Fig. 1: The conceptual implementation of road fragmentation in ORCHIDEE-SPITFIRE.

a Fragments are conceived of as circles whereby edge effects on fuel moisture, wind infiltration, and human ignition (v1-3) are defined by the ‘edge depth’ through which there is a gradient from fragment edge to interior. This is shown in the transect at the bottom, where each gradient decreases /increases towards the fragment interior, and values for the edge depth are shown. b The ratio of the total ‘edge area’ (blue shading) versus the ‘non-edge area’ (green) is defined by the edge depth and the surface area of the individual patch, which depends on fragmentation extent and so the number of patches per grid cell. This surface area also limits the maximum size of any individual non-extreme fire. Note that grid cell surface area varies with latitude, affecting AED (see Eq. 1, Methods). c (top) Fragmentation extent is proxied by road length to generate average circular patch area and radius (‘average edge distance’), determining the size and number of fragment patches in a grid cell. (bottom) The result of these conceptual implementations alters a given variable (var1-3) in direct proportion to the edge area entailed by a–c. See Figs. S1 and S2 for greater detail on how these implementations affect the sequence of processes represented in the model.