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The Seedbed:

Road slopes are a giant seedbed beyond the pavement on which
seeds will germinate under conditions of sunlight, moisture and
sufficient nutrients to support growth.

Moisture which reaches the root systems of plants is in part a
function of steepness. The steeper the slope and the smoother the
texture of the soil surface, the faster will be the rate of runoff.
Runoff will penetrate the soil surface over a longer time period
on flatter slopes because the rate of acceleration is lower. We
know that snowpack will be retained longer on north and east facing
slopes than on south and west facing slopes. Therefore, the former
will be wetter longer.

We know, too, that moisture is a factor in the success of
volunteer species which will occupy a slope; and that dry site
species will occupy the upper slopes and that wet site species will
occupy the lower slopes. In the Yellowstone we find that lodgepole
pine germinates quickly on slopes exposed to sunlight, and that fir

and spruce will germinate on duff covered, moist shaded slopes.
The succession of species following forest fires is the same.