Evolutionary and Demographic Approaches to the Study of Whole Plant Senescence

There have been two distinct definitions of “plant senescence” which have developed within the literature. First, physiologists and cell biologists use the term senescence to describe the continual turnover of cells and plant parts that occurs within an individual as part of an internally controlled program of development. In cases of monocarpy (semelparity), this program can be responsible for the death of the whole organism. The details of this program of “physiological senescence” within individuals are addressed in the other chapters of this book. The second, alternative approach to senescence is termed “evolutionary senescence” and it addresses theories and experimental evidence explaining variation in mortality patterns among individuals within populations and between species. Senescence, as viewed by most animal and evolutionary biologists (see Chapter 1), is a decline in age-specific survival and reproduction with advancing age. The evolutionary theories of senescence are designed to explain why this senescence occurs in most species, and to explain the variation in the rates of evolutionary senescence between different species. It is this, evolutionary, population-level, approach to senescence that will be considered in this chapter. This chapter will apply a demographic approach to senescence that has traditionally been used exclusively by animal biologists and theoreticians, to the study of plants and the determination of mortality patterns and longevity.