Abstract
Tropical rainforests are recognized as one of the terrestrial tipping elements which could have profound impacts on the global climate, once their vegetation has transitioned into savanna or grassland states. While several studies investigated the savannization of, e.g., the Amazon rainforest, few studies considered the influence of fire. Fire is expected to potentially shift the savanna-forest boundary and hence impact the dynamical equilibrium between these two possible vegetation states under changing climate. To investigate the climate-induced hysteresis in pan-tropical forests and the impact of fire under future climate conditions, we employed the Earth system model CM2Mc, which is biophysically coupled to the fire-enabled state-of-the-art dynamic global vegetation model LPJmL. We conducted several simulation experiments where atmospheric CO_2 concentrations increased (impact phase) and decreased from the new state (recovery phase), each with and without enabling wildfires. We find a hysteresis of the biomass and vegetation cover in tropical forest systems, with a strong regional heterogeneity. After biomass loss along increasing atmospheric CO_2 concentrations and accompanied mean surface temperature increase of about 4 ^circ C (impact phase), the system does not recover completely into its original state on its return path, even though atmospheric CO_2 concentrations return to their original state. While not detecting large-scale tipping points, our results show a climate-induced hysteresis in tropical forest and lagged responses in forest recovery after the climate has returned to its original state. Wildfires slightly widen the climate-induced hysteresis in tropical forests and lead to a lagged response in forest recovery by ca. 30 years.
Highlights
Ature stress, as well as increasing fire regimes threaten the survival of large areas of tropical forests [6,7,8,9,10]
Similar mechanisms were found for atmospheric moisture recycling [17] and deforestation [18]. Such system hysteresis is often accompanied by the existence of tipping points, where relatively small disturbances can cause a transition from one system state to another
We aimed to investigate the potential for a climate-induced hysteresis and multiple stable states using the fire-enabled dynamic global vegetation model (DGVM) LPJmL, coupled to the Earth system model (ESM) CM2Mc
Summary
Ature stress, as well as increasing fire regimes threaten the survival of large areas of tropical forests [6,7,8,9,10]. Similar mechanisms were found for atmospheric moisture recycling [17] and deforestation [18] Such system hysteresis is often accompanied by the existence of tipping points, where relatively small disturbances can cause a transition from one system state to another. Another study identified three stable states (forest, savanna and grassland) by analyzing remote sensing data (Hirota et al [16]) They found that deforestation to the threshold of 60% tree cover might lead to a self-propagating shift to an open savanna state. Using integrated remote sensing data, a hydrological model and atmosphere moisture tracking simulations, they emphasized the importance of moisture recycling in forests for the spatial extent of tropical forests Most of these studies rely on conceptual models, uncoupled simulations or remote sensing data—hardly ever on Earth system models. Investigating the tropical hysteresis using a fire-enabled and stateof-the-art DGVM coupled to an Earth system model (ESM) still remains a challenge, because of the complexity of such a models and computational demands
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