Fuels or microclimate? Understanding the drivers of fire feedbacks at savanna–forest boundaries

Abstract

The higher flammability of tropical savanna, compared with forest, plays a critical role in mediating vegetation-environment feedbacks, alternate stable states, and ultimately, the distribution of these two biomes. Multiple factors contribute to this difference in flammability, including microclimate, fuel amount and fuel type.To understand this transition in flammability, we studied fuel characteristics and microclimate across eight savannaforest boundaries in south-central Brazil. At each boundary, the environment was monitored for one week with automated measurements of near-surface wind speed, air temperature, relative humidity and presence of dew. Manual measurements were performed to quantify fuel amounts and fuel moisture. These data were used to parameterize the fire behaviour model BehavePlus5 in order to simulate fire behaviour over the savanna-forest boundary. There were strong gradients across the boundary in all variables with the exception of total fuel load. During the day, savannas had higher wind speed and air temperature, and lower relative humidity and fuel moisture than forests. Although fuel loads were similar in savanna and forest, savanna was characterized by lower fuel bulk density, largely because of the presence of grasses. Based on these measurements, the fire behaviour model predicted savanna fires to be faster, more intense, and with greater flame lengths, relative to forest. A sensitivity analysis indicated that the primary cause of these differences was the low fuel bulk density characteristic of grassy fuels, with lesser contributions from wind speed, fuel moisture and total fuel load. These results indicate that the dominance of grassy fuels is the primary cause of the high flammability of savanna.

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…Typically, savanna fires are extinguished within a few meters of closed-forest margins. Both the lack of grassy fuels and the forest shade, and changes in the microclimate, especially a reduction in wind speed, have been identified as causes of forest resistance to burning [63]. Nevertheless, under extreme conditions (high wind speeds, high temperatures, low humidity, high grassy fuel loads) fires originating in savannas can penetrate deep into closed woody vegetation [64].…”

Sensitivity of woody carbon stocks to bark investment strategy in Neotropical savannas and forests

“…Thus, large trees and trees with a large β at a given height are more likely to survive than small trees and trees with a small β. Survivorship is also dependent on the amount of grass biomass present. This grass dependence comports with observations that an abundance of grass biomass results in higher ecosystem flammability and hotter fires that cause increased tree mortality (Hoffmann et al, 2012b). These ideas are incorporated in the following equation based on Pellegrini et al (2016a) derived from data in Hoffmann et al (2009)…”

“…(2), survivorship is the tree survivorship fraction of a given tree size class and functional guild, g b is grass biomass (in g C m −2 ), and 25 is the threshold for increased fire intensity based on grass biomass curves of savanna flammability from Hoffmann et al (2012b). In the model, fire intensity is based only on grass biomass present and grass survivorship after fire is always zero.…”

“…A positive feedback between flammable grass presence and tree fire mortality in open canopies has been identified as an important mechanism in maintaining savanna regions (Archibald et al, 2009;Hoffmann et al, 2012b;Staver et al, 2011a). Some trees can persist, despite high fire frequencies in many saPublished by Copernicus Publications on behalf of the European Geosciences Union.…”

Spread of common native and invasive grasses and ruderal trees following anthropogenic disturbances in a tropical dry forest

“…Disturbance can negatively influence resource acquisition and use by native trees and saplings (Hoffmann et al 2009), increase fuel loads for wildfires, and alter understory vegetation structure (Rojas-Sandoval and Acevedo-Rodríguez 2014;Hoffmann et al 2012;Cordell and Sandquist 2008;Brooks et al 2004). The process of natural regeneration after a disturbance relies on the local plant community's ability to survive in a resource-depleted ecosystem, and amidst competition from potential invasive species, before the system can regain its optimal functionality (Ammondt and Litton 2012;Rew and Johnson 2010).…”