Impact of Cashew Expansion on Land Use/ Land Cover Change and Carbon Stocks in the Forest-Savanna Transition Zone, North-East Côte d’Ivoire

Abstract

An increased production of cashew (Anacardium Occidentale L.) in the forest-savanna transition zone of Côte d’Ivoire calls for an understanding of the biophysical, ecological as well as climate change implications that may be associated with its production. This study aimed to deepen knowledge on how expansive cashew production impacts on land use/ land cover and its consequences on biodiversity and carbon stocks. Using the maximum likelihood classification algorithm and post-classification change detection procedure, the spatial-temporal dynamics of land use/ land cover was assessed based on a series of Landsat images for 1988, 2001 and 2014. Vegetation inventories were carried out to determine the composition and structure of cashew plantations and natural vegetation. From the diameter and species identification as well as soil samples data collected in inventory, the biomass and for that matter carbon stock was calculated. Multiple linear regression analysis was used to examine the main drivers of cashew expansion at household level, and GIS and multicriteria analysis were used to analyse the natural vegetations’ vulnerability to future cashew expansion. The results identified cashew expansion (a rate of 26.78% and 7.24% per annum for the periods 1988-2001 and 2001-2013, respectively) as major land use changes. From 1988 to 2014, 26.4% and 34.1% of forest/woodland and savanna areas respectively were converted to cashew plantations, given a natural vegetation conversion rate of 2.35 to 2.88% per annum. There was a decrease in tree species richness and evenness from natural vegetation to cashew plantations. In terms of vegetation structure however, mature plantation had basal area similar to forest/woodland. The total carbon stocks in Mg C ha-1 t/ha were low in cashew plantations, where mature stands had 21.826 ± 3.23 (Mean ± SE), young iii 25.927 ± 6.53 and juvenile 16.732 ± 2.96 compared with natural vegetation (forest/woodland 64.375 ± 12.43, tree savannas 23.94 ± 3.3 and tree/shrub savannas 21.012 ± 10.12). There was no significant difference in soil organic carbon and total soil carbon stocks under different land use types, except between forest (24.67 ± 5.37 Mg C ha-1) and tree/shrub savanna (8.92 ± 1.57 Mg C ha-1). The long drought episode of the early 1980s was identified as the trigger for cashew plantation establishment, whilst better income and lack of technical support were the main drivers for expansion. Plantation size of households was influenced by two demographic factors of age and gender farmers (P < 0.05 and 0.305 R2-adjusted). Natural vegetation vulnerability to future cashew expansion was in the descending order of forest/woodland (21.43%), tree savanna (11.87%) and tree/shrub savanna (8.27%). This implies that cashew expansion is of higher threat to more woody vegetation which has serious implication in terms of conservation and carbon emissions. There is therefore a need for a more sustainable management approach to cashew agriculture practices to ensure optimum production for farmers, while conserving the forest-savanna ecosystem.