Senescence is a manifestation of programmed cell death in plants (it's called apoptosis in animals). Senescence of plant tissues can occur as part of the plant's normal life cycle (eg. leaves of deciduous trees) or as a result of environmental stresses such as salinity, nutrient deprivation, or insect attack. As the senescing parts die, a lot of the cellular contents (mineral ions, organic metabolites) are released for the rest of the plant to reclaim.