

nge

(%)

Jackson Jackson et alet al. 2002. 2002

NM & ArgentinaNM & Argentina

TX & MNTX & MN

Venezuela & AfricaVenezuela & Africa

Species EffectsSpecies Effects TarbushTarbush(Jornada site)(Jornada site) Creosote bushCreosote bush

Temperature EffectTemperature Effect Temperate siteTemperate site 9% 9% ((ProsopisProsopis communities) communities) Subtropical siteSubtropical site 27-103%27-103%

Soils EffectSoils Effect Sandy loam soils 27-37%Sandy loam soils 27-37%(subtropical site)(subtropical site) Clay loam soilsClay loam soils 103% 103%

Land Use HistoryLand Use History Grazing, Fire, Brush Grazing, Fire, Brush Management, ErosionManagement, Erosion

Why the variable effects on SOC?Why the variable effects on SOC?

<10% >90%

1937 1999400 km400 km22

Net 30%Net 30%

Asner et al. 2003Asner et al. 2003

(Asner, et al. 2003)(Asner, et al. 2003)

19371937 19991999

ARC

WYOKNZ

VCR

LAC

VER SEV

JOR

Ongoing intercomparison of sitesOngoing intercomparison of sitesundergoing woody plant proliferationundergoing woody plant proliferation

ARC

WYOKNZ

VCR

LAC

VER SEV

JOR

Site Biome type MAP/ MAT (mm/ ºC)

PET (mm)

Growing season length

Arctic, AK (ARC) Tussock Tundra 291/-12.5 114 55 days Jornada, NM (JRN) Chihuahuan Desert 264/14.4 794 255 days

Konza, KS (KNZ) Tallgrass Prairie 859/12.9 793 180 days La Copita, TX (LAC) Subtropical Thorn Woodland 680/ 22.0 1448 289 days Sevilleta, NM (SEV) Chihuahuan Desert 242/13.3 747 210 days

Vernon, TX (VER) Temperate Savanna 655/17.0 1487 220 days Virginia Coast Reserve,

VA (VCR) Temperate Barrier Island 1065/ 14.2 786 245 days

Sierra Madre, WY (WYO) Sagebrush Steppe 259/ 6.2 483 100 days

ARC

WYOKNZ

VCR

LAC

VER SEV

JOR

Site Biome type Dominant Grassland Species

(photosynthetic pathway)

Dominant Shrub Species E = evergreen, D = deciduous

N = N-fixer Arctic, AK (ARC) Tussock Tundra Eriophorum vaginatum

(C3) Betula nana (D), Ledum palustre (E)

Jornada, NM (JRN) Chihuahuan Desert Bouteloua eriopoda (C4) Larrea tridentata (E), Prosopis glandulosa (D, N)

Konza, KS (KNZ) Tallgrass Prairie Andropogon gerardii (C4) Cornus drumondi (D) La Copita, TX (LAC) Subtropical Thorn Woodland Chloris cuculatta (C4) Prosopis glandulosa (D) Sevilleta, NM (SEV) Chihuahuan Desert Bouteloua curtipendula

(C4) Prosopis glandulosa (D, N)

Vernon, TX (VER) Temperate Savanna Nasella leucotrica (C3),Bouteloua eriopoda (C4)

Larrea tridentate (E)

Virginia Coast Reserve, VA (VCR)

Temperate Barrier Island Spartina patens (C4) Myrica cerifera (E, N)

Sierra Madre, WY (WYO) Sagebrush Steppe Poa secunda (C3), Stipa comata (C3), Elysium smithii (C3)

Artemisia tridenta vaseyana (E)

ARC

WYOKNZ

VCR

LAC

VER SEV

JOR

Site Biome type Causes(?) Arctic, AK (ARC) Tussock Tundra Warming

Jornada, NM (JRN) Chihuahuan Desert Overgrazing/drought Konza, KS (KNZ) Tallgrass Prairie Fire frequency/intensity

La Copita, TX (LAC) Subtropical Thorn Woodland Overgrazing/Abiotic Sevilleta, NM (SEV) Chihuahuan Desert Overgrazing/drought

Vernon, TX (VER) Temperate Savanna Overgrazing/fire frequency/intensity

Virginia Coast Reserve, VA (VCR)

Temperate Barrier Island Frequency of storms (hurricanes)

Sierra Madre, WY (WYO) Sagebrush Steppe Fire frequency

ARC

WYOKNZ

VCR

LAC

VER SEV

JOR

Site Biome type Causes(?) Arctic, AK (ARC) Tussock Tundra Warming

Jornada, NM (JRN) Chihuahuan Desert Overgrazing/drought Konza, KS (KNZ) Tallgrass Prairie Fire frequency/intensity

La Copita, TX (LAC) Subtropical Thorn Woodland Overgrazing/Abiotic Sevilleta, NM (SEV) Chihuahuan Desert Overgrazing/drought

Vernon, TX (VER) Temperate Savanna Overgrazing/fire frequency/intensity

Virginia Coast Reserve, VA (VCR)

Temperate Barrier Island Frequency of storms (hurricanes)

Sierra Madre, WY (WYO) Sagebrush Steppe Fire frequency

Compare/contrast changes Compare/contrast changes accompanying WP increases in:accompanying WP increases in:

ANPPANPPBiomassBiomass

LAILAISOCSOC

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.