Choice of substrate varies by species as well. Moss species can be classed as growing on: rocks, exposed mineral soil, disturbed soils, acid soil, calcareous soil, cliff seeps and waterfall spray areas, streamsides, shaded humusy soil, downed logs, burnt stumps, tree trunk bases, upper tree trunks, and tree branches or in bogs. Moss species growing on or under trees are often specific about the species of trees they grow on, such as preferring conifers over broadleaf trees, oaks over alders, or vice versa. While mosses often grow on trees as epiphytes, they are never parasitic on the tree.