Phenological Variation in Flowering and Fruiting of Asclepias

Abstract

Phenological studies of flowering and fruiting in experimental plots and in field sites demonstrated variation within and between populations of Asclepias incarnata, A. verticillata and A. syriaca. Flowering of A. incarnata and A. verticillata occurred in July and August, and overlapped at least 30 days. In contrast, A. syriaca typically flowered in June or early July. Umbels of these species displayed open flowers for 5-9 days; 70-95 %o of marked flowers within umbels opened within 1 day of the first open flower. Individual variation in flowering within a species was considerable, and the times of initial flowering of individuals of A. incarnata were significantly correlated between years. Second-year plants varied more than 1st-year plants in peak flowering time, abundance and duration, but not in initial flowering date. Mean percentage fruit set per umbel for late-flowering plants was about twice that of early flowering plants for all species in 1978, despite shorter duration of flowering and lower average umbel abundance. The phenology of fruit maturation and seed release revealed greater synchrony in seed release, as well as longer periods for follicle maturation and dehiscence, for A. syriaca relative to the other species. INTRODUCTION Systematic studies routinely seek to detect and document diagnostic characters that are relatively invariant within species but are discontinuous among species. Reproductive characters are often considered the most useful taxonomically because of their general constancy (Jones and Luchsinger, 1985). Recently, though, we have been made aware of intraspecific, and even intrapopulational, variation in reproductive systems with respect to floral coloration (Paige and Whitham, 1985) and scent (Galen and Kevan, 1983), levels of self-compatibility (Denton, 1979; Kephart, 1981; Perkins et al., 1975), and even the relative placement of stamens and pistils (Waser and Price, 1984). Estes et al. (1983) further argue that monographers often have only cursorily examined potential floral variation, thus missing important factors contributing to species evolution and classification. Flowering time has been used as a reproductive character in taxonomic keys and in classification (Davis and Heywood, 1973); yet most studies have focused on its importance in species isolation (Gentry, 1974; Whitaker, 1944), and in competitive or mutualistic interactions (e.g., Mosquin, 1971; Waser and Real, 1979) rather than on intraspecific variability. Variational patterns among individuals are infrequently reported and only seldom are data available to ascertain the effect of the flowering time on seed production (e.g., Augspurger, 1981; Gross and Werner, 1983; Primack, 1980; Schemske et al., 1978). Few comparative studies of floral variation in both natural populations and experimentally planted areas exist. The present study measures individual and interpopulational variation in flowering and fruiting for three sympatric species (Asclepias incarnata L., A. syriaca L. and A. verticillata L.) grown in common experimental plots and observed in field populations for several seasons. Although flowering periods vary annually for these species (Willson and Price, 1977), little is known of intraspecific variability in flowering among populations and individuals within a local area. Because such variability in flowering time is known to be heritable in other taxa (e.g., Cruciferae, McIntyre and Best, 1978; Caprifoliaceae, McMillan and Pagel, 1958; Leguminosae, Stanford et al., 1962), it is of potential evolutionary significance.