15 Demographic Senescence in Herbaceous Plants

For herbaceous plants, in contrast to many higher animals but similarly to other modular and sedentary life forms, it is not so much a question of how senescence progresses as it is a question of whether senescence occurs at all for most species. Overall, both the empirical evidence and theoretical predictions provide contrasting evidence. Monocarpic species and other semelparous life forms are an extreme example of senescence. For polycarpic, iteroparous plants, however, there is only limited evidence that senescence occurs at all. Our review of studies with herbaceous plants also shows that there are only a few studies with detailed age-based demographic data, and here too the evidence for senescence is limited. Moreover, the detrimental effects of ageing are hard to detect in the observational studies upon which most of our current knowledge is built. Our theoretical expectations suggest that it is likely that the evolutionary pressure that shapes senescence in other organisms also acts on plants, but there are also several aspects of plant biology that conflict with some of the assumptions in classical models of the evolution of senescence.