Riparian forest loss and landscape-scale change in Sudanian West Africa

Jeremy I. FisherJohn F. MustardGeological SciencesBrown UniversityAugust, 2004 http://porter.geo.brown.edu/~jfisher

dynamics

Sudanian West Africa• Precipitation in W.

Africa controlled by annual movement of ITCZ – zones of

increasing dryness to the north

– occasional catastrophic drought

• Culture and climate linked:– Pastoralists in the

north– Agriculturalists in

the south• Sudanian zone

forms climate and land-use margin

West AfricaPrecipitation Isohets (mm / yr)

Sudanian ClimateZone

PastoralistsSheep Goats Cows

AgriculturalistsMillet Sorghum Maize Groundnuts

Sudanian West Africa• Environmental stress

– Natural population gain– Immigration from Sahel

during the droughts of 1968-1973 and 1983-1984

• Question:– Under intense land use, can

we separate the influences of climatic change and anthropogenic change?

– Control one variable (land use) in a natural experiment

Floodplain cleared for agriculture

Intense grazing

Nouhao Valley, Burkina Faso

100 km

Burkina FasoDepartments

• Burkina Faso– North = pastoralists– South = agriculturalists

• Nouhao Valley– Abandoned in mid-

century due to Onchocerciasis (River blindness)

– Resettled in 1984 after Onchocerciasis Control Program

– Experimental division of Pastoralists and Agriculturalists

Nouhao ValleyBurkina Faso

10 km

Ghana

Burkina Faso

PastoralZone

PastoralZone

AgriculturalZone

AgriculturalZone

Nouhao ValleyBurkina Faso

10 km

Ghana

Burkina Faso

PastoralZone

AgriculturalZone

10 km

PastoralZone

AgriculturalZone

Methods• Natural experiment

– Abandonment and resettlement leave impact on land cover

– Opportunity to observe spatial patterns of change due to land uses

• Satellite analysis– Two scales of observation

• High temporal resolution (fast repeat time)

• High spatial resolution (detailed imagery)– Look for patterns of vegetative change

• Regional patterns = climate induced • Local patterns = land cover change

LandsatReal color

2002

ND

VI

Year

NPP (M

g C ha-1 y-1)

Cum

ulat

ive

Rai

nfal

l (m

m)

TotalRainfall

NPP

Satellite proxy for NPP• AVHRR instrument

– collects daily global images of NDVI

– 1981 to 2000 – 8 km resolution

• NDVI ~ photosynthetic greenness

• Sum of all NDVI in a growing season is proportional to NPP

• NPP tracks rainfall closely, slope of NPP indicates– Changing climatic conditions– Changing species

compositions

Average Rainfall in Nimay, Niger

Avg

. Rai

nfal

l (m

m)

Avg. N

DVI

Date

NDVI(photosynthetic greenness)

NDVI area

NPP

Peak of growing season

Dry season

NDVI

Rainf

all

Net Primary Productivity Slopes• Slope of NPP indicates:

– Regionally increasing vegetation 1980 to 2000

• tracks increasing rainfall• Recovery from drought

– Nouhao Valley has pronounced vegetation increase

20-20 g C m-2 y-2

Slope of Net Primary Productivityfrom AVHRR 1980 – 2000

PastoralZoneAgricultural

Zone

Towns

Nouhao Valley

Year

NPP (M

g C ha-1 y-1)

Cum

ulat

ive

Rai

nfal

l (m

m)

TotalRainfall

NPP

Year

NPP (M

g C ha-1 y-1)

Cum

ulat

ive

Rai

nfal

l (m

m)

Landsat Drivers of Land Cover• Landsat Thematic Mapper

– 30 meter resolution– Scenes acquired in transition

season (October)• Grasses senesced• Trees leaf-on

• Spectral unmixing– Determine % ground cover

• Soil• Non-photosynthetic

vegetation• Green vegetation = shrub and

tree abundance• Multi-temporal analysis

– Analyze trajectories of shrub and tree % cover through time

– 1984, 1989, 1999, 2001, 2002

Ground transects

Spec

tral

unm

ixin

g m

odel

Y = 0.97X - 0.19R2=0.366

Percent cover oftrees and shrubs

Real Color ImageLandsat Bands 321 (RGB)October 20th 2002

Vegetation fractionalabundance from LandsatOctober 20th 2002

PastoralZone

AgriculturalZone

Display inset

1.0

0.0

Frac

tiona

l A

bund

ance

1984 1989 1999 2001 2002

1984

Pastoral ZoneAgricultural Zone

1984

Pastoral ZoneAgricultural Zone

1986

Pastoral ZoneAgricultural Zone

1988

Pastoral ZoneAgricultural Zone

1990

Pastoral ZoneAgricultural Zone

1992

Pastoral ZoneAgricultural Zone

1994

Pastoral ZoneAgricultural Zone

1996

Pastoral ZoneAgricultural Zone

1998

Pastoral ZoneAgricultural Zone

2000

Pastoral ZoneAgricultural Zone

2002

Pastoral ZoneAgricultural Zone

Forest Gallery Loss or Gain from 1984 to 2002

Pastoral ZoneAgricultural Zone

Slope of vegetation gain multiplied by vegetation abundance in 1993Note - general increase in vegetation abundance- pronounced gain in pastoral riparian zone- dramatic loss in agricultural riparian zone.

loss

gain

No changeor little

veg

Riparian areas

Non-Riparian areasDeforested AfforestedYear

Afforestation orDeforestation

Land use zone

Perc

enta

ge s

hrub

and

tree

cov

er c

hang

e ov

er 2

0 ye

ars

Non-riparianRiparian

ConclusionsPatterns and Drivers

• ~ 10% increase over 2 decades due to increased wetness

• Agricultural area cleared in 1980s, thus relative loss of trees and shrubs

• Agricultural riparian area decrease– Firewood use– Loss of margins through repeated fire

• Pastoral riparian area increase– Shrubs and trees favored by

• No Fire• Bovine herbivory

– Increased nutrient load from bovines• Signal is strong and consistent• Satellite multi-platform and multi-

temporal analysis is an effective means of segregating climatic and anthropogenic land cover influences

http://porter.geo.brown.edu/~jfisherManuscript available