Annual expansion of V. velutina. The picture depicts the two cumulative linear distances (right axis) between the invasion origin and the successive limits of the invasion front to the south (black line) and east (dark grey line). The southern and eastern limits of each year were measured by simply drawing a tangent to the southernmost point and the easternmost point, respectively. The left axis refers to the yearly cumulative invaded area (in km2) resulting from diffusion dispersal and depicted as a gray area.