Fire regimes and forest resilience: alternative vegetation states in the West African tropics
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Terrestrial ecosystems, including tropical forests, are hypothesized to have tipping points beyond which environmental change triggers rapid and radical shifts to novel alternative states.
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 pushed some forests 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 remotely-sensed data 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 four forest reserves. We analyzed precipitation trends to determine if shifts in vegetation and fire regime reflected a shift to a drier climate.
Two of the reserves experienced forest loss, were impacted by frequent fires, and transitioned to a vegetation community dominated by shrubs and grasses, which was maintained by fire–vegetation feedbacks. The other two reserves experienced less fire, retained higher levels of forest cover, and resisted fire encroachment from surrounding agricultural areas. Precipitation remained relatively stable, suggesting a hysteresis effect in which different vegetation states and fire regimes coexist within a similar climate.
There is potential for human land use and fire to create novel and persistent non-forest vegetation communities in areas that are climatically suitable for tropical forests. These disturbance-mediated regime shifts should be taken into account when assessing future trajectories of forest landscape change in West Africa.
KeywordsRegime shift Tipping point Tropical forest ecosystem Tropical forest fire Upper Guinean forest
This work was supported by a NASA Earth and Space Science (NESSF) Fellowship awarded to Francis Dwomoh. We also acknowledge support from the USDA Forest Service, Southern Research Station through Cooperative Agreements 11‐CA‐11330136‐098 and 14-CA-11330136-015. Dr. Mark Cochrane, Dr. James Vogelmann, and two anonymous reviewers contributed helpful comments on earlier drafts of this manuscript. Many thanks to Dr. Zhihua Liu for his technical assistance.
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