Lichens on a tree. Two types of lichen (Parmelia sulcata, green/grey, centre, and Xanthoria sp. yellow/green, centre right and lower left) growing on the branch of a larch tree (Larix decidua). Lichens are mutualistic (mutually beneficial) associations between a fungus and an alga. The alga produces carbohydrates for the fungus by photosynthesis, whilst the fungus protects the alga from harsh conditions. Lichens are found growing on the surfaces of trees and rocks. On trees they are epiphytes: they live on, but do not harm, their host. Parmelia sulcata is common throughout the northern temperate zone.