Riparian forest loss and landscape-scale change in Sudanian West Africa Jeremy I. Fisher John F. Mustard Geological Sciences Brown University August, dynami cs

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 Africa Precipitation Isohets (mm / yr) Sudanian Climate Zone Pastoralists Sheep Goats Cows Agriculturalists Millet Sorghum Maize Groundnuts

Sudanian West Africa Environmental stress –Natural population gain –Immigration from Sahel during the droughts of and 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 Faso Departments 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 Valley Burkina Faso 10 km Ghana Burkina Faso Pastoral Zone Pastoral Zone Agricultural Zone Agricultural Zone

Nouhao Valley Burkina Faso 10 km Ghana Burkina Faso Pastoral Zone Agricultural Zone 10 km Pastoral Zone Agricultural Zone 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 Landsat Real color 2002

NDVI Year NPP (Mg C ha-1 y-1) Cumulative Rainfall (mm) Total Rainfall 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. Rainfall (mm) Avg. NDVI Date NDVI (photosynthetic greenness) NDVI area " NPP Peak of growing season Dry season NDVI Rainfall

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 g C m -2 y -2 Slope of Net Primary Productivity from AVHRR 1980 – 2000 Pastoral Zone Agricultural Zone Towns Nouhao Valley Year NPP (Mg C ha-1 y-1) Cumulative Rainfall (mm) Total Rainfall NPP

Year NPP (Mg C ha-1 y-1) Cumulative Rainfall (mm) 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 Spectral unmixing model Y = 0.97X R 2 =0.366 Percent cover of trees and shrubs

Real Color Image Landsat Bands 321 (RGB) October 20th 2002 Vegetation fractional abundance from Landsat October 20th 2002 Pastoral Zone Agricultural Zone Display inset Fractional Abundance

1984 Pastoral Zone Agricultural Zone

1984 Pastoral Zone Agricultural Zone

1986 Pastoral Zone Agricultural Zone

1988 Pastoral Zone Agricultural Zone

1990 Pastoral Zone Agricultural Zone

1992 Pastoral Zone Agricultural Zone

1994 Pastoral Zone Agricultural Zone

1996 Pastoral Zone Agricultural Zone

1998 Pastoral Zone Agricultural Zone

2000 Pastoral Zone Agricultural Zone

2002 Pastoral Zone Agricultural Zone

Forest Gallery Loss or Gain from 1984 to 2002 Pastoral Zone Agricultural Zone Slope of vegetation gain multiplied by vegetation abundance in 1993 Note - general increase in vegetation abundance - pronounced gain in pastoral riparian zone - dramatic loss in agricultural riparian zone. loss gain No change or little veg Riparian areas Non-Riparian areas Deforested Afforested Year Afforestation or Deforestation

Land use zone Percentage shrub and tree cover change over 20 years Non-riparian Riparian Conclusions Patterns 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 Manuscript available