Grass Species Flammability, Not Biomass, Drives Changes in Fire Behavior at Tropical Forest-Savanna Transitions

Abstract

Forest-savanna mosaics are maintained by fire-mediated positive feedbacks; whereby forest is fire suppressive and savanna is fire promoting. Forest-savanna transitions therefore represent the interface of opposing fire regimes. Within the transition there is a threshold point at which tree canopy cover becomes sufficiently dense to shade out grasses and thus suppress fire. Prior to reaching this threshold, changes in fire behavior may already be occurring within the savanna. Such changes are neither empirically described nor their drivers understood. Fire behavior is largely driven by fuel flammability. Flammability can vary significantly between grass species and grass species composition can change near forest-savanna transitions. This study measured fire behavior changes at eighteen forest-savanna transition sites in a vegetation mosaic in Lopé National Park in Gabon, central Africa. The extent to which these changes could be attributed to changes in grass flammability was determined using species-specific flammability traits. Results showed simultaneous suppression of fire and grass biomass when tree canopy leaf area index (LAI) reached a value of 3, indicating that a fire suppression threshold existed within the forest-savanna transition. Fires became less intense and less hot prior to reaching this fire suppression threshold. These changes were associated with higher LAI values, which induced a change in the grass community, from one dominated by the highly flammable Anadelphia afzeliana to one dominated by the less flammable Hyparrhenia diplandra. Changes in fire behavior were not associated with changes in total grass biomass. This study demonstrated not only the presence of a fire suppression threshold but the mechanism of its action. Grass composition mediated fire-behavior within the savanna prior to reaching the suppression threshold, and grass species composition was mediated by tree canopy cover which was in turn mediated by fire-behavior. These findings highlight how biotic and abiotic controls interact and amplify each other in this mosaicked landscape to facilitate forest and savanna coexistence .

Frequent fire reorganizes fungal communities and slows decomposition across a heterogeneous pine savanna landscape

“…Recurrent fires maintain and structure many terrestrial ecosystems, including prairies, chaparral, savannas, and some coniferous forests. In many of these 'pyrogenic' systems, dominance of flammable grasses in the ground layer promotes the spread of fire (Beckage et al, 2011;Staver et al, 2011;Cardoso et al, 2018), which prevents ecosystem transition to a different state (Beckage et al, 2009;Callaham et al, 2012;Dantas et al, 2016;Pausas & Bond, 2019). Unlike high-profile, destructive wildfires (Chen, 2006;Hood et al, 2018), frequent low-intensity fires are essential for maintaining biotic and abiotic components of 'pyrogenic' biomes (Fill et al, 2015;He & Lamont, 2018) that include some of the most diverse plant communities on Earth (Bond et al, 2005;Noss et al, 2015;Peet et al, 2018).…”

Reviewing Fire, Climate, Deer, and Foundation Species as Drivers of Historically Open Oak and Pine Forests and Transition to Closed Forests

“…Tree and shrub encroachment has occurred in the Great Plains grasslands and South America, Africa, Europe, Australia, and Asia (e.g., Eldridge et al, 2012). In tropical zones of the Afrotropics, Neotropics, and Australia, in regions with moderate precipitation, researchers are examining alternative ecosystem states of savannas and closed canopy forests (e.g., Cardoso et al, 2018).…”

“…Because alternative ecosystem states and species can occur within the same climate range, or even outside of climate boundaries (Svenning and Skov, 2004;Cardoso et al, 2018), climate alone poorly predicts the global distribution of species and ecosystems (Bond et al, 2005). Additionally, annual precipitation both above and below the long-term mean occurred during the past century and millennia (Herweijer et al, 2007).…”

Climate change and deforestation boost post-fire grass invasion of Amazonian forests

“…As a result, the risk of reaching a critical canopy cover threshold beyond which highly flammable shade-intolerant grasses can establish in the forest understory is expected to increase. (Hoffmann et al 2012; Silvério et al 2013; Cardoso et al 2018). Once this threshold is crossed, both fire intensity and frequency are likely to increase abruptly, as the high cover by low bulk density grass fuels in the understory dramatically increases fuel flammability (Hoffmann et al 2011).…”

“…To avoid being arrested in a fire trap (Murphy et al 2010; Grady & Hoffman 2012; Trauernicht et al 2016), the forest must be resilient, that is, recover canopy cover quickly enough to exclude shade-intolerant flammable grasses before the next fire (Hoffmann et al 2012). Many studies in both Africa and South America have identified a Leaf Area Index (LAI) of 3 as the critical canopy cover threshold at which shade-intolerant C4 grasses are excluded from the forest understory (Hoffmann et al 2012, Dantas et al 2016, Cardoso et al 2018). How climate change and deforestation will affect post-fire grass invasion and the resilience of forests to grass-fire feedback is not currently known.…”