Flowering Ecology of Some Spring Woodland Herbs

Abstract

Fecundity characteristics, phenology, and behavior of insect flower—visitors were studied for 7 early flowering woodland herbs: Claytonia virginica, Dentaria laciniata, Dicentra canadensis, Dicentra cucullaria, Erythronium albidum, Isopyrum biternatum, and Sanguinaria canadensis. Sanguinaria canadensis is facultatively autogamous, the Dicentras are obligate outcrossers, and the remainder are self—compatible, at least within a stem. All are insect pollinated except sometimes S. canadensis. The numbers of ovules per flower and flowers per stem tended to be inversely correlated, and large—seeded species (S. canadensis, E. albidum, I. biternatum) had lower numbers of potential seeds per stem than did small—seeded species. Flowering of all species typically occurred during the first prolonged period of weather suitable for pollinator activity and ceased by the time the canopy closed. Annual differences in flowering times were associated with differences in average temperatures (i.e., early blooming in a warm, early spring), but cumulative degree—hours or degree—days of air or soil temperatures were not well correlated with flowering times. Other constraints on flowering phenology are discussed, including the predictability of suitable conditions, a proposed "fail—safe" mechanism that may assure flowering before canopy closure even if temperatures are abnomally low, and the importance of nontemperature factors in defining suitable conditions. Flowering time was not very finely tuned to the temperature regime and pollinator activity; flowers blooming during the flowering peak often had low seed production and the fertilization rate of most species was low. Evidence that seed production may have been pollinator limited for several species was obtained by comparing the success of hand pollination and of natural pollination, rarity of certain specialized pollinators, and estimates of the abortion rates of fertilized ovules. We suggest that flowering in early spring is a high—risk option in terms of insect—mediated sexual reproduction. Certain flower—visiting insects favored D. laciniata out of proportion to its abundance, but no effect on seed set of other species was detectable. Honeybees were abundant and active flower visitors with the potential for disrupting ecological/evolutionary relationships between native insects and flowers.