Climatic and Human Influence on Tropical Forest Degradation in West Africa

Abstract:

The Upper Guinean Forests of West Africa encompass one of most fragmented and endangered tropical forest ecosystems on earth. However, relatively little is known about the extent and causes of forest degradation in this heavily human affected region. We used the tasseled cap wetness (TCW) index to detect forest degradation using dry-season imagery from 2001 to 2015. TCW was calculated from MODIS (or Moderate Resolution Imaging Spectroradiometer) Nadir BRDF-Adjusted Reflectance (NBAR) at 500 meter resolution. The TCW index was compared with field data and high-resolution imagery and was found to have a strong relationship with different level of forest degradation, with lower TCW index values indicating higher levels of degradation. Mann-Kendall tests were used to detect the monotonic decreases in TCW index. Results indicated that about 12.9% of tropical forest in West Africa has been degraded during past 15 years, and most of these areas were located in isolated forest patches and on the periphery of intact forest. The boosted regression tree (BRT) method was then used to assess the influence of climate and human influence on forest degradation. BRT analysis indicated that climatic variations, human activities, and their interactions were important contributors to forest degradation. Major drivers of forest degradation included distance to cropland, rainfall trend, and rate of population growth.. Our results show that times series of coarse-resolution MODIS imagery can be used for broad-scale assessment of tropical forest degradation, and human activities need to be taken into account when assessing potential forest responses to climate change in this region.