Abstract
The Upper Guinea Forest (UGF) region of West Africa is one of the most climatically marginal and human-impacted tropical forest regions in the world. Research on the patterns and drivers of vegetation change is critical for developing strategies to sustain ecosystem services in the region and to understand how climate and land use change will affect other tropical forests around the globe. We compared six spectral indices calculated from the 2001–2015 MODIS optical-infrared reflectance data with manually-interpreted measurements of woody vegetation cover from high resolution imagery. The tasseled cap wetness (TCW) index was found to have the strongest association with woody vegetation cover, whereas greenness indices, such as the enhanced vegetation index (EVI), had relatively weak associations with woody cover. Trends in woody vegetation cover measured with the TCW index were analyzed using Mann–Kendall statistics and were contrasted with trends in vegetation greenness measured with EVI. In the drier West Sudanian Savanna and Guinean Forest-Savanna Mosaic ecoregions, EVI trends were primarily positive, and TCW trends were primarily negative, suggesting that woody vegetation cover was decreasing, while herbaceous vegetation cover is increasing. In the wettest tropical forests in the Western Guinean Lowland Forest ecoregion, declining trends in both TCW and EVI were indicative of widespread forest degradation resulting from human activities. Across all ecoregions, declines in woody cover were less prevalent in protected areas where human activities were restricted. Multiple lines of evidence suggested that human land use and resource extraction, rather than climate trends or short-term climatic anomalies, were the predominant drivers of recent vegetation change in the UGF region of West Africa.
Highlights
Information about the regional patterns and drivers of tropical forest dynamics is critical to anticipate the impacts of these changes on ecosystem services, including carbon storage, biodiversity conservation and climate regulation [1]
Given that tropical forests are highly sensitive to water availability and its seasonal changes [39], the patterns of canopy moisture captured by the shortwave infrared (SWIR)-based vegetation indices appear to be more sensitive indicators of variations in forest canopy density than the patterns of chlorophyll abundance measured by greenness indices, which may explain the different geographic patterns of vegetation trends detected by tasseled cap wetness (TCW)
Our findings confirmed that greenness trends are not a reliable indicator of changes in woody vegetation cover in the wetter regions of West Africa dominated by forests, cropland mosaics and woody savanna
Summary
Information about the regional patterns and drivers of tropical forest dynamics is critical to anticipate the impacts of these changes on ecosystem services, including carbon storage, biodiversity conservation and climate regulation [1]. There is a need for research on vegetation dynamics in the more climatically marginal and human-impacted tropical forest regions. These drier forests are generally found close to the minimum rainfall threshold that separates closed-canopy forests from open woodlands and savanna and are often located at the frontier of deforestation and degradation resulting from human land use [6,7]. Research on the dynamics of drier tropical forests is critical to support regional conservation efforts, as well as to further our understanding of the effects that future global change will have on other areas of intact tropical rainforest
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