Evolution of senescence in iteroparous perennial plants

We applied four tests to detect evidence of the evolution of senescence in life tables and fecundity schedules for 65 species of iteroparous perennial plants. Test 1 determined the pattern of variation in age-specific mortality with age (mu (x)). Fifty-five percent of species showed an increase in, or maximum value of, mu (x) at the end of life. In test 2, we tried to separate mortality into initial or baseline mortality and senescent mortality by fitting the survival data of these 65 species to Weibull functions. Unlike published results with animals, the rate of senescence was independent of initial mortality rate. However, a positive relationship was found between rate of senescence and reproductive lifespan, suggesting increasing risk of death with successive reproductive events. It has been suggested that a decline in reproductive value with age is a better diagnostic of senescence, but (in test 3) this occurred in only 9% of species (6/65). Our fourth test detected a positive correlation between age at first reproduction (a) and mean reproductive lifespan (L-a), as predicted by the theory that senescence is due to a trade-off between adult survival and reproduction. Comparing species within the two largest families present in the data set, we found a correlation between a and L, among the Liliaceae, which was largely represented by ramet life tables, but not among the Poaceae, which was largely represented by genet life tables. Clonal growth, which is common in plants, is a necessary but not a sufficient condition to prevent the evolution of senescence. We predict that clones that fragment are more likely to escape the evolution of senescence at the genet level than clones that remain physiologically integrated.