Supplementary Figure 1. Physical mechanisms in patterned-ground. Frost et al – ERL Supp. Figure 1

circle inter-circle inter-circle circle circle circle circle Frost et al – ERL Supp. Figure 1

silt loam blocky blocky organic organic circle inter-circle inter-circle circle circle circle circle Sharp microsite contrasts in soil conditions develop due to differential frost-heave (Taber 1929; Peterson and Krantz 2003) Frost et al – ERL Supp. Figure 1

Initially, on a barren surface, a more or less hexagonal system of cracks develops due to seasonal freezing. Frost et al – ERL Supp. Figure 1

Cracks collect seeds and retain moisture, so vegetation and organic matter accumulate there. This creates a strong gradient in soil thermal properties. Frost et al – ERL Supp. Figure 1

In early winter, downward freezing occurs faster in exposed mineral soils that lack surface organics. Frost et al – ERL Supp. Figure 1

Increased water flux to circle Pore-water is drawn to the advancing freezing-front in circles due to cryostatic suction. Frost et al – ERL Supp. Figure 1

Differential frost-heave Increased water-flux leads to stronger frost-heave at circles, as the water expands into ice. Frost et al – ERL Supp. Figure 1

Cryoturbation is strongest at the circle center. Tundra vegetation grows too slowly to withstand annual disturbance, so circles remain barren. Frost et al – ERL Supp. Figure 1