Historical patterns of forest cover loss in West African tropical rainforest: Has the tipping point been reached?

Background/Question/Methods

Tropical forests are critical components of the Earth system providing vital ecosystem functions. The West African tropical forest (referred to as the Upper Guinean forest, UGF), is a global biodiversity hotspot that also provides ecosystem services crucial for people’s livelihood and socio-economic development. The UGF has become one of the most human-modified forest ecosystems in the tropics with substantial forest decline in recent decades, but the magnitude and pattern of these changes is not well understood. Land cover changes are rapid and usually occur at a fine grain difficult to detect with coarse-resolution satellite images. The use of optical remote sensing for vegetation monitoring in the UGF is further hampered by persistent cloud cover. The aim of this study was therefore to map and characterize forest cover and forest cover change in the UGF over a four decade period using high spatial resolution data from the Landsat archive. Image compositing techniques were used to compensate for missing data due to persistent cloud cover and data gaps in Landsat ETM+ SLC-Off images. Advanced classification algorithms including spectral unmixing, fractional analysis and classification and regression trees were used for land cover mapping.

Results/Conclusions

Metadata analysis of the Landsat archive obtained 3,090 image tiles spanning four decades (1972 to 2012) for Ghana. Forty-nine percent of these images had cloud cover ≤ 30%. A pilot study was conducted in which forest cover and change maps were generated across one Landsat scene for four epochs centered on 1980, 1990, 2000 and 2010. Analysis of the classified images indicated monumental loss of forest cover, especially in areas outside protected areas (PAs) and forest reserves (FRs). Despite this pattern, the integrity of some FRs and PAs were compromised. Among FRs and PAs forest loss nearly followed a rainfall gradient, with highest losses occurring in the moist and dry forest areas. Major land transitions involved forest loss to agriculture and grassland/shrubs. The patterns of forest cover loss and transitions identified in this study suggest that the interaction of climatic stress, land use change, and fire may push these forests past a tipping point at which rapid and permanent forest loss occurs. Thus further research aimed at understanding these major drivers of change and their relative influence in the UGF region is strongly recommended.