Riparian forest loss and landscape-scale change in Sudanian West Africa
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Transcript of Riparian forest loss and landscape-scale change in Sudanian West Africa
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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
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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
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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
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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
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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
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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
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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
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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
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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
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1984
Pastoral ZoneAgricultural Zone
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1984
Pastoral ZoneAgricultural Zone
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1986
Pastoral ZoneAgricultural Zone
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1988
Pastoral ZoneAgricultural Zone
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1990
Pastoral ZoneAgricultural Zone
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1992
Pastoral ZoneAgricultural Zone
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1994
Pastoral ZoneAgricultural Zone
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1996
Pastoral ZoneAgricultural Zone
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1998
Pastoral ZoneAgricultural Zone
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2000
Pastoral ZoneAgricultural Zone
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2002
Pastoral ZoneAgricultural Zone
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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
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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