Plants can decrease in size by shedding components of their modular structures. In herbaceous perennials, we suggest shrinkage may be the result of two non-mutually exclusive biological processes: plasticity in response to adverse environmental conditions, and senescence. Due to their longevity, it is likely that perennials will face periods of fluctuating resource availability driven by changing environmental conditions, such as drought; shrinking may be an adaptive response to decrease resource demands. Additionally, if this decrease in size occurs consistently after maturity and is related to a decrease of fertility, it may indicate an age-related decline in plant vigor. Despite the potentially important evolutionary and ecological consequences of shrinkage, little is known about the frequency with which herbaceous perennial plants shrink, nor why it occurs. The occurrence of shrinkage was examined in 155 published transition matrices of herbaceous perennials. Of these, at least 65 species show shrinkage, but only in 35 cases is shrinkage explicitly acknowledged as a biological process per se. Shrinkage may actually happen more frequently than reported in these studies, both due to the possibility that some studies had been only carried out during favorable growing periods, and because many transition matrix models lack the precision to identify shrinkage. We present a new transition matrix model which clearly distinguishes all possible processes structured in three submatrices containing transition probabilities, asexual contributions, and sexual contributions respectively. This model allows for comprehensive analysis and interpretation of all biological processes occurring in the life cycle of the species of interest.
