Ongoing global changes have the potential to cause catastrophic ecosystem transitions with potentially severe consequences on the Earth’s climate, water and energy balance that are pivotal to life. Consequently, threats of regime shifts in terrestrial ecosystems are a major ecological concern due to escalating stresses from climate change, land use change, and other disturbances including fire. Of particular concern is the hypothesis that tropical forests have tipping points beyond which environmental change triggers rapid and radical shifts to novel alternative states. In this study, we explored the overarching hypothesis that fire-mediated alternative stable states exist in the semi-deciduous tropical forest zone of Ghana, and that increased fire activity has compromised forest resilience by pushing some forests past a tipping point to a new state in which a novel ecosystem with low tree density is maintained by fire. We combined a 30-year time series of Earth observations with field measurements to assess land cover trends, the effects of fire on forest vegetation, and the reciprocal effects of vegetation change on fire regimes, in selected forest reserves. We analyzed precipitation trends to determine if shifts in vegetation and fire regime reflected a shift to a drier climate.
Results/Conclusions
Widespread fires in the 1980s fostered a declining trend in forest cover in some forest reserves. In the ensuing decades, these reserves experienced substantial forest loss, were impacted by repeated fires, and transitioned to a vegetation community dominated by shrubs and grasses, which was maintained by fire-vegetation feedbacks. Human activities such as logging and agriculture helped to create conditions that resulted in increased fire susceptibility and led to the observed regime shifts. Other reserves had less pressure from human activities and experienced less fire, and they retained higher levels of forest cover and resisted fire encroachment from surrounding agricultural areas. Precipitation remained relatively stable across the entire study area suggesting a hysteresis effect in which different vegetation states and fire regimes coexist within a similar climate. These results suggest that fire and human land use can create novel and persistent non-forest vegetation communities in areas that are climatically suitable for tropical forests. Thus, there is the need for consideration of disturbance–mediated regime shifts when assessing future trajectories of forest landscape change in West Africa. This study demonstrates the potential vulnerability of tropical forests to state transitions in response to frequent fires, and adds to our understanding of fire–mediated regime shifts in terrestrial ecosystems.