Vegetation seasonality in the North (>50N) may be defined as the opposite of plant growth - simple explanation.
We used 30 years (1982 to 2011) of satellite measurements of vegetation greenness (figure to the left) to evaluate vegetation seasonality.
We first determine the growing season from the freeze/thaw state of the ground.
Plant growth is then approximated by the total vegetation greenness during the period when the ground is thawed (figure to the right). Vegetation seasonality is the inverse of plant growth - the shaded region.
Vegetation seasonality thus defined for broad latitudinal belts around the Earth depends only on the Sun-Earth geometry (Earth's axial tilt and Orbit around the Sun), as in the case of temperature.
This definition is only valid if plant growth decreases with increase in latitude, which is true in the North.