ANDERSON, GREGORY J.* and JAMES D. HILL. Department of Ecology and Evolutionary Biology, University of Connecticut, U-43 Storrs, Connecticut, 06269-3043 USA.. - Many To Flower, Few To Fruit: The Reproductive Biology of Hamamelis virginiana (Hamamelidaceae).
Hamamelis virginiana flowers late September to late November.
In 1977, we began studying the reproductive biology of this eastern
USA tree, including: floral phenology and rewards, pollen-ovule
ratios, breeding system, pollination, pollinator and resource
limitation, and seed dispersal. The homogamous, self-incompatible
flowers emit a faint odor, bear nectar with sucrose ratios typical of
bee and fly flowers, and produce abundant sticky pollen. Flowers were
visited infrequently by insects representing six orders. Flies were
the most common floral visitors, specifically members of the genus
Bradysia, whereas small bees carried high percentages of
Hamamelis pollen. Despite high P/O ratios (11 445
grains/ovule), bees and flies are likely pollinators as experiments
indicate wind pollination is much less likely. Pollen quantity and
resource availability did not appear to limit reproductive output, but
pollen quality did. Tests of > 40 000 flowers showed natural fruit set
to be < 1%. The flowering time, breeding system, and clumped
dispersion of plants due to limited seed dispersal combine to yield
this remarkably low fruit set. Because all other Hamamelis
flower from late winter to early summer, it may be that H.
virginiana evolved a fall flowering phenology to avoid competition
for pollinators with the closely related H. vernalis.
Key words: Breeding System, Floral phenology, Fruit/Flower Ratios, Hamamelis, Pollen-ovule Ratios, Pollination, Pollinator Limitation, Reproductive Limitations