Tribe Neottieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 70

Fruiting Success and Limiting Factors
Thornhill ( 1996 ) reported fruit set values of 40-60% in natural populations and
suggested that E. gigantea might be pollinator limited. This possibility is supported
by Wilson's ( 2009 ) observation of a statistically significant difference in fruit set
between open-pollinated control plants (62%) and hand-pollinated outcrossed plants
(72%). She tracked individual flowers and found that 100% of those visited by
syrphids produced fruit and seed. She believes that pollinator limitation may in
this case be compensated by facultative autogamy, probably as a backup in older,
unvisited flowers.
According to Coleman ( 1986 ), almost all the flowers in some California popula-
tions set fruit, whereas other nearby populations remained unpollinated. The reason
for the difference is not known, but where fruiting is successful seedling recruitment
can be high.
Mantas ( 1993 ), in a 2-year study in northwestern Montana, found that the per-
centage of ramets producing flowers was very low, from 3.2 to 4.8%. Each flower-
ing ramet produced 4.2 (1-9) flowers. At the same time, the yearly percentage of
flowers producing mature fruits varied widely from 6.8 to 63.5%. Mantas attributed
the variation to high levels of rainfall during the blooming period of 1 year, reducing
pollinator visitation rates. Wilson ( 2009 ) also observed variation in fruit set among
sites and years and speculated that the differences might in part reflect differential
supplemental pollination by other insects, such as ants and aphids.
Mantas ( 1993 ) also reported that seed production was higher in open stands,
which routinely produced taller ramets and more flowers than dense stands. The
latter were more likely to reproduce vegetatively. Arditti et al. ( 1981 ) found that
germination rates averaged 20% under laboratory conditions, that a shady environ-
ment was needed for germination, and that seeds from immature capsules germi-
nated better than those from ripe fruit. Although the latter could imply that seeds
decrease in viability during maturation, it also suggests that mature seeds may pass
through a period of dormancy prior to germination, a phase often absent in orchids
(Arditti et al. 1981 and references therein). The seeds are able to tolerate a range of
variation in soil acidity, retaining a constant rate of germination (20%) over a pH
range of 5-7.5. Apparently, however, the plants must survive a fairly long period of
development before reproducing. Myers and Ascher ( 1982 ) report that the interval
from seed to flowering is 39 months.
Variation occurs in the genetic diversity of populations and may reflect differ-
ences in the relative importance of clonal versus sexual reproduction. Nevertheless,
clonal growth appears to be the most frequent overall reproductive strategy in
E. gigantea (Thornhill 1996 ) . At the same time, the production of numerous small
seeds permits long-distance dispersal and self-compatibility, plus genetic variation
among populations allows colonization of a wide variety of geographically isolated
habitats.
Little is known about the effects of predation and disease on this orchid. Mantas
( 1993 ) found that browsing by slugs, insects, and ungulates occurred at one site in