Response of plant communities to fire in an Acacia woodland and a dry Afromontane forest, southern Ethiopia

To understand the role of fire in degrading tropical dry forest ecosystems of north-east Africa, fire potential was studied within two vegetation types: Acacia woodland and dry Afromontane forest. Fire behaviour was analysed in experimental fires late in the dry season (February) and the potential response of the soil seed bank was studied on samples taken before and after fire treatment. Bark thickness was also measured, as an indication of fire-resistance, on a suit of tree/shrub species representing the two vegetation types and an ecotone between them. The total fuel biomass differed only marginally between the two vegetation types. But biomass of the litter fuel differed significantly, and the biomass of litter fuel was two times greater in the Acacia woodland (673 g m−2) than in the dry Afromontane forest (308 g m−2). In contrast, the biomass of woody fine fuels was four times greater in the dry Afromontane forest (300 g m−2) than in the Acacia woodland (76 g m−2) and the biomass of live fine fuels was also several times greater in the dry Afromontane forest (110 g m−2), compared with the Acacia woodland (15 g m−2). During experimental fires in the Acacia woodland, fuel consumption was 100% for all burned plots, but in the dry Afromontane forest fuel consumption ranged between 10 and 40% of the available fuel within the plots. The results from the soil seed bank study indicated little if any impact from fire. There was a gradual increase in bark thickness of trees when going from the dry Afromontane forest over the ecotone towards the Acacia woodland site. Considering all species analysed, bark thickness of trees 15 cm in diameter at 1 m height ranged from 2.4 to 15.4 mm. Therefore, based on the differences in microclimate, fuels, fire behaviour and bark thickness the Acacia woodland was determined to be the most fire prone as well as fire resilient ecosystem. In contrast, sustained combustion was unlikely at the studied dry Afromontane forest site. Spread of fire was not possible due to the high fuel moisture content and poor fuel bed. Forest fragmentation and intentional burning of grasslands, followed by an altered microclimate, should however increase the probability of fires penetrating the dry Afromontane forest.