Why don't lichens grow well on trees or walls?

Lichens may struggle to grow on trees or walls due to factors like limited access to essential nutrients, moisture, and light. Trees often have a canopy that blocks sunlight and can create varying microclimates that aren't ideal for lichen growth. Additionally, the bark of trees may be too smooth or have a chemical composition that inhibits lichen attachment. On walls, particularly those treated with chemicals or painted, the surfaces might be inhospitable for lichen colonization.