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