Foliose lichen on top, crustose on side of wall

Foliose lichen on top, crustose on side of wall

Observation - Foliose lichen on top, crustose on side of wall - UK and Ireland. Description: Yellow and white-grey foliose lichen species on the top of a wall which is rough and seems fairly new. However, on the side of the wall, it seems only a yellow crustose species can grow very well. Is there much of a reason why?

Yellow and white-grey foliose lichen species on the top of a wall which is rough and seems fairly new. However, on the side of the wall, it seems only a yellow crustose species can grow very well. Is there much of a reason why?