Varying relationships between fire radiative power and fire size at a global scale
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Abstract. Vegetation fires are an important process in the Earth system. Fire intensity
locally impacts fuel consumption, damage to the vegetation, chemical
composition of fire emissions and also how fires spread across landscapes. It
has been observed that fire occurrence, defined as the frequency of active
fires detected by the MODIS sensor, is related to intensity with a
hump-shaped empirical relation, meaning that occurrence reaches a maximum at
intermediate fire intensity. Raw burned area products obtained from
remote sensing can not discriminate between ignition and propagation
processes. To go beyond burned area and to test if fire size is driven by
fire intensity at a global scale as expected from empirical fire spread models,
we used the newly delivered global FRY database, which provides fire patch
functional traits based on satellite observation, including fire patch size,
and the fire radiative power measures from the MCD14ML dataset. This paper
describes the varying relationships between fire size and fire radiative
power across biomes at a global scale. We show that in most fire regions of the
world defined by the GFED database, the linear relationship between fire
radiative power and fire patch size saturates for a threshold of intermediate-intensity fires. The value of this threshold differs from one region to
another and depends on vegetation type. In the most fire-prone savanna
regions, once this threshold is reached, fire size decreases for the most
intense fires, which mostly happen in the late fire season. According to the
percolation theory, we suggest that the decrease in fire size for more
intense late season fires is a consequence of the increasing fragmentation of
fuel continuity throughout the fire season and suggest that landscape-scale
feedbacks should be developed in global fire modules.
