Dynamics of forest cover conversion in and around Bwindi Impenetrable Forest and impacts on carbon stocks and soil properties

Abstract

This study quantified the extent forest cover changed in and around Bwindi impenetrable forest (0˚53΄–1˚08΄N; 29˚35΄–29˚50΄E) and identified drivers of change from 1973 to 2010, and determined the effect of forest cover change on wood C stocks and soil properties. Land cover change was determined through unsupervised and supervised classification of Landsat images for 1973, 1987 and 2001 and the land use/cover map for 2010 reconstructed from the 2001 image. Drivers of land use/cover change were identified through focus group discussions and key informant interviews and literature review on policies and institutional arrangements during for the period studied. Carbon stocks and soil properties were determined from three paired sites that included natural forest with each of the major land uses; potato, tea and grazing land on two slope positions; back slope and foot slope on four slope faces (North, South, East and West). Each paired site was replicated thrice, resulting in a total of 72 sites in three altitudinal ranges (2100-2500, 1800-2100, 1450–1800 m.a.s.l) unequally replicated. Sampling was from plots of 20 m × 50 m each further divided into four sub-plots of 5 m×50 m. Tree species in the subplots were identified and diameter at breast height (DBH) of four trees (height > 1.3 m) were measured. A total of 87 wood samples (~100g each) were collected from above the DBH for carbon content determination. A total of 192 composite soil samples also were collected for physical and chemical analysis from depths of 0-15 cm and 15-30cm. Overall, spatial and temporal changes in the land uses were in general not significant over the period of analysis because of the national policies that enhanced the protection of the gazetted natural forest. However the natural forest outside protected areas declined by 70.7% because of a mix of factors including population growth and emerging opportunities for cash through tea cultivation. Wood C densities were larger by 68.7 Mg ha-1 on the footslopes than on the backslope and signficantly (P<0.001) increased

with time following forest conversion across the footslope positions. The interaction effects of land use, landscape position, aspect and slope accounted for 61% of wood carbon density changes with time while other factors which could include management, soil quality, and climate accounted 39%. Soil pool contributed about 75% of the total C density. C stock decreased over the 38 years (1973 – 2010) from 15370.20 to 15211.44 Gg. Total C stock relative to 1973 had declined by 3.18% and 51.97% in 1987 and 2010 respectively. Cultivation effects coupled with landscape position, slope face and sampling depth significantly (P<0.05) impacted on SOC stocks following forest conversion but times since conversion make no detectable impact. Interestingly, there was no significant relationship

between SOC in the top and sub soils. Forest change to tea resulted in significant (P<0.001) increases in SOC and total N at depths 0-15 cm and 15-30cm. Total N was different (P=0.016) between forest and potato at 0-15cm depth, extractable P was significantly (P<0.001) lower in paired sites of forest and tea at 0-15 and 15-30cm depths. Exc. K is larger (P<0.001) in tea compared to forest within the paired sites. The interaction effect between land use, landscape position, slope face and sampling depth was significant (P<0.05) for SOC, KSat, total N, Available P, Mg, Na, %silt and %clay. The remaining natural forest surrounding Bwindi needs to be collaboratively protected and with advances in climate change mitigation through REDD (+, ++), possibilities for payment for ecosystem services to maintain these endangered forests should be explored because forests are beneficial for carbon sequestration, water quality control and biodiversity conservation.