Varying relationships between fire intensity and fire size at globalscale

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 but 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 intensity. Raw burned area products obtained from remote-sensing can not discriminate between ignition and propagation processes. Here we use the newly delivered global FRY database, which provides fire patch functional traits including fire patch size from satellite observation, to go beyond burned area, and to test if fire size is driven by fire intensity at global scale as expected from empirical fire spread models. We show that in most regions of the world the linear relationship between fire intensity and fire patch size saturates for a threshold of intermediate intensity fires. The value of the threshold differs from one region to another, and we suggest that it might be driven by drought, and the amount of available biomass. In some regions, once this threshold is reached, we also observe that fire size decreases for the most intense fires, which mostly happen in the late fire season. According to the percolation theory, we suggest that this effect is a consequence of the increasing fragmentation of fuel continuity along the fire season so that landscape-scale feedbacks should be developed in global fire modules.