Location of the 40 seed families of Arabidopsis lyrata collected in Saugatuck Dunes State Park, Michigan, USA, with information on the four environmental variables studied: distance from the crown edge of the nearest tree (in metres, m), position on dunes (0 = top of dunes, 1 = bottom of dunes), per cent coverage by herbs and grasses (vegetation cover) and density of A. lyrata plants (per quarter m²).