PS 32-30
Understanding fire regimes and tropical forest resilience: Fire-mediated alternative stable states in the West African tropical rainforest
The tropical forest ecosystem is a potential tipping element likely to shift to alternative states; with dire consequences for ecosystem functions, human wellbeing, the climate system and terrestrial biodiversity. In this process, the interactions between climate change and land-use pressures could alter fire regimes thereby hastening such ecosystem state transitions. Particularly, frequent and severe fires could culminate in the replacement of forest by fire-dependent vegetation in a positive feedback loop. Thus, the West African tropical forests are vulnerable to widespread fire-mediated state transitions because they are embedded in a hotspot of global warming, human population pressure, degradation, and fragmentation, all of which increase vulnerability to fire. Here, we hypothesize that, in the dry deciduous forest zone, increased fire activity has compromised forest resilience by pushing the system past a tipping point to an alternative stable state in which a novel ecosystem with low tree density is maintained by fire. We analyzed historical Landsat and MODIS imagery to map and quantify fire severity patterns and land cover change; and assessed fire severity effects on canopy recovery after a historic large fire in 1989. We conducted vegetation survey to assess changes in floristic composition and flammability traits of plant species.
Results/Conclusions
Our preliminary analyses of forest reserves in the dry deciduous forest region in Ghana suggest that increased fire activity has compromised forest resilience by pushing the system past a tipping point to an alternative stable state in which a novel ecosystem with low tree density is maintained by fire. Our results indicate that the historic 1980s El Nino-driven fires were sufficiently extreme to remove forest canopy dominants, thereby triggering a landscape-scale alternative stable states. This change has been maintained through time as evidenced by persistent land cover change. Thus, the catastrophic fires initiated a positive fire-vegetation feedback loop such that severely burned forests were replaced with fire-dependent vegetation, which is maintained through frequent burns. Finally, our vegetation survey, 25 years after the initial fire, indicates that there has been a floristic composition shift towards more pyrogenic vegetation in the fire-impacted forest reserves. This study expands our knowledge on tropical forest disturbance regimes and ecological resilience. Lessons learned will be useful for climate change mitigation and forest restoration efforts in the region.