Measuring Woody Encroachment along a Forest–Savanna Boundary in Central Africa

Abstract

Changes in net area of tropical forest are the sum of several processes: deforestation, regeneration of previously deforested areas, and the changing spatial location of the forest?savanna boundary. The authors conducted a long-term (1986?2006) quantification of vegetation change in a 5400 km2 forest?savanna boundary area in central Cameroon. A cross-calibrated normalized difference vegetation index (NDVI) change detection method was used to compare three high-resolution images from 1986, 2000, and 2006. The canopy dimensions and locations of over 1000 trees in the study area were measured, and a very strong relationship between canopy area index (CAI) and NDVI was found. Across 5400 km2 12.6% of the area showed significant positive change in canopy cover from 1986 to 2000 (0.9% yr?1) and 7.8% from 2000 to 2006 (1.29% yr?1), whereas <0.4% of the image showed a significant decrease in either period. The largest changes were in the lower canopy cover classes: the area with <0.2 m2 m?2 CAI decreased by 43% in 20 years. One cause may be a recent reduction in fire frequency, as documented by Along Track Scanning Radiometer-2/Advanced ATSR (ATSR-2/AATSR) data on fire frequency over the study area from 1996 to 2006. The authors suggest this is due to a reduction in human pressure caused by urbanization, as rainfall did not alter significantly over the study period. An alternative hypothesis is that increasing atmospheric CO2 concentrations are altering the competitive balance between grasses and trees. These data add to a growing weight of evidence that forest encroachment into savanna is an important process, occurring in forest?savanna boundary regions across tropical Africa.