As necrotrophs (which acquire nutrition from dead cells or tissues) and biotrophs (which obtain nutrients from living cells or tissues), fungi directly interact with trees. These interactions can take the form of parasites or mutualistic mycorrhizas. The great majority of plants in nature depend on mycorrhizas for their health because they provide them with water, mineral nutrients, and protection against root diseases. By digesting dead organic materials and releasing the nutrients that were contained therein, fungi have an indirect impact on trees because they are saprotrophs. The interactions between fungi, other microorganisms, and soil invertebrates that alter mycelial growth and function make the relationships between fungi, soil, and trees complex. The outcome of these interactions will probably change with a changing environment, and consequently, so will the relationship between fungi and trees.