Life-history variation and the control of flowering in short-lived monocarps

Traditionally short-lived plants are classified as annual, biennial or perennial. Although this distinction has proven to be useful in horticulture there are problems when it is applied to natural populations. First: not all annuals and biennials are monocarpic but some can show repeated flowering (thereby acting as perennials e.g. Poa annua L., Senecio jacobaea L., Digitalis purpurea L.) or reversely perennials may act as monocarps under unfavourable conditions (e.g. Scabiosa columbaria L.) Second: monocarpic "annuals" and "biennials" often do not behave as their names suggest. Especially these socalled biennials show large variation in the timing of reproduction, sometimes being annual, sometimes being monocarpic perennial. Furthermore, the traditional classification is merely descriptive. In this context there is a clear need for a more refined classification based on the processes that determine the onset of flowering in short-lived monocarps. Such a classification would have the advantage that it is not merely descriptive but that it can be used to predict what life-history can be expected under different environmental conditions and it would be helpful in unravelling the processes determining population behaviour. In trying to understand the processes involved in the onset of reproduction in shortlived monocarps, much attention was given to the ageversus-size dependency of flowering (Werner 1975, Werner and Caswell 1977, Hirose 1983, Lacey 1986a). So far almost all studies indicate that the probability of flowering is most strongly affected by size (Tab. 1). Comparatively few studies show that flowering is to some extent also age related (in bamboos, Janzen 1976; Cirsium vulgare (Savi)Ten., Klinkhamer et al. 1987). Of equal importance but receiving far less attention by population ecologists are the need of vernalization (sensu Chouard 1960) and the interplay between size dependence on the one hand and vernalization and photo-induction on the other hand. It is the aim of this paper to show how insight in the latter processes may help us to understand the life-history variation found within and between species and may guideline us in making a classification of short-lived monocarps that is more refined than "annual" and "biennial". Because, at this point, we have no better alternative we shall speak of annuals as species that complete their life-cycle within one year, of strict biennials as species that complete their life-cycle within two years, and of delayed biennials as species that can complete their life-cycle in two years but usually delay flowering to a later year.